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Experiments
DOES AIR TAKE UP SPACE? 1. MATTER: AIR CAN YOU FILL THE EMPTY BOTTLE? Stuff a large handkerchief or some crumpled newspaper into an empty glass or jar. Make sure the handkerchief won’t fall out when you turn the glass upside down. Then, fill a pot with water. Holding the glass so that its mouth is down, put the glass deep into the pot of water and hold it there. After a minute or two, pull the glass out of the water and remove the handkerchief. You will see that: The handkerchief is dry. Explanation: Water cannot fill the glass because the glass is already filled with air. The “empty glass" is full of air. So, air takes up space. Air is a gas. It has no size or shape of its own but will fill every space it can.  Place a funnel in the neck of an empty soda bottle. Pack clay around the neck of the bottle so that there is no space between the bottle and the funnel.  Pour water into the funnel. Notice what happens.  Then take the clay off the bottle and funnel.  You will see that: While the clay is there, the water remains in the funnel or enters the bottle only in slow spurts. When the clay is removed, the water flows freely into the bottle.  Explanation: The clay seals the neck of the bottle outside of the funnel. When water flows into the funnel, the air cannot escape except by going through the water very slowly. The air in the bottle takes up space and prevents the water from coming in. When the clay is removed and air is able to leave around the neck of the bottle, then water can flow in. This proves that air takes up space. DOES AIR WEIGH ANYTHING? WHICH IS HEAVIER, HOT AIR OR COLD? Drill holes (or make notches) is needed. Its length is 6 inches from each end of a narrow, 3-foot length of wood, such as a yardstick. Then, make a hole in the exact center of the stick, 18 inches from each end. Place a cord or wire through the center hole and suspend the stick from a chair back or a rod. Blow up a large balloon or beach ball. Tie its mouth tight and hang it from one of the end holes of the stick. Then, suspend a small can or box (such as a baking powder container) from the other hole. (See illustration.) Put a little sand or rice in the can until the stick balances. Then, let the air out of the balloon. You will see that: The can sinks down as the air is let out of the balloon. Explanation: When the air leaves the balloon, the balloon becomes lighter. Air has weight. At sea level, air weighs 1.25 ounces per cubic foot. (See if you can find a carton, or stack up books, to measure 1 foot wide, 1 foot long and 1 foot deep. Then you will know the space taken up by 1~ ounces of air. ) On a mountaintop, air is a little thinner and weighs less. Balance an “empty” baby bottle on one end of your yardstick and a tin can on the other. Put sand or rice in the can if needed. Hold a candle flame for one minute near the mouth of the bottle. Remove the flame and balance the scale again. You will see that: The bottle goes up when heat is applied to the air in it. You must remove sand or rice from the can on the other end to balance the scale. Explanation: Warm air weighs less than cold air occupying the same space. WHAT HAPPENS TO WARM AIR? WHAT IS WIND? Rinse one jar with very cold water, and rinse another jar with hot water. Dry them both thoroughly. With a cardboard between them, place the jars mouth to mouth with the warm jar on the bottom. Ask someone to blow puff of cigarette smoke into the bottom bottle, as you lift the cardboard. Let the smoke fill the bottom jar, and then pullout the cardboard. You will see that: The smoke will rise from the lower to the upper jar. Explanation: The smoke raises as the warm light air rises and the cold heavier air sinks. Try the experiment with the cold jar on the bottom and the warm onion top. What happens this time? Sprinkle talcum powder on a cloth. Shake a little of the powder off near a lamp with a light bulb which is not lighted. Notice what happens to the powder. Then light the bulb and give it a few minutes to get hot. Shake some more powder off the cloth. You will see that: Before the bulb is turned on, the powder sinks slowly down through the air. After the bulb is hot, the powder rises. Explanation: When the air gets warmed by the lighted bulb, it rises, carrying the lightweight talcum powder with it. The cooler heavier air is pushed down. This flowing of cooler air to take the place of hot air happens outdoors too. We know it as wind. AIR PRESSES IN ALL DIRECTIONS CAN AIR HOLD UP WATER? Cover the wide mouth of a funnel with a piece of rubber from a balloon or from a rubber sheet. Tie the rubber on tightly. Suck some air from the narrow end of the funnel and notice what happens to the rubber. Turn the funnel upside down and suck in again. Then turn the funnel side- ways and suck in. You will see that: When you suck in the air, the rubber is pulled in. The same thing happens whatever the direction of the funnel. Explanation: You are removing air from the inside of the funnel by sucking in. The outside push of air is then greater than the push inside, even when the funnels held upside down or sideways. Air pushes equally in all directions. The push-or pressure—of air is almost 15 pounds per square inch at sea level. (There’re 15 pounds of air pressing on this picture of square inch.) Fill a glass or jar with water. Place a piece of cardboard or stiff paper on top of the glass. Hold the cardboard in place and turn the glass upside down over a sink or basin. Then take your hand away from the cardboard. You will see that: The water stays in the glass until the cardboard be- comes soaked. Explanation: The water is held in the glass because of the pressure of air outside the glass against the cardboard. This air pressure is greater than the pressure of water against the cardboard. If the experiment doesn’t work the first time, try again. This time, fill the glass to the very brim and make sure no bubble of air enters between the cardboard and glass as you turn the glass over. A TRICK BOTTLE HOW DOES A STRAW WORK? Punch a small hole near the bottom of an empty canthat has a screw top (a floor-wax can, for instance). Fill the  can  with  water  and  cap  it quickly. Noticewhat happens. Then remove the top. You will see that: As long as the top is on, the waterwill not flow from the hole. When you take off thetop, the water flows freely. Explanation: Air presses up harder than the waterpresses down until you remove the top. Then the airpressure on top plus the pressure of the water makethe down- ward pressure greater. Color a few ounces of water with vegetabledye. Place a paper or glass straw in a glasswith the colored water. Suck up a little of thewater into the straw. Then hold your fingeracross the top of the straw and pull the strawout of the liquid. What happens? Then remove your finger from the top ofthe straw. You will see that: While your finger coversthe top of the straw, the liquid remains in thestraw. When you remove the finger, the waterflows out. Explanation: With your finger you arelessening the pressure of air over the straw.The greater pressure of air under the strawcan hold the liquid inside the straw.HOW DO SUCTION CUPS WORK? You will need two sink plungers for this experiment. Ask a friend to bring a sinkplunger from his kitchen when he comes to visit you. Using your own, too, press thecups together. Now try to separate them. Each of you can pull hard. You will see that: It takes great effort to separate the plungers. Press one of theplungers against a smooth kitchen chair. Try to lift it. You will see that: The chair can belifted with the plunger. Explanation: You have forced out the air from the inside of the plunger and thusreduced the air pressure from within. The pressure from the out- side is then morepowerful. Suction is actually a difference in air pressures. Now you know why suction-capped arrows stick to a smooth board or wall. Try topress a suction cup to a window screen or to a grate. Why doesn’t it hold? THE SIPHON Place a tall jar almost full of water on the table and anempty jar of about the same size on a chair alongside thetable. Fill a rubber tube or shower hose with water andhold the water in by pinching both ends of the tube or byusing clothes- pins as clamps. Stick one end of the tubeinto the jar on the table and place the other in the jar on thechair. Remove the clothespins, or open the tube ends.Notice what happens. When the water stops flowing, reverse the position of thejars. Then try both jars on the table. You will see that: The water will flow as long as the levelof water in one jar is lower than the level of water in theother Explanation: Gravity-the pull to the center of the earth-causes water to flow from the hose and reduces thepressure within it (at B). The air pressure is greater at Aand water is forced into the hose. A siphon, then, is a tube which uses air pressure andgravity to run water up over a high place. Try to use thesiphon without filling the hose with water. Does it work? HOW TO COMPRESS AIR AIR CAN HOLD A STICK DOWN Hold a glass with its mouth down and push itinto a deep bowl of water. You will see that: The water enters the glass alittle way. No bubbles of air escape. Explanation: The water forces the air into asmaller space. The small particles of air-the airmolecules—are forced closer together, orcompressed. Releasing com- pressed airfurnishes power, and many machines work onthis principle. Place a stick about the size of a yardstick on the table so that about a foot extendsbeyond the edge. Strike down on the free end. Notice that the other end of the stickpops up into the air. Then lay a sheet or two of newspaper over the section of the stick that rests on thetable. Smooth down the newspaper carefully by stroking from the center of the paperto the edges. Hit the uncovered end of the stick a sharp glancing blow with a hammer. You will see that: The covered stick won’t move up. If you hit the end of the stickhard, it will break.Explanation: When you smooth down the newspaper, you press all the air out fromunder the paper. The portion of the stick covered by the newspaper is held down bythe air pressing down from above. AIR SLOWS THINGS DOWN SOME SURPRISES ABOUT AIR PRESSURE Take two pieces of ordinary paper-newspaper will do. Crumple one into aball. Lift your arms high and drop bothpieces of paper at the same time. You will see that: The crumpled paperdrops right to the ground. The flat sheetfloats slowly down. Explanation: Air resists the movement ofobjects. The larger the surface pressed onby the air, the harder it is for the object tomove through the air. The flat, wing- likesheet of paper has a larger surface than thecrumpled ball. Cars, trains and planes are streamlined toreduce the amount of surface to be moved 1. Place two books 4 or 5 inches apart,and lay a sheet of paper over the books tocover the space between them. Blowthrough the space under the paper. You will see that: The paper sinksbetween the books. 