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December 31[edit]

Metal versus cloth[edit]

My grandson came up with this the other day and so far we haven't been able to find an answer. Probably because I'm not searching correctly. Why does a piece of material feel warmer than a piece of metal at the same temperature (−25 °C (−13 °F)). To make sure we put a some metal and some cotton outside overnight, about 10 hours, and when we checked in the morning we both thought that the cotton felt warmer to the touch than the metal. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 09:18, 31 December 2016 (UTC)[reply]

Because the metal, being a conductor, will conduct the heat away from your fingers, while the cotton, being an insulator, won't? 80.5.88.48 (talk) 09:40, 31 December 2016 (UTC)[reply]

... so the cloth material in contact with your hand really is warmer because of the temperature gradient of the heat flow from your hand. Dbfirs 09:53, 31 December 2016 (UTC)[reply]

No, the two items really will be the same temperature. But your temperature receptors don't measure the temperature of what they are touching - they measure the change in their own temperature. A piece of cloth doesn't draw heat as quickly, so your hand stays warmer, and thus feels warmer. The cloth isn't hotter than the metal, but the hand touching cloth is hotter than the hand touching metal, and your brain misinterprets. Someguy1221 (talk) 10:53, 31 December 2016 (UTC)[reply]

Yes, there really will be a temperature difference because of the temperature gradient in both the hand and the cloth (also see Thermal conduction), but I agree that temperature sensors in the skin measure their own temperature, and also the rate of heat flow, so this temperature difference will not be the predominant cause of the sensation. Dbfirs 12:03, 31 December 2016 (UTC)[reply]

At those temperatures polar explorers have found to their cost that the skin of their fingers bonds to the metal and they can't let go. 80.5.88.48 (talk) 10:03, 31 December 2016 (UTC)[reply]

Not so. Neither my grandson or ice bonded to the metal. At that sort of temperature you will get minor frostbite on your arms if it comes into contact with metal for a few seconds but not fingers. After 40+ years in the Arctic I have never seen anybody bond their fingers to metal. I've frozen my fingers several times over the years and have nerve damage and I think I have met about 5 people who have had one or more fingers amputated because of frostbite but again not from bonding to metal. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 14:31, 31 December 2016 (UTC)[reply]

There' an old wintertime tale about some kid licking a metal pole and their tongue sticking to it. I don't know if there's any truth to that, either, but it would make more sense, as the moisture from the tongue could freeze. There's relatively little moisture on arms and fingers. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:58, 31 December 2016 (UTC)[reply]

That's true. I used to go to regional education board meetings and one member mentioned several times that kids in Kugaaruk would lick and freeze to the metal railing going down the stairs at the school. You would think that one kid would do it once but it was 2 or 3 different kids each year. Once, when I had only been in the Arctic a few months I put a set of keys in my mouth when I ran out of hands. They came out with little problem. A few years later we were out fishing/camping one spring, around −20 to −25 °C (−4 to −13 °F), and I was cutting pieces from a block of Velveeta, the only blocks of cheese we got at the store, and putting them in my mouth. Of course the knife stuck to my tongue and instant reaction was to pull it out. You wouldn't believe how much blood comes out of the tongue, how amusing it is for others and how little sympathy you get. By the way tongue freeze metal. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 16:02, 31 December 2016 (UTC)[reply]

A Christmas Story is the canonical example. Is there such a thing as a "[single] dog dare"? Tevildo (talk) 16:09, 31 December 2016 (UTC)[reply]

@Tevildo: Not in Mississippi, but that is what made them peculiar. At least in 1893.[1] For future reference, if you find yourself in that position you say I double black dog dare you and hope the other kid doesn't come from Mississippi. Wnt (talk) 04:36, 1 January 2017 (UTC)[reply]

