Wikipedia:Reference desk/Archives/Science/2012 April 2

= April 2 =

What is the effective population size of the United Kingdom?
And what about the world? Anyone who fails to read the question properly and gives me the population will be shot. Egg  Centri  c  01:19, 2 April 2012 (UTC)
 * Maybe you could read the cited article and provide the answer yourself? :) ←Baseball Bugs What's up, Doc? carrots→ 02:15, 2 April 2012 (UTC)
 * I doubt it. I seriously doubt that the data exist to allow the question to be answered.  Even if that's wrong, the answer would require fancy analysis of genetic frequency databases for the UK population. Looie496 (talk) 03:11, 2 April 2012 (UTC)
 * The data is there, someone just needs to crunch it. The number is much lower than you'd expect. Tenesa et al (2007) estimate the effective population size of the entire non-African human population at 3100 individuals.--Itinerant1 (talk) 08:48, 2 April 2012 (UTC)

Glow leak
How do you break open a glow stick? I couldn't find any information in Wikipedia's article about breaking open a glow stick. Also—when broken—are the chemicals toxic? If so, in what way (e.g., when touched, inhaled, swallowed, etc.)? 71.146.8.88 (talk) 06:08, 2 April 2012 (UTC)


 * From the article you've already linked to....
 * "Glow sticks contain hydrogen peroxide, and phenol is produced as a byproduct. It is advisable to keep the mixture away from skin and to prevent accidental ingestion if the glow stick case splits or breaks. If spilled on skin, the chemicals could cause slight skin irritation, swelling, or, in extreme circumstances, vomiting and nausea. Some of the chemicals used in older glow sticks were thought to be potential carcinogens.[12] The sensitizers used are polynuclear aromatic hydrocarbons, a class of compounds known for their carcinogenity."
 * So, I wouldn't recommend breaking one, but if you must, it's presumably just a matter of brute force, or a sharp blade. HiLo48 (talk) 06:19, 2 April 2012 (UTC)


 * Put it in a sealed plastic bag, on the sidewalk, and pound on it with a hammer, I suppose. This should minimize your exposure.  Wear goggles, just in case. StuRat (talk) 06:31, 2 April 2012 (UTC)
 * Thanks. 71.146.8.88 (talk) 02:17, 3 April 2012 (UTC)
 * Thanks. 02:10, 3 April 2012 (UTC)
 * Why would we provide information on how to break something that A) isn't meant to be broken, B) is potentially hazardous if broken, and C) is easily broken open by any sharp kitchen knife? We don't provide instructions on how to cut open a baseball to see what is inside that...  Dismas |(talk) 10:52, 2 April 2012 (UTC)
 * Exactly Dimas. I'm amazed such a trollish question was answered Richard Avery (talk) 15:30, 2 April 2012 (UTC)
 * Seriously? Now people are accused of trolling for asking how to break open a glow stick?  What's next - accusing OPs of trolling for wanting to do anything you haven't done before and don't personally see a reason for?  --140.180.39.146 (talk) 16:36, 2 April 2012 (UTC)


 * I didn't say they were trolling. I just didn't (and still don't) understand why we would cover something like that in an encyclopedia.  Dismas |(talk) 19:11, 2 April 2012 (UTC)