2. Hang two balloons a few inches apartand blow between them. You will see that: The balloons movetogether. Explanation: By causing air to move, youlessen the air pressure. The faster airmoves, the less pressure it has. Airplanescan rise from the ground because of this. A PAPER HELICOPTER MAKE   AN  ATOMIZER  Make a slit in a paper straw about 1/3 from one end. Bend the straw at the slit andplace the short section into a glass of water. Make sure the slit is no more than 1 4 of an inch above the surface of the water. Blow hard through the far end of the longsection of the straw. You will see that: Water will enter the straw from the glass and possibly come outthrough the slit as a spray. Explanation: The stream of air blowing over the top of the short section of strawreduces the pressure at that point. Normal pressure underneath forces the water up inthe straw. The moving air blows the water off in drops. Now you know how perfume atomizers, window-cleaning sprays, and other suchdevices work. Cut a sheet of paper so that you have astrip about 2 inches wide and 6 or 7 incheslong. Hold the paper lengthwise and fold10 to 12 narrow (1 4-inch) strips on one end so that this endof the paper is weighted. (See illustration.)Then, starting from the other end, cut thepaper in half lengthwise for a distance of 3inches. Fold one half forward and the otherback to make flaps. Raise the helicopter above your head andholding it by one of the flaps, let go. You will see that: The helicopter will whirlaround until it reaches the ground almostdirectly beneath the spot from which it wasdropped. Explanation: Air flowing past the bladescauses them to whirl. If the motor of ahelicopter in flight failed, this is the way thewhirling blades would break its fall. FOOD IS MOSTLY WATER 2. MATTER: WATER WATER COMING OUT OF THE AIR Grate a potato or apple, or squeeze anorange or a piece of raw meat. Let a lettuceleaf stand in the air. You will see that: Water (juice) will bepressed or squeezed out. The lettuce will wiltand grow smaller as the water in it dries up. Explanation: Most of our foods contain largequantities of water. Potatoes are 3 4 water. Green vegetables, such as lettuce,are 95% water. Beef is more than 3 5 water. Men and animals are made up of60% to 70% water. Water is necessary tosustain life. Do you know now why dehydrated foods-foods with the water removed-are used whenit is necessary to save space? Remove the label from an empty tin can. Fill it with iceand add water and a few drops of vegetable dye. Let itstand on the table for a short while. You will see that: The can seems to be sweating,” fordrops of water form on the outside. Explanation: The drops are not colored and so theycould not come from ice water leaking out of the can.The water comes from the air. Water vapor (water in theform of gas) in the air around the can has been cooledby the ice. The small particles of air, the air molecules,are slowed down when they become cold, so they movecloser together (see chapter on heat) and change intoliquid form. This is known as condensation. Clouds are formed when large numbers of these dropsof water collect on dust particles as the air is cooled.The drops fall to earth, as rain or snow, when theybecome too heavy to be held up by the pressure of air. WATER GOING INTO THE AIR 1. Place an  equal amount of water intwo jars. Cap one of them. Place both onthe table overnight. You will see that: There is less water inthe open jar than in the capped jar. Explanation: Even at room temperature,the tiny particles or molecules of watermove fast enough to fly out and escapeinto the air. When the jar is uncapped,this is exactly what happens. Some of thewater turns into an invisible gas andescapes into the air. This process isknown as evaporation. Do you understand now how puddles disappear after the rain stops? 2. Place an equal amount of water in a large flat dish and in a deep narrow jar. Placeboth, uncovered, on the table to stand. You will see that: There is less water in the flat dish than in the narrow Explanation : The molecules of water escape only from the surface. Therefore water evaporates faster from a large surface than from a small one. Now you know why a large shallow puddle dries more quickly than a deep narrowone. 3. Hang two wet handkerchiefs to dry. Fan one with a cardboard, but let the other drywithout fanning. You will see that: The handkerchief that is fanned dries first. Explanation: By replacing the moist air near the handkerchief with drier air, fanningspeeds up evaporation. This is one of the reasons a windy day is a good day fordrying clothes. 4. Half fill two dishes with water. Place one in the sun or on the radiator, and the otherin the shade or another cool place. You will see that: The dish in the sun loses its water first. Explanation: The warmer the water, the greater is the speed of the molecules. Themolecules move off into the air faster and speed up the rate of evaporation. When evaporation takes place very quickly, it is known as boiling. (For more aboutevaporation and heat, see further pages) THE STRANGE STORY OF WATER’S SIZE  1. WATER EXPANDS WHEN HEATED Fill a jar with water to the brim. Heat it gently in a saucepan containing an inch or twoof boiling water. You will see that: The water overflows. Explanation: Water, like other liquids, fills more space when heated. The moleculesbounce against one another more rapidly and spread out. 2. WATER CONTRACTS WHEN COOLED TO 390 FAHRENHEIT Fill a jar (to the brim) and cool it in the refrigerator. You will see that: The jar is not quite full. Explanation: Until it goes down to 390 Fahrenheit, water contracts- takes up lessspace-as it gets colder. The molecules move more slowly and closer together. 3. BUT WATER EXPANDS ON FREEZING Fill a jar full of water and cap it with a piece of cardboard. Place it in the freezer ofyour refrigerator until it freezes. You will see that: The cardboard cap is forced off. Explanation: When water goes below 39 0 to its freezing temperature of 32 0, itexpands-takes up more room. It is one of the few things to behave this way. If you use a tight cap on the jar as you freeze it, you will break the jar. Have you everheard of water pipes busting because water froze inside’? 4. ICE IS LIGHTER THAN WATER Place an ice cube or two in a glass of water. You will see that: The ice cubes float. Explanation : Because water expands as it freezes, ice is actually lighter than water. Itis only 10/11 th as heavy. This lucky fact speeds up the melting of ice in the warm layerof ice on the surface also slows down the freezing of the rest of the water in the lakeand pond and protects the fish and other life there. WATER ISN’T PURE Place 5 tablespoons of tap water in a small glass dish andallow to stand. You will see that: A white ring is left after the waterevaporates. Explanation: The white ring is formed by minerals whichhave dissolved in the water as it flowed through the soil. Look at the inside of an old teakettle. Do you see themineral deposit? That ring around your bathtub is not somuch a ring of dirt as a ring of minerals from the water itself. Try evaporating rain water. Does it contain minerals?WHAT IS HARD WATER? WHAT HAPPENS WHEN SOMETHING DISSOLVES? Make a powder out of a piece of chalk by grinding it with a stone. Add the powderedchalk to a jar full of water. Stir the mixture and filter it by pouring it through ahandkerchief used as a strainer. Pour half of the mixture into another jar and add 1tablespoon of washing soda or borax. Add the same amount of soap powder to both jars. Shake them. You will see that: The water to which you added washing soda produces more suds. Explanation : You made hard water by adding chalk (or limestone) and then softenedpart of it with the washing soda. Certain materials such as limestone (the chalk) makewater “hard.” Hard water does not mix well with soap. The washing soda added to onejar softened the water so that it mixed more easily with soap than the hard water. The name “hard water” is said to have been given during the Civil War. When soldiersfound their beans were hard after being cooked in a particular water, they left behindsigns, “Hard Water. Do you have hard water? You can test your tap water by comparing the amount ofsuds it produces with that made in the hard and soft waters of this experiment. Fill a glass with water to the brim. Slowlyshake in salt, stirring carefully with a thinwire or a toothpick. See how much salt youcan add without making the water overflow. You will see that: If you are careful, youcan add an entire shaker of salt to the fullglass without spilling any water.Explanation: You are making a solution ofwater and salt. It is believed that as the saltdissolves, molecules of salt separate and allthe spaces between the molecules of water. INVISIBLE INK MAKING A CRYSTAL To a tablespoon or two of salt, gradually add asimilar amount of hot water. Then, dip a clean pen or a small stick (the clean endof a used match is fine) into the solution. Write yourmessage on a sheet of paper. At first, your message can be seen. Let the paperstand for half-hour or so and the writing disappears. Rub over the sheet of paper with the side of a softpencil. You will see that: Your message will be clearlyvisible. Explanation: The water evaporates from yoursolution, leaving the small particles of salt clinging tothe paper. These make the paper rough and uneven,but they are too small to be seen. When you rub overthe paper, the pencil lead darkens them and causesthe particles of salt to stand out. Gradually stir 1/4 cup of sugar into hotwater until the water is too full to acceptany more. Then hang a string in thesolution and let it stand for several daysor a week. You will see that: A crystal forms on thestring. Explanation: The water evaporates in theair, leaving behind the sugar, in the formof a solid crystal. WATER PRESSURE WHICH WAY DOES WATER RUN? Punch 3 or 4 small holes, one above the Other,along the side of an empty milk carton or a largecan. Cover the holes with a long strip ofadhesive tape and fill the carton with water. Thenplace the can in the sink or a basin and pull offthe tape. You will see that: The stream from the lowesthole travels farthest.Explanation: The water at the bottom of thecarton has the force exerted by the pressure ofthe water above it. Like air, water has pressure. Water pressure depends, as your experimentshows, on the water’s depth. Many cities pumpwater into raised tanks. This is done to give thewater enough force to run up into people’shomes from pipes beneath the ground. Remove the cover of a quart-size can. With a nail, punch holes around the can about2 inches from the bottom. Cover the holes with a circle of adhesive tape. Fill the can with water. Center it on a sheet of newspaper in a sink or basin and stripoff the tape. You will see that: The water travels the same distance from each of the holes. Yourstreams of water make a circle on the newspaper. Explanation: Pressure is the same at the same depth. Water pressure is the same in alldirections if the depth is the same. PRESSURE AND SHAPE  AND  SIZE WATER SEEKS ITS OWN LEVEL Punch a hole 1 inch from the bottom of anempty, frozen orange juice can and do the same toa much taller can. Cover each hole with a strip oftape. Fill both cans with water to the same level. Ofcourse, it will take more water to bring the largercan to the same depth. Place the cans in a sink or basin and pull off thestrips of tape. You will see that: The streams of water shoot outto the same distance. Explanation: Hard as it is to believe, the pressureof the water does not depend on the size or shapeof its container but on the depth of the water. Insert a funnel into one end of a 2- or 3-footstrip of rubber tubing or a narrow hose. Intothe other end of the tubing, insert a glassstraw or tube. Holding both the funnel and the glass tubingupright, as in the illustration, pour water intothe funnel. You will see that: The level of the water in thefunnel and in the glass tube will be the same. Explanation : The same pressure pushes onboth and so the depth of the water is thesame. Try raising and lowering the funnel a littleand notice what happens. MEASURING WATER PRESSURE Connect two glass or clear plastic straws with a short length of rubber tubing. Attachthe straws to a support, as in the illustration. Use adhesive tape to bind them to theblock of wood. Color some water with vegetable dye and pour it into the tubes until the straws havewater to their halfway mark. Cover a funnel with a circle of thin rubber (from a balloon or old rubber sheet).Stretch the rubber taut and tie it tightly with thread or a rubber band. Attach the funnelto one of the straws with a long length of rubber tubing. With this gauge, or manometer, you can now measure water pressure. Fill