There is an Oscar-nominated 1991 Czech film The Elementary School (cs: Obecná škola) by Jan Svěrák, which contains a minor subplot about those physics between tongue and deeply cooled metal: At peak of winter a school principal cautiously issues a schoolwide warning against "licking metallic objects" and (of course because of the inadverently inspiratory effects of his warning :-)) shortly thereafter he has to admonish several unobedient boys stuck by their tongues to a railing, while they are being, one by one, "separated" (a janitor is heating the part of the railing besides their respective heads with a blowtorch). The episode ends well, but not without some blood-spitting and graphic detail of tongue surface tissue spots remaining on the railing. The lesson seems clear: If one (despite the well-minded warning above) wishes to experience the effect with his/her own tongue, then perhaps one should begin with a sufficiently small and conveniently shaped piece of metal, of which the railing in front of local school building obviously isn't the best choice. GCZPN3 (talk) 13:16, 2 January 2017 (UTC)[reply]

Metal doesn't always feel "colder" or "hotter", it will tend to feel "more so" (either colder or hotter). If its temperature is less than the body, it feels cold. As it has both greater thermal conductivity and greater heat capacity, then more heat will flow from the hand to the metal than for cotton - so the metal feels as if it's more cold than the cotton, as the heat transfer is greater, even though the temperature is the same.

As body temperature is greater than ambient temperature, most metals touched will be colder. If the metal (and cotton) is hotter, then the metal feels hotter too. You'd only notice this in Dubai or in a blacksmith's workshop though. Andy Dingley (talk) 12:29, 31 December 2016 (UTC)[reply]

Or in a parking lot. If your car has been out in the sun all day during summer, then you might expect the car body to feel warmer than the cloth on the seats (assuming you have cloth-covered seats). ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:36, 31 December 2016 (UTC)[reply]

Here in California, on hot summer days, metal door handles can literally get too hot to touch -- so we notice this effect here as well! 2601:646:8E01:7E0B:F88D:DE34:7772:8E5B (talk) 12:39, 31 December 2016 (UTC)[reply]

In Arabia the steering wheel gets too hot to touch. They drive with oven mitts on. Sagittarian Milky Way (talk) 04:48, 1 January 2017 (UTC)[reply]

Thanks all for the answers. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 14:31, 31 December 2016 (UTC)[reply]

Even here in the UK, I've had my fingers gently stuck to metal, though we don't get the really cold temperatures that you get. Perhaps the very humid atmosphere here has some effect? Dbfirs 17:05, 31 December 2016 (UTC)[reply]

I suggest the following experiment (substituting wood for cloth, as the air gaps in cloth introduce a new variable we want to avoid):

1) Drill 2 holes each in identically shaped and sized piece of wood and metal, one hole 6 inches from the end and one 7 inches from the end.

2) Place temperature sensors in each hole.

3) Place both outside, in the cold, until they reach equilibrium temp (you can use a freezer in summer).

4) Bring them both inside, and place them in two identical bowls you just filled, to the same depth, with boiling water, so each is sitting on the bottom, partially submerged, in 5 inches of water, so the one sensor is 1 inch above the waterline and the other is 2 inches above the waterline. Make sure the items are long enough so that the wood doesn't float when so submerged (test this first). There should also be thermometers in the bowls.

5) The temperature at the sensors in the metal should rise more quickly and you should see less difference between them than in the wood, showing that the heat is being conducted away form the bowl of hot water more quickly. Meanwhile, the temperature should drop more quickly in the bowl with the metal in it.

This requires lots of thermometers/temperature sensors, but somehow I suspect you may have a few at your disposal. :-) StuRat (talk) 19:47, 31 December 2016 (UTC)[reply]

This experiment could be a lot simpler by using just a single thermal camera. DrChrissy ^(talk) 20:05, 31 December 2016 (UTC)[reply]

That's a good experiment to show the temperature gradients. Dbfirs 20:15, 31 December 2016 (UTC)[reply]

Alternatively, to eliminate temperature gradients which could confound results, the bowls could contain a stirring flea. DrChrissy ^(talk) 20:27, 31 December 2016 (UTC)[reply]

That would eliminate any significant gradient in the water, but I was thinking of the gradients in the wood and the metal. The end of the wood in the water would be warmer than the corresponding metal end. Dbfirs 20:41, 31 December 2016 (UTC)[reply]

Maybe I am misunderstanding something - I thought both the wood and metal were totally submerged. DrChrissy ^(talk) 21:15, 31 December 2016 (UTC)[reply]

... "waterline"? Dbfirs 21:20, 31 December 2016 (UTC)[reply]

