Wikipedia:Reference desk/Archives/Science/2013 December 31

= December 31 =

Hair dryer
Hello all, I have a hair dryer with a built-in ionizer (which makes for faster and more even drying by breaking up the water droplets -- or at least, so they say). Well, a couple days ago, when using the dryer with the ionizer turned on (essential because my hair is very curly and doesn't dry well), I happened to look into the hot-air outlet and I saw what looked like a momentary blue spark deep inside the appliance -- it looked almost like a transient electrical arc. Is this normal? Should the hair dryer be replaced immediately? Thanks in advance! 2601:9:3200:467:F539:5282:3708:AF9F (talk) 05:10, 31 December 2013 (UTC)
 * Could this be related to the sparks I often observe in my electric drill? Something to do with the electrical contacts of a brush motor. --78.148.110.243 (talk) 05:50, 31 December 2013 (UTC)


 * I've never heard of such dryers before, but over at Answers.com a guy says that some of them produce ions by means of "internal electrical devices that create high voltage rather like the peizo sparker on a gas cigarette lighter". Perhaps that is the sparking you saw. Deor (talk) 12:23, 31 December 2013 (UTC)


 * The ioniser is likely pyroelectric tourmaline. The sparks would not be from that, but maybe you need to clean the filter. Graeme Bartlett (talk) 12:34, 31 December 2013 (UTC)


 * An ionic hair dryer should not spark. Sparks produce positive ions – a hair dryer  needs to give out  negative ions.  It probable does need a good clean but it is a domestic appliance. The consumer should  not fiddle about with it, unless the instruction booklet that came with it tells them exactly what to do. If you do not have this still – then email the manufactures for advice. If you get no reply after two weeks, dump it and buy a reputable-branded ionic hair dryer. If they don't reply,  it may be a cheap Chinese counterfeit -which does not comply to your country’s safety standards. Report it then, to to your local relevant consumer protection agency. It is your duty as a citizen to prevent harm coming to others by alerting them.--Aspro (talk) 14:40, 31 December 2013 (UTC)


 * In my experience ionizers often make sparks. They work by having two wires or plates with opposing charges, which ionizes the air between them.  The distance is set just beyond what would produce a constant spark.  However, even a speck of dust between the two can cause a spark to jump from one to the other.  Most likely a hair got in there and is causing the constant spark. StuRat (talk) 14:46, 31 December 2013 (UTC)


 * Correction: the spark was momentary and transient, not constant. And I think I forgot to say that this was right after coming out of a long hot shower, so the air in the bathroom was saturated (the other reason why my hair doesn't dry well) -- which would increase the conductivity and thus increase the chance of sparking, right?  Based on your advice, I wouldn't be too concerned, since the majority opinion seems to be that sparking in an ionizer is not dangerous. Thanks! 2601:9:3200:467:F539:5282:3708:AF9F (talk) 23:02, 31 December 2013 (UTC)


 * In that case, you might want to store and use the hair dryer outside the bathroom, to prevent the sparks. StuRat (talk) 15:29, 1 January 2014 (UTC)

What is the equation of J-integral in axisymmetric cylindrical coordinate(r-z plane)?
Dear all, I am doing research on fracture mechanics. I am stuck with how to calculate the J-integral of a crack in axisymmetric problem. Can anyone give some help. Thank you very much and happy new year. SongJie@NTU (talk) 08:08, 31 December 2013 (UTC)
 * Well the easiest way would be to put the origin of your coord system at the crack tip, then the path is a small circle with a crack through it, with r=constant. You will have to transform x to cylindrical coords, and you may use: :$$\mathrm d\mathbf{r} = \mathrm d\rho\,\boldsymbol{\hat \rho} + \rho\,\mathrm d\varphi\,\boldsymbol{\hat\varphi} + \mathrm dz\,\mathbf{\hat z}.$$ Graeme Bartlett (talk) 12:13, 31 December 2013 (UTC)

Thank you for the reply, can anyone give explicitly the equation? I am not familiar with J-integral. Just want to use it for a small project. Thanks a lotSongJie@NTU (talk) 13:16, 31 December 2013 (UTC)
 * The article here is J integral. I am not confident enough to do the conversion for you. Graeme Bartlett (talk) 20:00, 2 January 2014 (UTC)