a pail withwater and test the device. Put the funnel into the pail of water-first just below thesurface, then halfway down, then all the way under. You will see that: The colored water moves lower in the closed straw and higher in theopen one, as the funnel goes deeper into the pail. Explanation: The pressure of the water on the rubber of the funnel forces themovement of the colored water. With your manometer, compare pressure near the surface and toward the bottom ofthe water. Compare the pressure of the same depth of water in a milk carton and afrozen-juice can. Compare the pressure of the same depth of water and other liquidsabout the house-orange juice, rubbing alcohol, oil, milk. A HOT WATER BAG LIFTS BOOKS Fit a hot water bag with a 1-hole rubber stopper or cork. Punch a hole in the bottomof an open can or carton and fit the hole with another I-hole stopper or cork. Placeshort glass or plastic straws into each of the Stoppers. You will also need 4 to 5 feet ofrubber tubing to connect the hot water bag to the can. Fill the hot water bag with water and stopper it. Fit on the rubber tubing, attaching theother end to the glass tube of the can. Rest the bag on the floor and press it gently untilwater fills the tube. Then fill the can with water. Put a large, flat board on the hot water bag and then stack books or blocks on top ofit. Raise the can up. You will see that: As you raise the water, the books move. Explanation : The pressure increases at the bottom of the tube as you increase theheight of the tube. Increased pressure on one part of the enclosed water is carried bythe water in all directions equally. This is how the hydraulic press works. A barber chairis raised by a hydraulic press that uses oil as its liquid, and the hydraulic brake in theautomobile uses oil and alcohol. YOU ‘WEIGH” LESS IN WATER Attach a spring or a rubber band to a nail on aboard. Fill a small bottle or screw-top can with water. Puta string around the bottle and attach it to the rubberband. Note how much the rubber band stretches.Then lower the bottle into a pail of water and noticewhat happens to the rubber band. You will see that: The rubber band is stretchedless. Explanation: The  bottle  appears  to weigh lessbecause the water exerts a lifting force, known asbuoyancy. An object in water is buoyed up by aforce equal to the weight of the water it displaces. WHWHAT FLOATST FLOATS?A FLOATING OBJECT DISPLACES OWN WEIGHT Put an empty stopper medicine bottle in a pan ofwater. Observe what happens. Half fill the bottle ina pan of water, stopper it, and place it in the panagain. Fill it completely and watch again. You will see that: The empty bottle floats but asyou fill it with water it sinks lower and lower. Thefull bottle sinks. Explanation: Objects float in water if they arelighter than a quantity of water that would take upan equal amount of space. The bottle continues totake up the same amount of space as it getsheavier. When it is heavier than the water whichwould occupy an equal amount of space, it sinks.Place wooden, plastic and brass buttons in a glassof water. Which float? Weigh a small block of wood in a large dry can. You can use the yardstick balancedescribed earlier. Then take out the wood and place a smaller can into the larger one. Fill the small canto the brim with water. Carefully push the wood block into the water until no moreflows over into the larger can. Remove the small can carefully. Weigh the large can with its overflowed water on the yardstick balance. You will see that: The weight of the water in the large can equals the weight of thewood in the large can. Explanation: An object that floats displaces its own weight of water. A boat floatsbecause it displaces water that weighs as much as it does. BOTTLE  SUBMARINE FLOAT AN EGG Half fill a small medicine bottle or tiny glass with water. Then pour water into a tall jar or glass. Hold the water ina small bottle with your finger and put the bottle, upsidedown, into the glass bottle. If the bottle floats on top of the water, add water to thebottle. If it sinks, pour out a little. When the bottle is just barely floating, fill the tall jar withwater to its top. Cover the jar with a circle of balloonrubber. Stretch it taut, and tie it tightly. Hold the palm of your hand over the rubber and pushdownwards. Then release your hand. You will see that: the bottle dives down. When youremove your hand, the bottle floats again. Explanation: When you press with your hand, you forcethe air inside the bottle to compress—to occupy lessspace—since water cannot be compressed. This leavesroom for more water. When the added water enters thebottle, the bottle becomes heavier than the water which itdisplaces and sinks. Place an egg in a glass of fresh water. Noticewhat happens. Add salt to the water, stir gently,and observe what hap- pens. Put a tack in the eraser end of a pencil andplace the pencil in a glass of fresh water. Addsalt, stir gently, and notice what happens. You will see that: In the fresh water the eggand the pencil sink. As you add salt, they floathigher and higher. Explanation: A denser liquid has a greaterupward lift or buoyancy. Salt makes waterdenser. Now you know why ships ride higher inocean water than in fresh water, and why youfind it easier to swim in the ocean than in a lake. SURFACE TENSION  SOAP BOAT 1. Using a cardboard or a fork as a carrier,place a needle on the surface of water in adish. Carefully remove the carrier. You will see that: The needle will float. Explanation: The needle is heavier than theamount of water it displaces and should beexpected to sink. It ~oats, however, becauseof an invisible elastic skin. When water comesin contact with air, the molecules on thesurface of the water huddle closer togetherand form a thin film or skin over the surface. 2. Dip a piece of soap in your dish with thefloating needle. You will see that: The needle sinksimmediately. Explanation: The soap reduces the surfacetension. it is one of the reasons we use soapfor cleaning. By lowering surface tension,soap makes water able to wet greasysurfaces. Use a piece of soap as the fuel for a cardboardboat. Place a notch in your boat and insert a dabof soap. Put your boat in a tub or basin of water. The boat will sail until the soap reduces thesurface tension of all the water in your lake. HOLDING WATER IN A STRAINER HOW MANY DIMES WILL IT HOLD? Pour some liquid oil over a small strainer(a tea strainer will do). The oil coats thesharp edges of the wire. Shake the strainerso that the holes are open. Hold the strainerover a sink or basin. Carefully pour waterinto the strainer from a glass or pitcher,letting it run down the inside of the strainer. You will see that: The strainer fills. Thewater pushes through the openings butcannot get through. Explanation: Surface tension—theinvisible elastic skin-keeps the water fromrunning through. Touch the bottom of thestrainer with your finger and the water willrun through because your finger breaks thesurface of the water. Place a jar or a glass in a basin. Fill the jarto the brim with water. Drop in dimes orthin metal washers, holding them by theiredges. You will see that: You can drop asurprising number of coins into the jarbefore the water flows over. Explanation: Surface tension permits youto heap the water quite high before it breaksand the water runs over. Watch a ball bouncing, a train rushing by, the sun going through the Sky. See anautomobile wheel turn, an airplane fly, a screw boring into a piece of wood. All of these are examples of motion. The science that describes and explains these motions is called mechanics. 3. MECHANICAL ENERGY AND MACHINESWHICH FALLS FASTER? WHY DO THINGS FALL DOWN? Suspend various things from strings-amarble, a can, a fork, a toy. Hold each up ortie it to a rod. Cut each string. You will see that: The objects all fall. Explanation: The force of gravity pullsobjects down toward the center of the earth. This pull of gravity sometimes helps us andsometimes works against us. Gravity keeps usand everything around us from flying off intoSpace, but it makes it harder for us to send arocket to the moon. Compare how mucheasier it is to walk down a flight of stairs thanit is to walk up a flight. When we climb andwhen we lift something we need to make theupward pull or push greater than thedownward pull of the earth. Stand on a sturdy table or on a high chairand drop two objects at the same time—aheavy object and a light one. You will see that: Both reach the ground atthe same time. Explanation: The weight of an object doesnot affect its speed as it falls. But we know that a feather doesn’t fall asfast as a stone and that a man with aparachute falls more slowly than a manwithout one. The shape of the feather and theparachute are important because they offer alarger surface to the air and are slowed downby the air’s resistance.HOW DO YOU PITCH A  BALL? FALLING WEIGHT DOES WORK Throw a ball straight out as far as you can. Notice where it falls. Now with just asmuch energy, throw the ball slightly up and as far as you can. You will see that: The ball thrown slightly upward lands farther away.Explanation: The ball thrown upward has farther to fall before it hits the ground.Meanwhile it is also going away from the thrower. Therefore it has more time to go agreater distance before it strikes the ground. If two balls are thrown straight by boys ofthe same height, the balls will strike the ground at the same time. This is true even if oneboy uses more energy. His ball will go farther but will strike the ground at the same Lift a little pebble and a large stone from the floorand place each on a table. Lay a flat tin can on thefloor near the table. Push off the pebble so that itstrikes the can. What happens? Push off the largestone so that it strikes the can. What happens? You will see that: The large stone makes a large dentin the can while the pebble barely scratches it.Explanation: The large stone stores up more energy.It took more energy to lift it than it did to lift the littlestone. Objects which require more energy to lifthave more energy when they fall. Have you ever seen a pile driver work?SPRINKLERS AND ROCKETS CENTER OF GRAVITY With a hammer and small nail, make 4 small holes near thebottom of an empty can. The holes should be in a straightline about 1 4 inch apart. Run wire around the rim of the top of the can, or punch 2more holes near the top through which to thread the wire.Hang the wire from a piece of string, and then tie the stringto a hanger and support it on a ledge or rod (or in one hand)over a sink or basin. Pour water into the can. What happens? You will see that: The water goes out the holes in onedirection—and the can swings in the opposite direction. Explanation: For every action, there is an equal oppositereaction. As the water rushes out forward it causes the canto move backward. Revolving lawn sprinklers work in muchthe same way. When you row a boat, the oars push the water backward and the boat movesforward. Blow up a balloon and then let go of it. When the air escapes from the balloon, theballoon moves in the opposite direction from the escaping air. This is the law of motion that makes both rockets and let planes work. As hot gasesare forced out the back, the let or rocket shoots forward at high speed. Roll a ball on a level surface. Do it several times and noticewhat happens. Now stick some clay on the ball at one point.Roll the ball again. What happens? Repeat it a few times. You will see that: At first the ball keeps on rolling and stopsin any position. When the clay is fixed to one point, the ballalways stops rolling with the clay touching the surface onwhich the ball is rolled. Explanation: An object acts as though all of  its  weight  isconcentrated  at  one point. This is known as its center ofgravity. It is likely to be located at the part where most of theweight is. An object will tend to move until its center