Bring them both inside, and place them in two identical bowls you just filled, to the same depth, with boiling water, so each is sitting on the bottom, in 5 inches of water I think the 5 inches is an arbitrary depth. DrChrissy ^(talk) 21:27, 31 December 2016 (UTC)[reply]

Yes, I can see that the suggestion could be interpreted either way, but your thermal camera will work better if only the ends stand in the water. Dbfirs 21:47, 31 December 2016 (UTC)[reply]

Yes it would. However, the other way is to look at how quickly the water cools - which is why I suggested the stirrer. A sensitive enough thermal camera will pick up that the water in the bowl containing the metal cools more quickly than the bowl containing the wood. Either approach would give useful data. DrChrissy ^(talk) 22:21, 31 December 2016 (UTC)[reply]

I meant partially submerged, and have clarified my post. The holes are supposed to be 1 inch and 2 inches above the waterline. I also didn't assume any fancy equipment. StuRat (talk) 23:10, 31 December 2016 (UTC)[reply]

Because other editors have already commented, you would be best to (it might even be required) you use strike-through on the original content of your post.

I didn't want to leave it unclear. I did make the changes bold. They can view the history to see the prior version. StuRat (talk) 04:56, 1 January 2017 (UTC)[reply]

Think User:Dbfirs was just being rhetorical about placing an expensive thermal imager in water. Better to rely on the inbuilt Peltier effect rather than ruin one's warranty. Comes up with the right point though. Cloth might have the same temperature but as soon as one put one hands upon it, the surface warms quickly. When the air is ever so cold, the air is very dry to, so ones fingers don't stick, unless one is a newbie working in (say) a defense contractors proving cold chamber at -40 and whips off one's mitten an pick up a 1½ spanner with ones sweaty little hand. That spanner will stick! The uneducated repose is to immediate shake it free – loosing some skin in the process. Using liquid as a comparison can also be misleading because when working in R&D, used to inform graduates about the nature of liquid nitrogen (at minus −190 something) by dipping my fingers in it. Err hang on.. 1,2,3,4,5,6,7,8,9 10... Yep -still go all of my digits. PS don't try this at home folks Before any one ask, no never tried with O2 – to dangerous and although a flask of O2 poured over the garden barbecue charcoal lights it instantly (and very impressively too – crating exclamations of 'awesome') that can be dangerous at a venue where people may have already been imbibing alcohol. Just telling you this in case some bonzo turn up with a flask at your event who doesn’t know what he is doing. ( Bonzo means self-immolation and others might go up in flames at the same time) Cloth does not even have the specific heat capacity of metal. So it can't absorb as much as quickly. Frozen butter, ice cream, will feel cold for the same reason that unlike cloth it absorb thermal energy very readily. How does one explain that to a kid though? Practical demonstrations.--Aspro (talk) 23:53, 31 December 2016 (UTC)[reply]

As I understand, the barbecue lighting becomes much more impressive if you soak the charcoal in the liquid oxygen first. :) Wnt (talk) 00:07, 1 January 2017 (UTC)[reply]

Yes but don't ! It is not just that which is the problem but also people bring it along in a domestic vacuum flask--Aspro (talk) 00:12, 1 January 2017 (UTC)[reply]

If your master of ceremonies happens be a emeritus professor and wants to try this trick out, he will know what his doing (says me hopefully) and ensure that only his own eyebrows get singed if he miss judges it. Chemistry is fun so don't get me wrong. Yet, what if his car has a 'minor' shunt on the way? Should it be in a Thermos and that flask fractures - one has a fire. If he has his kids inside, that sudden heat pulse may become too great for survival--Aspro (talk) 00:33, 1 January 2017 (UTC)[reply]

Some Emeritus Professors are female. DrChrissy ^(talk) 00:46, 1 January 2017 (UTC)[reply]