 * I've opened maybe 100 glowsticks, use a razor blade or other sharp knife, cut off the top 10 mm, careful not to break the glass tube inside. Then turn around, hold above container, cut other end, once air can come in, the liquid and the glass tube will slide out. Don't do it above the container holding all the (colorless) liquid, one glass tube breaking will ruin all of it. The glass tubes are really thin an break easily. As far as I know, the description given in the article does not fit current glowsticks, there does not seem to be any reaction with hydrogen peroxide (tested up to 30% concentration). Also, I doubt here in the EU carcinogenic liquids would be allowed in childrens toys, and that's what they are, according to te packaging. Not posonous, not for children < 3 years. "Can cause discolorations on clothing and furniture." Now that clearly talks about the inside! I complained on the talk page about the inaccuracies, seems no one knows what's in todays glowsticks.... Ssscienccce (84.197.178.75) (talk) 18:25, 2 April 2012 (UTC)
 * May I ask in what way was my question “trollish”? 02:11, 3 April 2012 (UTC)
 * Thanks. 02:12, 3 April 2012 (UTC)
 * Why did you open 100 glowsticks ? StuRat (talk) 18:50, 2 April 2012 (UTC)
 * Wanted to make something out of it. Under UV light the glass tubes light up brightly like thiny neon tubes; you can mix the chemicals and put them in other stuff (with a syringe), heating the mix gives really bright light, the empty glass tubes can be bent in whatever shape you want by heating them with a lighter and filled with the mix (looks much better than glow sticks because the glass is transparent, not just translucent)... Just messin' around, you know... :-) Ssscienccce (84.197.178.75) (talk) 19:59, 2 April 2012 (UTC)
 * Heh, that sounds quite fun, to tell you the truth. :) (I noticed that you use emoticons.) 02:13, 3 April 2012 (UTC)
 * 100 glowsticks!!! Then you will love this . I'm just glad your not my next-door neighbour – no offence :-)--Aspro (talk) 21:16, 2 April 2012 (UTC)
 * The nurdrage vids are quite entertaining :-) I used to enjoy chem vids on youtube, after Myfanwy94 I had enough for a while... Ssscienccce (84.197.178.75) (talk) 01:34, 3 April 2012 (UTC)
 * Having just had a quick look at Myfanwy_94 I'm now hoping we live on different continents. Hope you don't have any access to glow-in-the-dark radio active isotopes – no ! don't tell me, or I wont sleep tonight ;-) --Aspro (talk) 01:52, 3 April 2012 (UTC)
 * Myfanwy94 died, probably from pulmonary edema. I think he was 16 :-( Ssscienccce (84.197.178.75) (talk) 03:27, 3 April 2012 (UTC)
 * Totally OR, but when I accidentally ate a naan bread with glowstick juice on it (long story but basically one of my housemates had decided to paint our kitchen with them, rave-style) I spent the next 24 hours throwing up. I'm assuming the two are linked, though it's always possible it was a coincidence. I didn't die at least, so unless I was very lucky, the lethal dose for the stuff is very high. Smurrayinchester 22:15, 2 April 2012 (UTC)
 * Very high?! °O° How much did you ingest then ?? Ssscienccce (84.197.178.75) (talk) 03:27, 3 April 2012 (UTC)


 * Um, I would think this is obvious **but if you almost died ingesting glowstick stuff please at least ASK a doctor if you're going to be alright**. Not here.  Call up a doctor, any doctor, and ask.  Tell them how sick you were and for what period of time and why.  Toxicity is not just instant death, and the adage "anything that doesn't kill you can only make you stronger" is obviously false, as people die from wounds later on all the time.   94.27.233.109 (talk) 18:46, 3 April 2012 (UTC)


 * Well, the adage doesn't say it has to kill you immediately. But, some things which don't kill people definitely leave them weaker, such as the quadriplegic. StuRat (talk) 20:39, 3 April 2012 (UTC)


 * From my personal experience, the flexible plastic walls of glowsticks are very hard to cut with a sharp blade. What I did when I needed the innards of ~100 glowsticks was to cut one end off with a hacksaw.  It was very difficult not to break the glass vial inside and trigger the glow reaction, but that was OK, because I wanted glowing liquid anyway.
 * I wore gloves and worked in a well ventilated area, and suffered no ill effects. The blue coating on the hacksaw blade was stripped off, however, which indicates that whatever the liquid is, it isn't pleasant. gnfnrf (talk) 19:34, 6 April 2012 (UTC)

Load in motor and current
Hi guys, one of the students I homework help got this (quite confusing, in my opinion) question for their test. I must admit I'm having a bit of trouble with it. Electricity was never my strong suit, unfortunately.

"A DC motor with a load attached is at full rpm. A voltmeter attached to the motor gave a reading of 400V. The load is then detached and a reading of 415V is then observed. Explain the change in voltage. In which case will the current be higher or lower, and explain."