ofgravity is at its lowest possible point. The center of gravity of the ball is at its very center. It isbalanced at any point since rolling neither raises nor lowersits center of gravity. When we put on the clay, however, thecenter of gravity is changed and the ball will tend to roll untilthe clay is at its lowest possible point. STOP AND GO MORE  ABOUT INERTIA Fill a toy wagon with blocks, or pile blocks on a skate. Start it slowly, pull it for atime, then slowly stop it. Then start the wagon quickly, pull it for a time, and stop it quickly. You will see that: It is harder to start the wagon than to keep it moving. The quickeryou want to start it, the harder you need to pull. The faster it is moving, the moreenergy you need to stop it. Also, the more quickly you want to stop it, the more forceyou need. Explanation : It takes more force (push or pull) to start and to stop an object than tokeep it moving. Objects that are moving tend to keep moving, and objects at rest tendto remain at rest. This is known as inertia. Draw a line on the floor and try to run to it and stop. You will find that you may beable to stop your feet but your upper body will continue to move forward. Now you know why you lurch forward when a car stops suddenly. Place a book on a sheet of paper. Thenjerk the paper suddenly. You will see that: The book doesn’t move. Explanation: When you pull the paperquickly, it is easier to move the paper fromunder the book than to move the book.Inertia—the tendency of objects at rest tostay motionless-is responsible. WHY USE WHEELS?WHAT IS FRICTION? Borrow an oil drum or small barrel for thisexperiment. Place the barrel in an up- rightposition and push it across the room. Then turn iton its side and roll it back. You will see that: It is much easier to roll, than topush the can. Explanation: There is less rolling friction thansliding friction. In sliding, the bumps on the roughsurfaces catch against each other. In rolling, thebumps of the wheel roll over the bumps of therough surface without rubbing as much. Tack a piece of sandpaper to half of a board. Thenplace a tack in a small but heavy block or other piece offinished wood. The tack should be free enough so thatyou can loop on a thin rubber band. Holding the block by the rubber band, pull it across thesmooth half of the board. Notice how much the rubberband is stretched. Then pull the block across thesandpaper. Again, watch the rubber band. You will see that: The rubber band stretches more whenyou pull your block across the rough sandpaper. Thegreater stretch of the robber band indicates you are usingmore effort. Explanation: When two things move in contact with oneanother, they resist moving. No two surfaces arecompletely smooth -look at something you think issmooth under a magnifying glass. Therefore, the bumpsof one surface catch against the bumps in the other. Theresistance that results when the surfaces rub against eachother is known as friction. The amount of friction depends on the kinds of surfaces in contact with one anotherand the force pressing them together. The rougher the Surfaces, the greater will be thefriction. The greater the weight of the Objects, the greater will be the friction. Some friction is necessary. New tires with deep, sharp treads are safer than worn-out“smooth” tires. The greater friction between the new tires and the road preventsskidding and spinning. But too much friction wastes energy, produces unwanted heat, and wears away parts. WHY DO WE OIL MACHINES? Slide two blocks of wood over each other. Then rub soap or petroleum jelly overeach surface, and slide the blocks over each other again. You will see that: The surfaces slide more easily after the soap is put on. Explanation: The soap fills in the low places of the surfaces of the wood and alsoforms a coat over the surfaces. The woods, therefore, do not touch one another andcannot rub. Instead, the soapy surfaces slide against one another with less friction. Trycoating a dull safety pin with soap. Notice how much more easily you can use it. Water, too, can act as a lubricant to smooth a surface. Coal chutes are sprinkled withwater to make the chutes smoother. In ice skating, a little of the ice melts under theskate and the skater is thus able to slide over a film of water. For most tools and machines, we use oil or grease to do the job the soap did on ourblocks of wood. The oil and grease smooth the surfaces so that there will be lessrubbing. They are used because they do not dry up as quickly as soap or water orother lubricants. Do you know now why a drop or two of oil will stop the squeak in a door hinge? It’shandy to know, too, that a little wax (a kind of hard oil) will help you open and closeyour desk drawers more easily. MACHINES A machine is anything which makes work easier because it helps us in Some way topush or pull. The machine may allow less effort on our part, or it may increase speed,increase distance, or change direction. All of our complicated machines are based on two or more simple machines whichhave existed for thousands of years. These are the lever, the wheel and axle, the pulley,the inclined plane, the wedge, and the screw. SEESAWS AND SCALES ARE LEVERS WHEEL AND AXLE Place a pencil under the 6-inch mark of a 1-footruler. Balance the ruler. Then place a Penny on each end of the ruler andnotice what happens. Cover the coin on the 12-inchmark with another penny. What happens? Move the two-coin weight closer and closer to thepencil. What happens? You will see that: The two coins (twice as heavyas the one coin) balance the one coin when theyrest on the 9inch mark of the ruler. When you movethe coins even closer to the pencil, one coin is ableto lift two coins. Explanation: This is exactly what happens on a seesaw. You can seesaw With aperson heavier than you if he is moved in close enough to the center. Instead of placing a pencil under a ruler, as you did in the experiment above, suspendthe ruler On a string and balance the coins. Both scale and seesaw are levers. A lever is merely a stiff bar able to turn about onepoint, known as the fulcrum. In the seesaw experiment, the ruler acts as the bar, thepencil as the fulcrum. Many levers have fulcrums at an end of the stiff bar, instead of inthe center. You may be familiar with such levers as the crowbar, the shovel, the baseball bat, butyou may not know that pliers, scissors, tin shears and nut crackers are pairs of levers.Our fingers, arms and legs are levers. So are knives, forks,. rakes and brooms. Howmany more levers can you think of? Take off the cover of a pencil sharpener. Tie alength of string around the axle of the sharpener, asin the illustration. Attach several books to the freeend of the string. Turn the handle of the sharpeneruntil the books are raised to the desk or table onwhich the sharpener is mounted. Untie the booksand lift them the same distance by hand. You will see that: You use less effort lifting thebooks when they are attached to the sharpener. Explanation: You are using the sharpener as a wheeland axle (wheel and rod) to lessen the force neededto lift the weight. This is really a lever that spins in acircle. Other examples of a wheel and axle are adoorknob, a key and a windlass. BOTTLE-TOP GEARS HOW A PULLEY WORKS Thread a stiff wire through a spool and shape the ends into a hook, as in theillustration. You can use a metal clothes hanger, bending the wire back and forth until itbreaks. Suspend the spool (pulley) from a rod or hook. Place a piece of string several feetlong over the spool and attach a small paper box to each end. Place several coins inone of the boxes. Then add various coins and find out what weight you need to lift theother box. When the two sides are balanced, pull down one box 2 inches. Whathappens to the other box? Collect three bottle caps. Be sure they are not bent. Punch ahole through the center of each with a nail. Place them on ablock of wood close enough to one another so that theytouch. Tack the caps down loosely with thin nails so that theyturn easily. Turn one of the caps with your finger or with a pencil andnotice what happens to the others. You will see that: When you turn one cap, all three turn, Explanation : The ridges of each cap act like the teeth of agear and interlock or mesh with the teeth of the gear next to it. You will notice that each gear turns in the opposite directionto the gear next to it. When the gear in the middle turnscounterclockwise, the two on the ends turn clockwise, forexample. Thus gears can be used to change the direction ofthe turning of an axle. A good example of this is when a car isshifted into reverse to make the rear wheels turn backward. In addition to changing direction, gears also are used to You will see that: Equal weights are neededto balance the boxes. When you pull one boxdown 2 inches, the other box moves 2 inchesup. Explanation: You are using your spool andstring as a single fixed pulley. It gives noincrease of force but simply changesdirection. In this case it also allows youto pull down in order to lift up. Pulleys help us to raise windows, get theflag to the top of the pole, move clotheslines.Nearly all cranes, hoists, and elevators makeuse of one or more pulleys. change force or speed. Speed is increased when a small gear is turned by a large one,and force is increased when a large gear is turned by a small one. The teeth on the rimof the gears are to prevent slipping. You can see then that gears are a form of the wheeland axle Examine the gears of an egg beater and of an old clock. Also, notice thechains that connect a bicycle’s gears. BLOCK AND TACKLE SOMETHING ABOUT RAMPS This is an experiment for you and your parents or twofriends. Give a length of broomstick or doweling to each of thegrownups and ask them to stand a few feet apart. Then tiedown one end of a length of clothesline or strong rope to oneof the sticks and weave the rope in and out around the sticks,as in the illustration. You pull on the free end of the rope. You will see that: You will be able to pull the two stickstogether although strong adults try to keep them apart. Explanation: you have formed a combination of pulleys. Theforce you apply is increased by the number of ropes holdingthe weight. In this experiment, you It,, increase your forceeach time you wrap the rope around the broomstick. A smallforce moving a long distance results in a greater force movinga shorter distance. A group of pulleys, called a block and tackle, is used forloading ships lifting shovels of cranes, tightening fences on afarm, lowering and lifting lifeboats, pianos, safes, machinery. Prop rulers of different lengths on a pile of books. Attach a thin rubber band to asmall toy automobile or tack the rubber band to a block. Pull the object up the differentramps and notice how far the rubber band is stretched in each case. Then pull the toystraight up to the books with- out using a ramp. Notice how far the rubber band isstretched. You will see that: The longer the ruler, the less the rubber band is stretched. The bandis stretched most when the object is pulled straight up into the air to the height of thebooks. Explanation: The ramp or inclined plane is a machine that makes it possible to climbgradually. The object rises more slowly but with less effort. When you lift the objectstraight up, it takes more force over a shorter distance, but you do the same amount ofwork. When going up the ramps, less force is used but the distance traveled is longer.The longer the ramp, the less force used. But the object must travel farther to reach thesame height. Gangplanks, winding roads up a mountain, even stairs are all examples of inclinedplanes. Watch the next time a truck man has to raise a heavy load from the ground tothe truck. See whether he uses a ramp to make his job easier. NAILS AND KNIVES SCREWS AND SCREW TOPS Hammer a nail into a block of wood. Pull itout with the claw of the hammer. Then