Was simply using the 'neuter generic' -Professor Chrissy. Know also that Madam Curire won the Noble Prize not once but twice. Suggest that universities go back to requiring that only Classical Greek is spoken during lectures which would avoid quasi gender taxonomy confusion...(OK, they can continue using dorsal and planter etc., for anatomical things) If you have noticed, I often reply to posts using s/he or individual or editor where gender is not evident whilst wishing to be inclusive to all editors and contributors. Also, some editors wish to hide their gender for fear of harassment and I have to respect that too. Gosh . My cat went loopy at my typing the word neuter. Can't get her off my keyboard as she keeps pressing her paws on TOM, TOM, TOM again and again. Have the same problem when I type in any words that begin with V.E.T. Strange ! Any way, and more to the point: What are you doing up so late tonight, you should have been in bed long ago. Happy New Year. --Aspro (talk) 02:09, 1 January 2017 (UTC)[reply]

Thanks for the explanation - Happy New Year to you too. DrChrissy ^(talk) 20:36, 1 January 2017 (UTC) [reply]

Why stop with LN2? Go for molten lead![2] (ref also discusses holding red-hot Space Shuttle tiles). DMacks (talk) 05:22, 3 January 2017 (UTC)[reply]

Exactly DMacks. These demos are what really teaches physics. Think it was probably with a lower temperature lead alloy and being a classical guitar player, had long finger nails and so in one demonstration must have had some liquid water under them. Sufficient to turn it into super-heated-steam thus splattering molten metal out -which a I was not expecting. Needless to to say, never made the same mistake again.--Aspro (talk) 21:24, 4 January 2017 (UTC)[reply]

OK. We had a discussion about the fingers bonding to metal earlier. Turns out that I was wrong up about that. My fingers start to hurt quickly that I let go of anything below freezing very quickly. I was also thinking about people years ago. People were quite happy to work on their snowmobiles without gloves and not having their hands stick to the metal, don't try this. It turns out that younger people are able to touch the metal long enough, without pain, to cause moisture to form and their hands stick to the metal. StuRat. I really don't have many thermometers left. Two mercury and one digital hand held. None of them go above 40 °C (104 °F) so it wouldn't work. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 03:51, 1 January 2017 (UTC)[reply]

Wow, if there was one thing I expected you to have in quantity at a weather station, that's thermometers. I suppose you could make the water bowl room temperature, if you must. The effects will be less dramatic, but perhaps a bit safer. You can also do the experiment on one material at a time, if you don't have enough thermometers to do them together. (BTW, that 40°C limit might be hit soon, what with global warming and all, prepare for them all to explode.) StuRat (talk) 04:45, 1 January 2017 (UTC)[reply]

Not really. The weather equipment at the airport has a built in sensors and we have an emergency electronic thermometer. Before that we had a remote temperature unit that also used sensors (on the left and right of the two mercury thermometers) and several mercury, minimum and maximum thermometers. We never really had much use for them, except when the power was out, and even with little use they would get broken and not replaced. Some time after the new equipment (HWOS) but before the the arrival of the hand held electronic we had three mercury thermometers broken in a week including both in here. Correction the thermometeres go to 45 °C (113 °F). I also see we have a maximum and minimum thermometer I had forgotten about. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 15:06, 1 January 2017 (UTC)[reply]

I know this is pretty much resolved, but the answer to this question is also why you can quite easily put your hand into an oven which is at 200 C for several moments, but if you as much as touch the wire rack inside it will give you a burn immediately. Vespine (talk) 22:19, 2 January 2017 (UTC)[reply]

True, but it's easy to see why the thermal conductivity of a gas is lower than a solid, what with far fewer molecules to transmit the heat. It's not so obvious that various solids would transfer heat at dramatically different rates, too. StuRat (talk) 18:25, 4 January 2017 (UTC)[reply]

I think Prince Albert is mostly known nowadays for what happened after his tutor left him alone with a hole punch. ;) Wnt (talk) 13:28, 3 January 2017 (UTC) — Preceding unsigned comment added by 79.73.132.73 (talk) — Preceding unsigned comment added by 81.151.101.189 (talk) [reply]

Feynman Lectures. Lecture 42. 42–2Thermionic emission [3][edit]

...
In order to get an electron out of a piece of metal, it takes a certain amount of energy or work to pull it out. This work varies with the different kinds of metal. In fact, it varies even with the character of the surface of a given kind of metal, but the total work may be a few electron volts, which, incidentally, is typical of the energy involved in chemical reactions. We can remember the latter fact by remembering that the voltage in a chemical cell like a flashlight battery, which is produced by chemical reactions, is about one volt.