So I think that the change in voltage would be caused by the back EMF. But it wouldn't explain why the drop is so low. "Load" is an ambiguous term to- does it have an innate resistance? I thought for a while and I'm just confusing myself now.

For the second part, wouldn't the back EMF in both cases limit EMF to close to 0? So torque 0, and by t=nBIA since B, A, constant, I=0 in both cases. Or have I just completely forgotten my high school physics somewhere. But then I think of Power in case 1=Power in case 2, so VI case 1 = VI case 2, so the 400V one would have a higher current. Then I think of V=IR and a load would give more resistance. Yeah I'm confused.

Sorry for this huge mess of random musing, it's late and my brain is tired. Hopefully I can get a definite answer

Thanks in advance, 27.32.104.185 (talk) 12:41, 2 April 2012 (UTC)
 * An ideal voltage source has zero internal resistance, so you wouldn't measure any difference between load or no-load operation. Since no source is really ideal, you measure some voltage drop when load is attached, caused by the current flowing through the resistance of the power source... Ssscienccce (84.197.178.75) (talk) 18:41, 2 April 2012 (UTC)


 * This is obviously a homework question. So I don't think we will answer it. As a clue though. This is inductive not a 'pure' resistive load. So it not right to think V=IR but V= back EMF + I R. Once the load is taken off the back EMF drops and thus the kilo-wattage drops. So you have to add something back to the other side of the equals sign. That's the terminal voltage returning upwards towards  the supply voltage  at open circuit  less  the voltage drop due to the conductance through the motor...   Depriving oneself of sleep it not a good thing.--Aspro (talk) 19:56, 2 April 2012 (UTC)
 * "Once the load is taken off the back EMF drops and thus the kilo-wattage drops" : that seems to fit with $$P = I \cdot V_{cemf}$$ in Brushed DC electric motor, but in my opinion, that formula is wrong; back-emf rises so P drops, I think ... Ssscienccce (84.197.178.75) (talk) 20:21, 2 April 2012 (UTC)


 * That article's talk page is the place to query that formulae. --Aspro (talk) 21:06, 2 April 2012 (UTC)

Thanks for the responses. I did say it was a homework question, and I agree that the resistance of the load would cause the voltage drop. But I don't understand why "once the load is taken off, the back EMF drops". The second case without the load is still at maximum rpm, so wouldn't the back EMF almost cancel out the input? Also, how do you know the kilowattage drops (is this just power)? 27.32.104.185 (talk) 22:17, 2 April 2012 (UTC)


 * Assuming you've read through the various article and your mind is still 'stuck' I will have a go at explaining it verbally – but be warned – over simplification can often lead to buggering the whole thing it up. So lets take your second question first: “The second case without the load is still at maximum rpm, so wouldn't the back EMF almost cancel out the input?” In a perfect motor, with no bearing friction, wind shear dragging the rotor, and an infinite amount of poles; the conductors will not be cutting any lines of force, as they will be in perfect synchronization with the magnetic fields – so there will be no back, forward or any other  EMF. OK so far? As soon as some load is applied , the rotor retards (with regard to the fields) and some  EMF is induced in the conductors. Yet, the more it retards the more each brush cover more than one commutator section. Now the motor winding has current flowing through more than one winding – in other word there is more copper to carry the current – more conductance – more 'adjacent' magnetic flux to attract the poles – i.e., more juice going through  to drive it round - more juice less voltage (and if some now asks about 'Q' at this point, I shall scream because were talking DC but Henry's  are OK because that's valid).  Also, due to the retardation,  back EMF is now produced due to them cutting  lines of force as they are now  out of perfect synchronization. The formula’s  explains the relationships  far as the units of V, I, R, P  etc. are concerned but they don't explain how these ruddy things goes round and round. --Aspro (talk) 23:31, 2 April 2012 (UTC)