bluntthe end of the nail by filing it down. Try tohammer it into the same block of wood. You will see that: You have more difficultyhammering the filed nail in. Explanation: The end of the nail is a wedgeuntil you file it down. A wedge is two rampsor inclined planes back to back. As the nail isforced into the block, its sloping surfacesmake the job a more gradual one. You neednot bang as hard to get the nail in. Knives, axes, stakes, needles, pins, chiselsare among the common wedges. Cut out a triangle (or ramp) from a sheetof paper, as shown in the illustration. Rollthe ramp around a pencil. What do youhave? You will see that: You have made ascrew. Explanation: A screw is really a ramp orinclined plane wrapped around a roundform. You know about screws that holdtogether pieces of wood or metal. Nowexamine the jars in the house. Do some ofthe covers screw on? Do you know that a piano stool is liftedby a screw? Other examples of the screware a food chopper, electric fan, airplanepropeller, skate clamp and vise.4. HEAT Heat isn’t a thing. It doesn’t occupy space. It has no weight. Like light, sound andelectricity, heat is a form of energy. Heat does work. It is energy that raises thetemperature of a thing by causing the molecules in that thing to move faster. CAN YOU TELL HOT FROM COLD? Prepare three bowls or pans. Half fill one with hot water—not hot enough to burn!Place lukewarm water in the second. Pour very cold water in the third. Set them in arow on the table, with the lukewarm water in the center. Place your left hand in the hot water and your right hand in the cold water. Keep themin for a few minutes. Then take them out, shake off the water, and put both into themiddle bowl. How do they feel? You will see that: Your left hand feels cold and your right hand feels warm. Explanation: When you put your hands in the center bowl, some heat from your lefthand leaves and goes to warm up the water, and so you feel a loss of heat-your lefthand feels cold. Heat from the water travels to your cold right hand, and so you feel again of heat-your right hand feels warm. HOW TO MAKE HEAT BY FRICTION 1. Feel a nail and hammer. Then hammer the nail intoa piece of wood. Feel both nail and hammer again. You will see that: Both nail and hammer are warm. Explanation: The energy of your muscle is given tothe moving hammer, and goes from the hammer to thenail. Because of the added energy, the molecules ofhammer and nail move faster and the heat is increased. 2. Put your hands on your cheeks to see how warmyour hands are. Now rub your hands together quickly10 times. Bring them to your cheeks. You will see that: After rubbing, your hands arewarmer than before. Explanation: Friction (rubbing) causes movement ofmolecules. So the temperature of your hands wasraised.HOW TO MAKE HEAT BY RADIANT ENERGY Pour a little cold water into a saucer. Place it on a window in the sun- light. Let itstand for a while, and then test it with your hand or a thermometer. You will see that: The water gets warmer and warmer. Explanation: The sun sends out rays of energy (infrared) which warm an object whenthey strike it. This is known as radiant energy. HOW TO MAKE HEAT FROM ELECTRICITY HOW HEAT BLOWS UP A BALLOON Feel an electric bulb (not fluorescent)which has not been used for a while.Then turn on the electricity and feel thebulb. (Don’t wait too long.) You will see that: The bulb feelswarm. Explanation : Part of the electricalenergy is converted to heat as it passesthrough the wires (filament) in the bulb.Toasters, irons and heaters make use ofthe same principle. Electric currentscan produce large quantities of heat asthey go through a wire coil. Stretch a rubber balloon over the neckof an “empty” bottle. Put the bottle intohot water, or light a candle and hold thebottle over the flame. You will see that: The balloon blows up. Explanation: When heat is added, themolecules of air in the bottle move fasterand farther apart and therefore the gas(air) occupies more space. As more andmore air flows into the balloon from thebottle, the walls of the elastic balloon arepushed out by the air. Heat has causedthe air to expand. What do you think willhappen when you remove the heat? Tryit. Now you know why it is necessary tocheck balloon tires during hot weather. WHY SIDEWALKS HAVE SPACESHOW A THERMOMETER WORKS Hammer a nail into a tin can. Ease the nailout. Put it in again to make sure that the holeis large enough for the nail. Then, holding thenail with a pair of pliers, scissors or forceps,heat the nail over a candle, in hot water, orover the stove. Try to put it into the hole inthe can. You will see that: The heated nail does not fitinto the hole in the can. Explanation: Heat expands solids. Themolecules in the solid move faster, spreadapart and occupy more space. Now you know why sidewalks are laid insections with spaces between, and why adoor is sometimes difficult to open and closeduring the summer. Fit a medicine bottle or small jar with a cork and tube.You can use a glass straw or the medicine bottle tube. Fillthe bottle to the brim with water colored with a drop ortwo of ink or vegetable dye, and cap it securely. Mark theline the water rises to in the tube. Place your bottle in a pot of hot water or hold it over aburning candle. Notice what happens. Cool the bottle and-watch the results. You will see that: The water rises into the tube whenheated. It drops lower in the tube when cooled. Explanation: Liquids expand when heated and contractwhen cooled. The mercury thermometer we use is basedon these facts. We do not measure temperature directly, but rather thechanges it produces. The liquid of the thermometer(usually mercury) absorbs heat and expands when itcomes in contact with anything warmer than itself. Theliquid of the thermometer grows smaller (contracts) whenin contact with something cooler than itself. Temperature isreally a measure of whether one object will give heat to orabsorb heat from another object. HOW HEAT CHANGES SOLID TO LIQUID 1. Put an ice cube into a tin can or a small pot and apply heat. 2. Heat sugar in a can or pot. 3. Put the paraffin of a candle in a can and apply heat. You will see that: Solids turn to liquid when heated. Explanation: As you add heat, you speed up the molecules of the substance so thatthe solid first expands and then changes to a liquid in which the molecules can moveabout more freely. We call this process melting. Have you ever seen pictures of the pots of red-hot molten steel over a furnace?Afterward, the liquid steel is poured into molds and solidifies- becomes a solid-as itcools.HOW HEAT CHANGES LIQUID TO GAS Heat a little water in a pot or jar and keepheating it. Measure the temperature with acooking thermometer from time to time. You will see that: You get steam, the gaseousstate of water, but the thermometer will not riseabove 212 0 Fahrenheit. Explanation: You speed up the molecules untilthey are flying about and form a gas. Thetemperature rises to the boiling point of 212 0 ,but not above 212 0, allowing all the water to boilaway. Hold a cold glass over the pot while it issteaming but after the heat is turned off. You’llfind that drops of water will form in the glass.Lessening the temperature to below 212 0 allowssome of the gas to change back to liquid form. HOW EVAPORATION COOLSFUN WITH ICE AND SALT 1. Put a tablespoonful of water in one dish, and atablespoonful of rubbing alcohol in another dish. Whichdisappears-evaporates-first? 2. Wet one hand with water and the other with rubbingalcohol. Fan both in the air. Which hand feels cooler?You will see that: Alcohol evaporates (turns into a vaporor gas) more quickly than water. Both alcohol and watercool, but alcohol cools more. Explanation: Heat is absorbed from the surface of yourskin as the water or alcohol evaporates. Therefore thetemperature of your body is lowered. The more rapidevaporation of alcohol results in greater coolness. This is why an alcohol rub is given to someone with ahigh fever. By making use of what you’ve learned about the transfer of heat, you can performscientific “magic.” Dip a string in water until it is thoroughly wet. Lay it across the top of an ice cube.Sprinkle a little salt along the line of the string. You will see that: In a few minutes you can lift the cube by the string. Explanation: Where the salt strikes the ice, it lowers the freezing point of ice (32 0 Fahrenheit) to a little below 32 0 and causes it to melt a little. As the ice refreezes, itencloses the string. FUN WITH ICE CUBES 1. Squeeze two ice cubes together in a towel and hold them for several minutes. You will see that: When you stop pressing, the two cubes are frozen together. Explanation: The pressure causes the ice to melt by lowering its melting temperature.The two cubes freeze together when the pressure is released and the freezing pointgoes up. DEGREE AND CALORIE 2. Tie stones or other weights to the ends of athin wire. Hang the wire and weights over an icecube or larger block of ice. You will see that: The wire passes through theice without breaking it, leaving a solid cube.Explanation: The line of ice directly under the wiremelts because the pressure lowers the meltingpoint, but the water freezes again as the wirepasses through. Does this explain how you skateover ice? 1. Place a small pan of water and a large pot of water on high flames on two stoveburners at the same time. At the point when each bubbles, put in a cookingthermometer and measure the temperature. You will see that: The small panful begins to boil (or bubble) long before the largepan. Both, however, show a temperature of 212 0 Fahrenheit or 100 0 Centigrade at theboiling point. Explanation: More heat is needed to boil the larger amount of water. 2. Place an open can of cold water in each pot. Watch their temperature to find outwhich can gets hotter. You will see that: The large pan will raise the can of water to a higher temperature. Explanation: Both pots of boiling water have a temperature of 212 0 But the large potcan give off more heat energy than the small pot. Total heat is measured by the calorie, the amount of heat needed to raise one gram ofwater one degree Centigrade. (The “Calorie” we mean when we talk about food isequal to 1,000 small calories. ) WHY NOT METAL HANDLES? 1. Put a silver spoon into a hot cup of chocolate. Feelthe heat of the spoon after a few seconds. 2. Melt candle wax. Knead it into lumps as it coolsand press it at various points onto a steel knittingneedle. Dig the point of the needle into a cork and usethat as a handle. Then hold the other end of the needle(not a plastic tip) over a burning candle or other sourceof heat. You will see that: The silver spoon gets hot. Theneedle gets hot enough to melt the wax. Explanation: The molecules of the hot chocolate,moving very quickly, bump into the molecules of thespoon. These bump into the molecules next to them,which bump into the molecules next to them until theheat energy is exhausted. The same thing happens withthe molecules of the needle and wax. Metals are goodconductors of heat. The molecules of the cork are not as easy to move asthose of the metal and therefore the heat energy is notas easily transmitted. Would you rather drink hot chocolate from analuminum cup or from a china cup? Would you prefer ametal or a wooden handle for your frying pan?HOW HEAT TRAVELS IN WATER AND AIR 1. Place some grains of sand, pieces of sawdust, or tiny bits of blotting paper in a jar.Fill the jar almost full of water and heat it in a pot of water. Feel or measure thetemperature. You will see that: As the water is heated, the hotter particles go to the top. The sandmoves so as to show how the currents are traveling from bottom to top. Explanation: When liquids are heated, they expand and take up more room. Thatmeans that they weigh less when warmed. This warm, lighter part of the water moves upward while the heavier, cooler partsinks. The current in the experiment continues as long as there is a difference oftemperature within. Now you can understand why furnaces are usually placed in the basement rather thanin the attic. 