— Feynman • Leighton • Sands, The Feynman Lectures on Physics, Volume I

We know that AA LR6 battery has energy 3 W•h or 10⁴ J or 6•10²² eV. I don't see the connection between the fact that voltage = 1,5 volts and the energy per electron = few eV.

We also have accumulators with voltage 12 volts, energy 600 W•h (automobile one), so accumulators have different energy per electron or what?

Username160611000000 (talk) 16:12, 31 December 2016 (UTC)[reply]

Automobile accumulator can maintain the current 50 amperes during 1 hour .

50ampere\cdot 1hour=50{\tfrac {coulomb}{sec}}\cdot 3600sec=180000coulomb

This is equivalent to

{\tfrac {180000\,coulomb}{1.6\cdot 10^{-19}\,coulomb}}=

1,125•10²⁴ electrons.

So energy per electron =

{\tfrac {600Wh}{1,125\cdot 10^{24}}}={\tfrac {1,35\cdot 10^{25}eV}{1,125\cdot 10^{24}}}=

12 eV.

Is it correct?

So is it correct that the energy per electron = voltage of the cell?

Username160611000000 (talk) 16:43, 31 December 2016 (UTC)[reply]

The basic reference is galvanic cell. Aside from real-world complications (e.g. internal resistance), one does expect the voltage of a simple galvanic cell to equal the energy liberated per electron transferred. Of course, such cells can be arranged in series to reach higher voltages, or combined in more complicated ways to achieve other effects. Dragons flight (talk) 18:45, 31 December 2016 (UTC)[reply]

...
$I=q_{e}nv=(q_{e}v/V_{a})e^{-q_{e}\phi /kT}.$

Now one electron volt corresponds to kT at a temperature of 11,600 degrees. The filament of the tube may be operating at a temperature of, say, 1100 degrees, so the exponential factor is something like e⁻¹⁰; when we change the temperature a little bit, the exponential factor changes a lot.

— Feynman • Leighton • Sands, The Feynman Lectures on Physics, Volume I

This part is also not very clear. Why did he decide that $q_{e}\phi$ = 1 electronvolt ? Username160611000000 (talk) 16:52, 31 December 2016 (UTC)[reply]

To begin with, a "battery" is a wikt:battery of galvanic cells, after analogy to a battery of cannon; even a 9V battery contains multiple cells in series. Now that said, we know that power can be converted from V1 x I1 to V2 x I2, where V1 = V2 * N and I1 = I2 / N; in other words, the power is voltage times current. Putting batteries in series is just one way to accomplish such a conversion.

Now if you convert the current from Coulomb (unit) per second (Amperes) to Faraday (unit) per second, you know how many moles of electrons are flowing, and you can multiply that current by Avogadro's number to get how many individual electrons are flowing, and you can then see how many volts they are flowing across. (Or come to think of it, you could just divide by the charge on the electron!) Wnt (talk) 00:03, 1 January 2017 (UTC)[reply]

(Or come to think of it, you could just divide by the charge on the electron!) - I have already done it above. Yes, I realize I can know the total number of electrons, driven by the cell. But it's hard to realize the idea. So stored energy in eV is (number of electrons)•(voltage). Then the stored energy per electron is (voltage) , always , and the technology can't make it higher than 1..3 volts per cell.

even a 9V battery contains multiple cells in series - I forgot the automobile accumulator has 6 individual jars 2 volts each. It seems Feynman is right.Username160611000000 (talk) 07:14, 1 January 2017 (UTC)[reply]

What is the reason that it's not recommended to bend while the knee is not bed too?[edit]

The most of people know that it's not recommended to bend (for example to take something from the ground) with locked knee. My question is why? 93.126.88.30 (talk) 23:50, 31 December 2016 (UTC)[reply]

Here is some info. The explanation given at that source is "to avoid compressing the spinal discs or straining your lower back". Check this out too: Why You Lift With Your Legs, Not Your Back. Bus stop (talk) 00:00, 1 January 2017 (UTC)[reply]