Long distance vacuum
I have a post and beam house with a cathedral ceiling which accumulates a good deal of dust on the beams. It's probably a normal amount but they're hard to clean so it seems to accumulate a lot. Anyway, it's a pain to bring an extension ladder in and lug a vacuum up to clean the beams. So, I was thinking that I could try to find a very long extension for my vacuum hose or modify a piece of PVC. I think the amount of suction at the end of the extension would be the same as if I was using the standard length hose. My wife seems to think that the amount of suction would decrease at that length. I can't see why it would if I am able to keep the extension airtight along the length of it. So, who's right? Dismas |(talk) 14:55, 31 December 2013 (UTC)


 * Yes, if there are no leaks, it should have the same vacuum at the far end as at the near end. However, it will take a bit longer for the vacuum to get to the far end, as it must first lower the pressure of all the intervening air.  So, when you turn it on, it might seem a bit weak, at first.  I've noticed a similar effect with fans: If you turn the fan on at the far side of the room, it takes a noticeable amount of time for the breeze to make it to the other side.StuRat (talk) 15:11, 31 December 2013 (UTC)


 * (Note that I neglected the effect of air resistance in my answer, since I don't think it would be significant over such a length.) StuRat (talk) 18:59, 31 December 2013 (UTC)


 * I should also mention that a lighter-weight alternative is a duster on a long stick. Static electricity makes the dust stick to the duster.  Some dust might just be knocked loose and fall down, though, although this is also true of the vacuum.  Taping a feather duster to a broom handle should hopefully be long enough. StuRat (talk) 15:40, 31 December 2013 (UTC)


 * The length of the line does make a difference. If you're familiar with electrical circuits, there's a very close hydraulic analogy between electricity and fluid flow that is often very helpful for thinking about vacuum systems.  The air pump in the vacuum cleaner is like a battery: it creates a pressure difference between its inlet and outlet.  Pressure difference plays the role of potential difference (voltage), and air flow plays the role of current.  Any hose that you attach has some impedance, depending on its length and diameter according to the Darcy–Weisbach equation.  That means that the pressure at the far end of the hose is not as low as the pressure right next to the pump.  For this setup to be effective, you want the impedance of the line to be much less than the impedance at the end of the hose.  (That's just like how you don't care too much about the resistance of a wire in a circuit as long as it's much lower than the load resistance.)  That will be true if you're able to aim your long hose well enough to face the beam flat, forming an impedance.  Some vacuum cleaner attachments also have a narrower part at the end, for the same reason.  If you're just putting the hose in the vicinity of the beam and counting on high air flow, you will lose some pumping power because of the long line.  --Amble (talk) 17:49, 31 December 2013 (UTC)


 * Length makes a difference, because of friction. You can notice this effect when pumping water through very long garden hoses (100-200 meters). Water near the pump will exit with noticeably more force than at the end of a very long hose. If you have irrigation sprinklers at the end of the hose, you can see very easily that the force of the water has decreased a lot. It's even worse with hoses that have very little diameter.


 * But water must have a lot less friction than air. And with 3-4 meters of length, the effect friction should be so small that it's not be noticeable.


 * My experience with vacuum cleaners is that leaks in the pipe will decrease your performance very quickly. Use a good pipe, check for leaks by sealing the ends of the pipe and submerging it underwater, then seal the leaks with duct tape or glue or whatever. --Enric Naval (talk) 18:33, 31 December 2013 (UTC)
 * Water has way much more friction than air does. Or at least resistance to motion. ←Baseball Bugs What's up, Doc? carrots→ 18:53, 31 December 2013 (UTC)

I think this website has the answer where it says: "Due to their large-bladed fans, and comparatively-short airpaths, dirty-air cleaners create a very efficient airflow from a low amount of power, and make great carpet cleaners. Their 'above-floor' cleaning power is less efficient, since the airflow is lost when it passes through a long hose." Richerman   (talk) 23:51, 31 December 2013 (UTC)


 * One thing that would make quite a difference is a smooth hose versus one with pleats (is that the right term ?) which will slow the air flow far more. StuRat (talk) 08:22, 1 January 2014 (UTC)