2. From a milk carton or piece of cardboard, cut a pinwheel and spiral, s illustrated.Mount each on a knitting needle or wooden stick. Hold ach above a hot radiator orlighted electric lamp. You will see that: The pinwheel revolves. The spiral seems to rise. Explanation: The gadgets are set in motion by air currents produced the warm airrises and the cold air sinks. Wind is simply moving air. HEATING BY RADIATION Punch small holes on each side of a large tin can. Blacken theinside half of the can, where one hole is, with paint or sootfrom a candle dame. Insert used matchsticks in each hole. Meltwax and let it harden on the ends of the sticks. Hold a lightedbulb in the center of the can. You will see that: The blackened side gets hotter; the wax onthe matchstick on that side melts first. Explanation: The dull black surface absorbs and radiatesmore  heat than  the bright shiny surface. Heat hitting the shinysurface is bounced back to the lamp; it is reflected. The sun is not the only source of radiating heat. Everythingradiates heat all the time. Do you see now why we wear light-colored clothes in the summer and dark clothes in the winter? 5. SOUND WHAT CAUSES SOUND? 1. Attach one end of a clothesline or Venetian-blind cordto a doorknob. Measure off about 4 feet of line and restyour foot on the line so that a portion is kept taut. Pluck it. 2. Say “ah-h-h” as you touch the sides of your throat. 3. Place paper clips or bobby pins on a drum (you canmake your own drum by encircling a coffee can withwrapping paper). Beat the drum top lightly. 4. Whisper “too” into straws of different lengths. 5. Strike a fork with another utensil and bring it close toyour ear. 6. Hold a steel knitting needle or yardstick on the edge of atable. Pull the needle or yardstick upward and let it snapdown quickly. You will see that: In each case, you hear a sound-and yousee or feel a movement to and fro. Explanation : In each of your experiments, you makesound by causing an object to vibrate-to move back andforth or up and down. The number of vibrations per second(known as the frequency) depends on the size, shape andmaterial of the object that is vibrating. Our ears cannot hear sound unless the object vibrates atleast 16 times per second and not more than 20,000 timesper second. We know, however, that certain insects andbirds can hear objects vibrating at a much faster speed. Youcan summon your dog with a special whistle which yourdog can hear but you cannot because it vibrates so fast. SEEING SOUND WAVES CAN SOUND TRAVEL THROUGH NOTHING? Attach dry cereal kernels (such as puffed rice) to threads bygluing, sewing or by merely wrapping thread around each kernel.Suspend the threads (close to one another) from a clotheshanger. Hook the hanger onto the back of a chair or shelf so thatyou need not hold it. Then stretch a rubber band out from yourclenched teeth and pluck the taut rubber band next to (but nottouching) the kernel or ball in the center. You will see that: The vibration of the rubber band causes theball next to it to move. As the ball moves to and fro, it hits a ballon each side, which in turn hits its neighbors. This continuesuntil the energy is spent. If the rubber band is plucked harder,more balls move. No one ball, however, moves very far. Explanation: This will give you an idea of how sound travelsfrom a vibrating object to your ear. When an object vibrates tomake sound, the object bumps the small invisible air or solid orliquid particles or molecules next to it on all sides. Before they If there is no air to carry a wave from a vibrating object toour ears, can we hear a sound? In this experiment you takethe air out of an “empty” bottle to create a vacuum. Use aglass coffeepot or quart milk bottle fitted with a cork so thatyou can close it tightly. You’ll also need a length of wire and asmall bell or two. Thread the wire through the bell and attachit to the cork. Be sure the bell is free to ring without hitting thesides of the bottle when you shake it. Tear a sheet of newspaper into shreds and put them into thebottle. Light a match and apply it to the paper. Quickly coverthe bottle with the cork. The burning paper will use up the airand create a partial vacuum. When the bottle cools, shake it and listen. Then open thecork and let in some air. Reseal the bottle and shake it again. You will see that: After you remove most of the air, you arenot able to hear the bell, though you see the clapper moving.When you let in the air, you hear the bell again. Explanation: If sound is to travel from a vibrating object toyour ear, there must be a substance to carry it. Sound cannottravel in a vacuum. Normally, the energy of sound travels inwaves through the air. Though air is sound’s most usualcarrier, it is not its most effective. bounce back, the molecules bounce into other molecules near by. These bump intotheir own neighbors. Thus, while each molecule moves but slightly, sound may travelgreat distances. Finally, the molecules of your ear are bumped, your eardrum vibrates,the nerve endings take the vibrations to your brain and there they are converted tosound. CAN SOUND TRAVEL THROUGH A LIQUID?CAN SOUND TRAVEL THROUGH A SOLID? Hold one end of a 12-inch ruler 1 inch from your ear and scratch on the far end of theruler. Note how loud the sound is. Then move the ruler 13 inches away from your earand scratch on the near end, so that you are making noise at the same distance fromyour ear as before. Com- pare the sound you hear. You will see that: The sound is much louder when carried by the wooden ruler than byair. Explanation: Many solids are much better carriers of sound than air or water. It isbelieved this is so because the molecules of a solid are closer together than those ofeither a gas or liquid. Get a friend or neighbor to tap out a rhythm on a radiator or pipe from a door aboveor below you. You will hear him very well. Metals are the best of all conductors ofsound. In some, sound travels 16 times as fast as it does through air. Indians and pioneer scouts knew that the solid earth is a better conductor of soundthan air. They put their ears to the ground to hear distant sounds. Click together two stones, two blocks, or twopot lids. Listen to the sound. Then submerge themin a basin of water-or take them into the bathtubwith you—and listen to the sound they make underwater. You will see that: The sound in water is clearer,louder. Explanation: Liquids carry sound farther andfaster than air, as you may have been aware if youhave ever heard sounds carry across a lake. Inwater, sound travels more than four times as fast asin air. Did you ever notice that sounds seem louderon foggy days than on clear days? SPEED OF SOUND ECHOES During the next thunderstorm in your area, you canhave fun with this activity. When you see a flash of lightning, start counting andcontinue until you hear the roar of thunder. Divide thenumber you get by 5. This will give you a roughestimate of the number of miles away the center of thestorm is. Explanation: Sound takes about 5 seconds to travel amile in air. (It travels about 1,100 feet a second, andthere are 5,280 feet in a mile.) Light, on the other hand,takes only a small fraction of a second to travel a mile.(It travels 186,000 miles per second. ) The lightningand thunder occur at the same time but travel to us atdifferent speeds. This accounts for the different timesthey reach us. Variation: If you’re too impatient to wait for a thunderstorm, you can set up a similarexperiment. Ask a friend to play a drum some distance from you, so that you can bothwatch and listen. Or watch your friend batting from far away on the baseball field.Notice there is  time lag between the time you see him hit the drum or ball and the timeyou hear the sound. Here’s an experiment you can perform in a large emptygym or auditorium. It’s also a perfect activity when you’rehiking in the country or the mountains. You can judge thedistance from one side of the gym to the other, or how faryou are from a cliff, barn or from a bridge overhead. Noteyour position and shout out a message. Then count thenumber of seconds it takes before you hear an echo anddivide this by 2, then by 5. This gives you the distance inmiles. The sound travels to the cliff and back to you so youdivide by 2. Because sound travels 1 5 Of a mile per second in air, you divide by 5. If your echo returns within 4 seconds, for example, thecliff is about 2 5 of a mile away. Explanation: When sound waves hit a solid object, somepass through, but some bounce back like a ball. Thereflected sound is heard as a separate sound (or echo) if thedistance is 40 feet or more. The human ear requires at least 1 15 of a second to hear separate sounds. Sound travels atabout 1,100 feet per second, so it takes about 1 15 of a second to travel a distance of 40 feet and back. The depth of water is measured on shipboard in the sameway. Sound, however, travels faster in salt water—4,800 feetper second.CONTROLLING THE DIRECTION OF SOUND Sound waves travel out in all directions from the source of sound. But, we canconcentrate sound energy in one direction, instead of permitting it to spread. Usingequipment around the house, you can see how a few of these devices for concentratingsound operate. MEGAPHONE Make a simple megaphone from a sheet of paper or from cardboard. Fold as in theillustration. Have someone speak into the narrow end while you listen from a distance. Listen tothe ticking of a watch at the narrow end while you stand a few feet away. SPEAKING TUBE All you need for this device is an old garden hose. You can convert it into an excellentspeaking tube merely by patching any holes and making sure both ends are open. Taketurns with a friend, talking and listening. You will be able to speak back and forth overa considerable distance because the sound is being channeled directly to your ears bythe air inside the hose. Many ships still use speaking tubes for communicating onboard. STETHOSCOPE Attach rubber tubing, perhaps an old shower hose, to the kind of funnel used to fillbottles with liquid. Then use your homemade stethoscope as your doctor does his. Putthe end of the tube in your ear and listen to the beat of your own heart. The funnel and tube concentrate the sound and therefore it is made louder.SOUND DIFFERENCES: PITCH VARIATIONS WITH STRINGS Pitch is the highness or lowness of a sound. 1. Hold the edge of a card against a bicyclewheel. Revolve the wheel slowly and thengradually faster and faster. Note how the sound changes. 