 * The question's phrasing is a bit vague and puzzling. "Full RPM" might mean a motor running at its rated speed with nominal voltage and no load, as opposed to the highest RPM at which it could turn without self-destructing when run at a very high voltage and minimal load. If you then apply a load to a DC motor, the speed typically drops, along with an increase in current, So perhaps the question writer meant that the motor is running under nominal conditions: rated voltage, rated load, rated speed.  The question then says the load is "detached," so the motor goes from some load to no load other than air resistance and bearing friction. I would expect its speed to increase compared to the initial condition while the current decreases. The voltage measured at the terminals increases from 400 to 415. The loaded voltage was 3.61% less than the no-load voltage. This change would be due to the source impedance (in this DC case, the source resistance) of the battery, power supply, or generator plus the resistance of the power system and conductors connecting it to the motor, assuming that there is no voltage regulation at work to increase and decrease the supply voltage. Why all the discussion about "synchronization" and "impedance?" Direct current was specified. DC is not AC, although sometimes pulsating unidirectional power sources are loosely termed "DC." What is to be "synchronized" if the energy is supplied by a battery or its equivalent in the form of a good power supply power supply? Edison (talk) 00:01, 3 April 2012 (UTC)


 * ? If your replying to me, If the speed increases if the load is taken off, isn't that the same thing as the 'rotor retarding' when load is applied? You don't seem to have read my posting properly. I pointed out that we're talking about DC. The synchronization I spoke of is in a 'perfect motor' where the angle of the live commutator segments are in phase (match) with the sum of the vectors of the magnetic field at max speed. What has AC got to do with that? Also I don't remember bringing in impedance – were talking DC – as you pointed out.--Aspro (talk) 00:50, 3 April 2012 (UTC)


 * The comment was a general one on the thread. "Synchronization" with respect to motors and generators generally implies AC, and seems meaningless when the source voltage could be constant, as from a battery. Maybe the term was used in your classes on DC motors, but I do not recall it being an aspect of DC motor operation. With AC there are synchronous motors and induction motors which  run under load at a lower RPM than a synchronous speed would indicate. Synchronization of AC generators is a job to be done when one is brought online, but is pretty meaningless with DC generators.  Edison (talk) 02:09, 3 April 2012 (UTC)


 * Are you being recalcitrant for the sake of it? This has nothing to do with AC motors.--Aspro (talk) 02:49, 3 April 2012 (UTC)
 * Since it is not related to AC, there is little reason for you to talk about "synchronization." Edison (talk) 14:39, 3 April 2012 (UTC)

Thanks Aspro and Edison. I have read through the responses and although I don't totally get it, (never did physics past high school), I think I understand most of it now. But I find it quite surprising that they would this confusing question in an exam for high school students. It's hard to believe how you can explain it in a paragraph. In regard to the 'perfect motor' with infinite poles, I think their syllabus only covers simple DC motors (i.e. two N/S magnets and a square coil with power), so I don't think that was the answer they wanted- so there will be a change of flux and induced EMF. And I do think the 'full RPM' needs to be disambiguated. Now I'm really interested in seeing their answers. 27.32.104.185 (talk) 10:56, 3 April 2012 (UTC)


 * You wrote Load is an ambiguous term too - does it have an innate resistance? I was puzzled by this.  In the question, load is not referring to an electrical load - it is referring to the mechanical load being driven by the DC motor.  For example, in an automobile the windshield wiper is operated by a DC motor.  The load is the reciprocating mechanism plus the resistance to motion as the wiper blades move back and forth across the windshield. Many variable-speed cranes and large machine tools such as lathes and boring machines have DC motors.  Dolphin  ( t ) 02:34, 4 April 2012 (UTC)

fine -structure
can the value of the fine structure of an atom placed in an external magnetic field be the same as the value measured in the absence of an external magnetic field ? What is the literature value of the fine-structure(splitting) of the mercury yellow spectral line ? — Preceding unsigned comment added by 197.255.118.206 (talk) 16:54, 2 April 2012 (UTC) Nimur (talk) 19:36, 2 April 2012 (UTC)


 * You may wish to read Zeeman effect and fine structure. You should find the literature for yourself if that is your assignment. Graeme Bartlett (talk) 06:39, 3 April 2012 (UTC)