 * I've actually seen such a thing as central vacuuming, with a vacuum set up in the basement and hoses running throughout from one end of a fairly longish house, with little low-voltage electrical lines to sense when any one of the outlets is lifted to turn it on. It was actually functional, if perhaps functionally insane  Wnt (talk) 05:16, 3 January 2014 (UTC)
 * Yes, those are kind of cool. I believe the plumbing from the pump to the outlet is rigid and smooth-sided, since it doesn't need to flex; and has a wider diameter than whatever hose you're likely to plug in at the wall outlet.  So the impedance to air flow won't be all that great.  In addition, the pump can be quite beefy since you no longer need to wheel it around your living space.  I suppose our OP can also use a smooth-sided, rigid tube of some kind for his vacuum extension, since he wants it to be stiff anyway to reach up to the ceiling.  That should help somewhat.  --Amble (talk) 17:59, 4 January 2014 (UTC)

Thank you for the responses, everyone! I might try to get some PVC and rig something up! Dismas |(talk) 22:43, 4 January 2014 (UTC)

Cat in the mirror
I have a cat. He is very timid, and does not like interaction with other cats. However, he is quite content to sit on my other half's dressing table and stare at himself (and even me) in the mirror with no sign of fear. Neither he nor I are far from the mirror and he can surely hear that I am speaking to him from behind. Can my cat see and recognise himself, or is something else going on? The Rambling Man (talk) 20:49, 31 December 2013 (UTC)


 * There is something called the Mirror Test designed to test what animals can recognize themselves in the mirror. Cats have never passed, but it's debatable what a failure of the test proves. (Passing the test without special training is usually taken as evidence of self-awareness.)
 * Perhaps if the cat doesn't recognize itself, it's at least smart enough to dismiss the mirror as an illusion. (Compare to many small birds that seem convinced there's a real bird somehow inside the mirror.) APL (talk) 21:08, 31 December 2013 (UTC)


 * (ec) You probably won't get an authoritative answer to this, but I'll give you my thoughts. The classic test of self-recognition is the so-called mirror test invented by Gordon Gallup, and the general finding is that cats don't pass it. But it seems likely to me that the majority of mammals are capable of a lesser form of mirror-recognition. The basic requirement for recognizing something as "self" is that it generates visual changes that are strongly correlated with the sensory inputs that the brain receives. It is possible to get people to perceive very strange things as parts of their own body by creating spurious correlations, as in the famous rubber hand illusion. When a cat watches itself in a mirror, there is a strong correlation between the movements it generates and the movements it perceives visually -- in principle the cat should be able to pick up on that correlation. But I have to acknowledge that this is original synthesis on my part, and I don't know of any literature that directly supports it. Looie496 (talk) 21:11, 31 December 2013 (UTC)


 * Thank you both. It's fascinating to me that this question hasn't been answered by all the might of science.  The fact that my cat looks me in the eye via the mirror but knows I'm behind him is astonishing to me.  The Rambling Man (talk) 21:18, 31 December 2013 (UTC)


 * This is going to be my own original research, but here goes. I have many cats, and they are all quite frightened of strangers (both human and cat). But they are not frightened of mirror images. So while they may or may not recognize their own image, they clearly recognize that the images are not real cats. However, one particular cat of mine used to stare at herself in the mirror. If I stood behind her and waved so that she could see my reflection waving, she would turn around to look at me. Incidentally, this is the same cat that grabbed the laser pointer out of my hand when I tried to fool her with the red dot. Someguy1221 (talk) 21:20, 31 December 2013 (UTC)


 * My cat reacted to his mirror image the first few times he saw it, but has ignored it at every opportunity over the last several years. I don't know if he recognizes himself in there; I think perhaps he just knows it's definitely not another cat (no foreign smell, no sounds, etc.). Mirrors confuse us quite readily because we're very visually oriented creatures; our sense of hearing is good, but limited, and our sense of smell is pathetic compared to most animals. If you can fool us visually, you can probably fool us totally; I don't think cats (or dogs) are so easily fooled on an ongoing basis. Matt Deres (talk) 22:45, 31 December 2013 (UTC)
 * Yep, maybe it's just my imagination that he's looking straight at me in the mirror, that it seems he should be capable of recognising me... it's really odd.  In any case, thanks to all of you who provided great replies.  The Rambling Man (talk) 22:49, 31 December 2013 (UTC)