2. Play a 33 r.p.m. phonograph record atthe different speeds on a 3-speedphonograph. Notice what happens to theshrill- ness of the sound. You will see that: The faster you spin thebicycle wheel, the higher the sound willbecome. Similarly, the faster the phonographrevolves, the higher the sound. Explanation: Pitch depends on the numberof vibrations per second. The morevibrations, the higher the pitch. String four rubber bands of various thicknesses around arectangular cereal box. Make a bridge from a thin block orpiece of plywood. Place it as in the illustration. Now youhave a box banjo. Pluck each of the strings in turn and compare sounds. Now shorten the rubber bands by moving the bridge. Is thesound lower or higher than with the longer band? Insert a thumbtack on the far side of the box. Use this tostretch one band tighter and tighter as you pluck. You will see that: The thinnest band produces the highestnote; the thickest, the lowest. The shorter the band, thehigher the note; the tighter you stretch the band, the higherthe note you get. Explanation: The pitch depends on the tension, length andthickness of a band or string. In general, the smaller the vibrating surface, the faster thevibrations and the higher the pitch. For instance, the thinband will produce a higher note than a thick one of samelength because the thick band has a larger surface andproduces slower vibrations. It has more molecules to set inmotion than the thinner one. STRIKING SOUNDS Take a handful of long wooden blocks and a jump rope. Also get two pencils andtwo empty spools. Make mallets by fitting together the pencils and spools. Then shape your rope in the form of a horseshoe. Arrange your blocks on top of therope so that the shortest is centered on the narrow ends and the longest is centered onthe wide part of the horseshoe. Each block should overhang the rope on each side 1 4 of its length so that it is free to vibrate. You will see that: You have made a ladder of sound, ranging from the low-soundinglong block to the high-sounding short block. Explanation: The pitch depends on the number of vibrations per second. The smallersurface can vibrate faster and therefore makes a higher sound. BLOWING SOUNDS Press the top edge of an empty bottle to your lowerlip and blow lightly across the top. Pour in a little water and blow again. Then add morewater and blow. You will see that: The more water you add, the higheryour sound will be. Explanation: You are vibrating the air in the bottle.When you add water you leave less room for air. Theless air there is in the bottle, the faster it vibrates andthe higher the sound. In the same way the higher noteson a musical instrument are made by shortening the aircolumn. In general, the larger the instrument, the lowerthe notes it can play and the smaller the instrument, thehigher the notes it can play.LOUDNESS AMPLIFYING LOUDNESS The loudness of a sound depends not on the speedof the vibrations but on the energy of the vibrations.Demonstrate this for yourself with the followingactivities: 1. Clap hands gently-and then vigorously. 2. Hold one end of a ruler over the edge of the table.Pull the other end down gently and then let go. Listento the sound. Repeat with a harder pull. You will see that: The more energy you apply, thelouder the sound is. The louder the sound, the fartherthe body vibrates. Explanation: More energy causes the molecules ofair over a greater distance to be moved back andforth. Strike the prongs of a fork in mid-air with a spoon.Listen to the sound. Repeat-but this time quickly pressthe handle of the fork to the table, holding the forkupright. Notice the difference in loudness. You will see that: Touching the vibrating fork to thetable makes the sound considerably louder. Explanation: Sounds can be made louder if otherobjects vibrate too. Ordinarily, the larger the vibratingsurface, the louder the sound. Many musicalinstruments have wood or metal sounding boards orboxes to make the sound louder. WHAT IS RESONANCE? Two milk or soda bottles will demonstrate resonance (or sympathetic vibration) foryou and a friend. Hold one of the bottles to your ear while your friend blows across the mouth of thesecond bottle until he produces a clear note. You will see that: Your bottle will vibrate in sympathy and sound a similar, thoughweaker, note. Explanation: Each object has a natural rate of vibration, depending on its nature, itssize and shape. When two objects naturally vibrate at the same rate, one object canmake the other vibrate. The two are said to be in resonance. Strike A on a piano and watch a nearby violin. It will vibrate sympathetically. Did you know that soldiers crossing a bridge deliberately march out of step? If theirsteps in unison should happen to match the natural rate of vibration of the bridge, itwould set the bridge in violent motion, and this might destroy it. SEASHELL RESONANCE A couple of seashells and two open cans serve as theequipment for this experiment. Choose a large and a small shell and put each in turn toyour ear. Do you hear any difference in sound? Now put a large and a small can, in turn, to your ear. Vary the setting for the experiment from indoors tooutdoors, from beach to city street. You will see that: You hear bass sounds from the large shelland high sounds from the small shell. You hear low soundsfrom the large can and soprano sounds from the small can.The sounds indoors differ from outdoors; beach differsfrom city. Explanation: Of course, the sounds coming from theseashells are not the “sounds of the sea.” They aresympathetic vibrations. The enclosed air in the shell vibratesin response to those sounds in the outside air thatcorrespond to the pitch of the shell. The particular rate ofvibration depends on shape, type of material, and amount ofair enclosed. 6. LIGHT We need light to see—natural light from the sun, or artificial light from a match, acandle, a lamp. Like heat, sound and electricity, light is a form of energy; it is capable of doing work. Anything will give off light if it can be heated enough before it changes to anothersubstance. It is believed that the heat excites the atoms of a material, and that some ofthe electrons of the atom lump out of place. When the electrons lump back into theirnormal place, bundles of energy shoot out. These bundles are sometimes calledphotons. A line of these photons forms a ray of light; a group of rays forms a beam oflight. Photons travel from all light-giving objects, strike the eye, and cause us to seelight. Another way scientists explain light is by the wave theory. It is believed that light issent out in the form of waves, similar to water waves. Light waves are very short,about 1 50,000 of an inch. Unlike sound, light can travel in a vacuum, in empty space where there is not even air.Light travels at a speed of 186,000 miles per second, the fastest speed known to man.CAN WE SEE IN THE DARK? Make a pinhole in the side of a shoe box or any box that can be closed tightly. Put aball and a pencil in the box. Cover the box and look through the pinhole. Do you seethe ball or the pencil? Do you see anything? Take off the cover of the box. Look through the pinhole again. Do you see anything? You will see that: You cannot see the pencil and ball when the cover is on and you cansee both when the cover is off. With the lighted flashlight inside, you can see flashlight,ball and pencil. Explanation: Without a source of light (such as the sun or the flashlight) you cannotsee. You cannot make out either shape or color. Light comes to our eyes in two ways. Light from the sun or the flashlight or otherluminous objects comes directly to our eyes. This is how we see the stars, lightning, anelectric bulb, a match or a candle. But we cannot see a ball or a pencil directly. Light from the flashlight hits the ball andbounces back (is reflected) to our eyes. We see people, chairs, trees because light isbounced off them. A PINHOLE CAMERA DUST HELPS US TO SEE Cut an opening about 3 inches square out of the bottom of a round cereal box. Overthis hole, paste very thin paper (tissue paper or onion- skin). Cut another square hole,about the size of a small postage stamp, in the center of the top of the box. Cover thiswith tinfoil. In the center of the tinfoil make a small hole with a pin. Cut out a paper doll and crayon it black. Tape the doll with cellophane tape to theglass of a flashlight. Hold the box about 2 feet away from the lighted lamp (preferably in a darkenedroom). Point the pinhole at the lamp and look at the tissue paper. You will see: An image of the doll is thrown on the tissue paper-upside down. Explanation: The rays of light travel in straight lines from the lamp to the image, asshown in the illustration. This is what happens in our eye. The image forms upsidedown on the retina at the back of the eye. Our brain turns the image right side up again. Arrange your window shades so that only asmall ray of bright sunshine comes into theroom. Follow the beam of light with your eye. You will see that: You can see dust moving inthe path of the ray of light. Explanation: Dust particles bounce back(reflect) light and help us to see indoors and inother places where the sun does not shinedirectly. Without dust, we would not have daylightinside our house except when we received thedirect rays of the sun. HOW LIGHT BOUNCES HOW DO YOU REALLY LOOK? In a dark room, place a mirror on thefloor. Cast a beam from a flashlightdirectly down to the mirror. Sprinkletalcum powder or chalk dust near thebeam. Follow the reflection on the ceiling. Then slant your flashlight and cast aslanting beam at your mirror. Observe thereflection. You will see that: The beam that travelsstraight down to the mirror bounces backstraight up to the ceiling. The beam thattravels on a slant down to the mirrorbounces back on an opposite slant to thewall.Explanation: A ray of light striking asurface is reflected at the same angle. Stand up two pocket mirrors and tape them togetherso that they form a right angle, as in the illustration.Face a clock toward the two mirrors. Try to read thispage in the mirrors. Look at yourself. Try to combyour hair. You will see that: You can read the clock and thebook. You look strange and you can’t seem to combthe side of the hair you mean to. Explanation: Light from the left side of your face hitsthe left mirror and is reflected to the right-hand mirror,which reflects it back to your eye. The same thinghappens on the other side of your face. Thus you seeyourself as others see you, instead of the way youusually look in the mirror. MAKE A PERISCOPEBENDING LIGHT RAYS Use a milk carton or make a cardboard or wooden box aboutthat size. Cut a hole on one side of the box, near the top, and a similarhole on the opposite side, the same distance from the bottom.Tape two pocket mirrors in place parallel to one another, at a 45-degree slant, as in the illustration. Hold the box up to your eye and look through the lower hole.Now go to a corner and hold the box so that one hole is stickingout. Look through  the-other hole. What do you see? You will see that: You can see what is above you and on theopposite side of the box. You can also see around corners. Explanation: Light is reflected by the mirror on top of theperiscope to the mirror on the bottom. An object facing the tophole can be seen through the bottom hole. Periscopes are used in submarines to see above water. In someparts of the world, theatres are equipped with periscopes so thatyou can see the stage even if the person in front is taller than you. Place a pencil, a ruler or a spoon in half a glassof water. Look at it from the top, bottom andsides. You will see that: When you look at the pencilfrom the side, the pencil appears to be bent orbroken at the point where it enters the water. Explanation: The light rays appear to be bentbecause the speed at which they travel in thethicker water is slower than in air. Light travels inair at the high speed of 186,000 miles per second.It travels ¾ of that speed in water. The bending oflight is known  as  refraction. READING GLASSWHAT: CAUSES A SHADOW? Pour water into a clean glass or jar. Hold itclose to this page and read through the side ofthe glass. You will find that: The print appears larger. Explanation: Because the glass is curved, thelight rays enter it on a slant and change directionas they go through the water. This is how amagnifying lens works. In a darkened room, shine a strong flashlight or ashaded lamp bulb on a white wall, or on a sheet tackedto the wall, as in the illustration. Place the lamp 5 to 10feet from the wall. Stand behind the lamp. Do you make a shadow? Hold up your hand, or stand between the lamp andthe wall. What happens? Move farther away from thelight and closer to the wall. What happens to theshadow? You will see that: You do not cast a shadow when youstand behind the light. You cast a big shadow when youstand near the light and far from the wall. As you movefarther from the light, the shadow becomes smaller. Explanation : You cast a shadow by blocking the raysof light. As you move away from the source of light,your shadow becomes smaller because you cut offfewer of the light rays. Any object that won’t permitlight to pass through creates a shadow, an area oflessened light. MAKING RAINBOWS 1. Stand a glass of water on a window ledge in bright sunlight. Place a sheet of whitepaper on the floor. What do you see on the paper? 2. Set a tray of water in bright sunlight. Rest a mirror upright against one edge of thetray. Look at the wall. 3. In a darkened room, hold a prism (a 3-sided piece of glass), a crystal doorknob, acut-glass bottle or even a milk bottle up to the sun or another source of light, such as alamp bulb or flashlight. Look at the wall, ceiling or floor. You will see that: You see the colors of the rainbow. Explanation: You are separating the various colors (the spectrum) that make up whitelight. When the light passes at a slant from the air through the glass or water, the rayschange direction. They are refracted. The different colors are bent differently: violet isbent the most and red the least. When the light comes out of the glass or water, thedifferent colors travel in slightly different directions and strike the screen at differentplaces. Rainbows in the sky are made when sunlight shines through water drops in theair. The water drops bend the sun’s rays to form a spectrum.MAGIC COLORS 1. With water colors or poster paints, color one side of a cardboard disk red and theother side blue. Punch small holes on opposite sides of the disk, as in the illustration.Thread short lengths of string through each hole. Hold the cardboard by its strings and twirl it around. You will see that: The color you see is purple. 2. Make a toy top by dividing a cardboard disk into alternating segments of blue andyellow. Thread a string through a hole in the center, as in the illustration. Then spin thedisk. You will find that: The color you see is green. Explanation: The disk reflects bothcolors. You see a third color when your eye and your brain mix the colors of therapidly whirling disks. This happens because the eye continues to see each color for ashort time after it has disappeared.7. MAGNETISM AND ELECTRICITY Both electricity and magnetism were known more than 2500 years ago. But it was notuntil the beginning of the 19th century, about 150 years ago, that experiments showed adefinite connection between the two. In 1819, Hans Oersted showed that electricity can produce magnetism. A few yearslater, in 1831, Michael Faraday proved that magnets can make electricity. From these experiments and those that followed came our modern electrical world-telegraph, doorbell, telephone, electric motor, generator, radio and television. We usemagnets to produce the electricity that lights our homes and factories. We have put magnetism and electricity to work for us. We are not sure, however,what causes either form of energy. We are still trying to figure out why they work. WHAT DOES A MAGNET D0? You will need a magnet-in the shape of a bar, Uor horseshoe. A ten-cent toy magnet will do. Jumble together a box of paper clips, pins orsmall nails with a collection of buttons or pennies.Use the magnet to separate the items. You will see that: The objects made of iron orsteel are drawn to the magnet. If your magnet is astrong one, some will even jump up to it. Theplastic buttons and copper pennies do not move,nor do pins of brass. Explanation: A magnet is an object that attractsiron and steel and certain alloys. A few othermetals-cobalt, nickel, aluminum and platinum- canalso be attracted but only by much more powerfulmagnets. Natural magnets are a form of iron ore called“magnetite,” or “lodestone” meaning “leadingstone.” Man-made magnets, such as thehorseshoe or bar you use, are usually either ironor steel. Very strong magnets are made of an ironalloy called alnico, which contains aluminum,nickel and cobalt. CAN MAGNETS ATTRACT THROUGH SUBSTANCES? WHERE IS A MAGNET THE STRONGEST? Lower a magnet of any type into a pile of nails or clips or pins. Try picking up thenails with the different parts of the magnet. You will see that: The nails cling to the ends of the magnet. Explanation: A magnet has the strongest attraction at its ends. These are known as thenorth and south poles of the magnet. In the horseshoe, or U magnet, the bar has beenbent so that the poles or strongest parts are close together. This increases its liftingpower. Assemble tacks, nails and clips. Then, with your magnet, try to attract the variousitems in the following ways: 1. Put several clips into an empty, dry drinking-glass. Move the magnet aboutunderneath the glass. 2. Put some tacks or nails on the table and cover them with a sheet of paper. Moveyour horseshoe magnet slowly over the paper. 3. Put brads (headless nails) into a dish of water. Place a magnet just above the water. 4. Put several nails into a “tin” can. Move the magnet about beneath the can. 5. Put a clip on top of a thin piece of wood, leather, rubber or cork. Move the magnetaround slowly underneath. You will see that: Magnets can act through glass, plastic, water, paper, leather, rubberand cork-but not through the can which is really an iron can coated with tin. Explanation: Magnets can act through most substances. Iron and steel and otherhighly magnetic materials, however, take up the magnetism themselves and prevent thepower from passing through. WHAT IS A SPARK? Rub a comb with a piece of wool or fur. Hold it near a water tap, metal radiator ordoorknob. You will see that: You will produce a small spark. Explanation: By rubbing the comb, you charge it with electricity. The spark is madewhen the charge jumps to the uncharged (or neutral) tap. A spark is the passage of anelectrical charge between two objects. You may have seen a similar spark when you rubbed your shoes on a rug and thentouched something. Or you may have heard a crackling while combing your hair. Theseare examples of static electricity. Lightning is a huge electric spark that results when charges lump from one cloud toanother or from a cloud to the ground.ELECTRICITY CAN ATTRACT Turn on the water faucet so that you get a fine, even stream of water. Rub a glassstraw with a piece of silk or a comb with a piece of wool or fur. Hold the straw orcomb near the stream of water. You will see that: The stream bends toward the charged glass or comb. Explanation:The charged object attracts the neutral stream of water. ELECTRICITY PRODUCES MAGNETISM Now you are going to produce magnetic effects without a magnet. Your equipment will include iron filings, a strip of heavy copper wire, a 3-foot lengthof covered (insulated) bell wire, a compass, and a dry cell battery. A flashlight batterywill serve instead of a larger dry cell, if you make a holder for it or strip off the outercardboard. 1. Connect the ends of the bare copper wire to the cell or battery as in illustration A.Dip a loop of the wire into the iron filings. Then quickly disconnect one end of the wireso that you don’t wear out the battery. You will see that: The iron filings stick to the wire. When you disconnect one end ofthe wire and stop the how of electricity, the filings soon drop off. 2. Scrape the covering from the ends of the 3-foot length of covered wire. Substitutethis for the bare wire, arranging it so that one length is vertical as in illustration B, butdon’t attach one end. Place the compass at the side of the wire. Rearrange the batteryand wires so that the needle is pointing toward the wire. Attach the loose end of thewire to the battery and note the results. Disconnect the wire at both ends, andreconnect them to the opposite posts, to reverse the direction of the electric current.Then observe the needle. You will see that: The compass needle moves first in one direction and then, when thecurrent is reversed, in the opposite direction. Explanation: When electricity flows through a wire, the wire acts like a magnet andproduces a magnetic field. The magnetism lasts only while the current is flowing. This was Oersted’s significant discovery of 1819. A wire carrying a current ofelectricity produces magnetism. MAKING AN ELECTRIC LAMP You can make your own electric lamp and get a bright, thoughbrief, glow. You’ll need two nails, a short length of thin iron wire(a strand of picture frame wire), an ordinary bottle or jar, a corkto fit the bottle, and about four dry cell batteries with a length ofcovered copper wire. Stick the two nails through the cork. Attach the iron wire to thenail points. Fit the cork into the neck of the bottle, allowing thenail heads to remain outside and the iron wire to go inside. Withthe covered wire, connect the dry cells to the heads of the nails,as shown in the illustration. You will see that: The thin iron wire gets hot enough to glowand you have made an electric lamp of the bottle. Soon,however, the iron wire gets so hot that it burns in· the air of thebottle. The iron breaks and the lamp goes out. Explanation: In our modern electric lamp, nitrogen (whichdoesn’t sup- port burning) is substituted for the air within thebulb. Tungsten is used for the inner (filament) wire because thismetal can get white hot and glow without melting. Since itrequires less heat to make a thin wire glow, an extremely thintungsten wire is used.CONDUCTORS AND INSULATORS Connect a dry cell to a flashlight bulb and socket, leaving two bare ends of copper wire, asshown in the illustration. Briefly touch these ends together to make sure that the bulb lights.You now have a tester with which you can find out whether certain materials allow electricity toflow. Touch the two bare ends of wire to two points on any of the following objects you haveavailable: a clip, fork, key, coin, piece of cloth, wood, glass, rubber band, leather heel, nails,pins, paper, chalk, covered wire. You can also try a number of solutions: salted water, lemon juice, vinegar. (You may needmore than one battery to provide the current for these.) Also try different kinds of wire-copper, iron, aluminum. You will see that: Metals are generally good conductors and will light the bulb. Non-metalswill not conduct electric current. (They are called “insulators.”) Solutions made with salts,acids or alkalis will conduct. Notice that the various kinds of wire differ in effectiveness. Thelamp burns brightest with the copper wire. Explanation: In producing static electricity, we used insulating materials such as glass andrubber which do not permit electricity to move freely. These insulators are valuable in helpingus keep electricity from going where it is not wanted. This is why we cover wire with rubber,cloth or thread. Electricity will how only if it makes the return trip to its source; it flows in acircuit. When we want electricity to move along a path, or circuit, we use conductors. ELECTRICITY CAN PRODUCE HEAT You know from your toaster, heater, iron, electric stove, and otherelectrical devices that electricity can be used to produce heat. If you wouldlike to do your own changing of electrical energy to heat, you can try thissimple experiment. Use a short length of thin bare iron wire-one thin strand of picture framewire will do. Connect one end to a dry cell. Then wrap the other end arounda pencil and hold it to the other cell terminal, as in the illustration. You will see that: The wire will get red hot and possibly even break if youdon’t disconnect it in time. Explanation: Different kinds of wire act differently when electricity flowsthrough them. The iron wire that we used in the experiment gets hot becauseit resists electric current. It does not conduct as well as copper or aluminumbut instead changes the energy to heat. When the same current flowsthrough two wires, the wire with the greater resistance to electricity getshotter. Thicker wire permits a larger current, for thin wire has more resistancethan thick. Similarly, a long wire allows less of the current being applied toflow than a short wire does. Heating elements in toasters and irons are made of alloys with a higherresistance than the copper wire in the insulated cord. End of book