# Questions about heat

Discussion in 'Physics & Math' started by Magical Realist, Oct 1, 2017.

1. ### Magical RealistValued Senior Member

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1) Why does pressure make an object get hotter instead of colder? Aren't the molecules moving less because of the pressure?

2) Is the heat you feel on your hands from a campfire being transferred thru convection or radiation?

3) How can sound vibrate the air without causing it to heat up?

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3. ### exchemistValued Senior Member

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1) Compression makes an object hotter. This is due to the work done on it (Force x distance) in order to compress it. Work is a form of energy. So the act of compression adds energy.
3) If a gas is compressed without losing heat to its surroundings (what is called "adiabatically") and is then allowed to return to its previous volume, the temperature will return to what it was before, so there will be no net heating. However in reality some heat is lost when the temperature is elevated due to compression, so sound waves do lose energy via heating effects. The effect is however generally too small to be noticeable.

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5. ### iceauraValued Senior Member

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Illustration: start a ping pong ball bouncing on a table, and compress the bounce with the paddle - bring the paddle closer to the table while keeping the ball between the two surfaces.
If your hands are to the side, radiation. If your hands are above the fire, convection contributes as well.

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7. ### arfa branecall me arfValued Senior Member

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Pressure is equivalent to the flux of momentum, increase the pressure and you increase the flux. There are more molecules interacting with the walls of a container after compression, so there is more heat per unit volume.
It does heat up the air, but also cools it since the air is compressed and expanded by the propagation of pressure waves (i.e. sound).

8. ### ajantaRegistered Senior Member

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In the present work, it has calculated the energy levels of GaAs parabolic quantum dot under the combined effects of external pressure, temperature and magnetic field. The eigenenergies have been obtained by solving the two electron quantum dot Hamiltonian using the exact diagonalization method. The obtained results show that the energy levels of the quantum dot depend strongly on the pressure and temperature. It has found that the energy levels enhance as the pressure increases for fixed temperature and magnetic field while the quantum dot energy levels decrease as the temperature increases for fixed pressure and magnetic field. The comparisons show that results are in very good agreement with the reported works.

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9. ### TheFroggerValued Senior Member

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Thermodynamics. Hot flows to cold.

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10. ### Magical RealistValued Senior Member

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Thanks for all your excellent answers. Here's some more questions.

4) Where do the air bubbles in the bottom of a pot of boiling water come from?

5) How do the vibrating molecules of heated matter create light?

6) A welder once showed me a neat trick. He heated the end of an aluminum bar with his torch until it was glowing. Then he stopped and over a period of several minutes the glowing spot moved all the way down to the other end of the bar! How does this happen?

7) How does the body heat itself? Special heating nerves?

8) Why doesn't shaking a bottle of water heat up the water to boiling point?

Last edited: Oct 1, 2017
11. ### iceauraValued Senior Member

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It will. And far past it. Insulate a strong bottle very well, fill it with water, put a good cap on it, shake it for a few months, take the cap off (careful don't burn your hand), and you can be successfully blinded by an eruption of phase changed H2O.

Your questions are similar to those asked by renaissance natural philosophers at the beginning of the scientific age. After hundreds of years of research some answers have been found, and you can find them compiled in the esoteric libraries of the top secret "Wikipedia" society. For access you will need the proper codes (ask an adept where you can acquire a "browser"), and a special entry device equipped with one or more "heat sinks" designed according to the principles of heat that have been discovered, the existence of which verifies for you the validity of those principles you will be given (you already possess one such device, clearly, since you are employing one to communicate through the aether at this moment).

12. ### Magical RealistValued Senior Member

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No thanks. I'd rather get the answers here from people who can explain it to a layman. Assuming ofcourse they can.

13. ### iceauraValued Senior Member

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That's a lot of work just to recap a Wikipedia article for somebody too lazy to make mouse clicks, and too unobservant to notice which way convection is taking the heated air from a campfire.

I'll bet you haven't even tried the ping pong ball illustration for developing an intuitive feel for compression heating, have you.

What's in it for me, in my attempts to get you to see, do, and comprehend against your will?

Last edited: Oct 1, 2017
14. ### exchemistValued Senior Member

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4) They are not air bubbles, but bubbles of water vapour.
5) They don't. Molecular vibrations emit in the IR region.
6) Cannot comment, never having seen this demonstration.
7) Expenditure of chemical energy in metabolism. For example, carbohydrates + oxygen -> carbon dioxide + water + energy. Mammals are endotherms, meaning that they have feedback loops that stimulate energy expenditure to maintain body temperature, by means of such reactions, when needed.

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15. ### Magical RealistValued Senior Member

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Don't worry about it troll. You'll be ok..Unlike you not everybody here has to consult Wikipedia to answer science questions.

Last edited: Oct 1, 2017
16. ### Magical RealistValued Senior Member

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Vapour is air with water molecules in it. Where does the air in the bubbles come from?

So a light bulb filament's molecules aren't emitting light when heated?

How does the brain regulate this process in the rest of the body?

17. ### sideshowbobSorry, wrong number.Valued Senior Member

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The vapour in the bubbles would be (almost) pure steam. If there was air dissolved in the water, it would be a small part of the bubbles.

18. ### Magical RealistValued Senior Member

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So the air in the vapour comes from the water itself? Tks. That makes sense. There must be a lot of air in water for it to bubble up so long.

Last edited: Oct 1, 2017
19. ### DaveC426913Valued Senior Member

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Vapour is more accurately gaseous water. As SSB said, almost pure steam.

Sure, water vapour may contain air, but that's not relevant here.

No. It is pretty much pure gaseous water.

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20. ### Magical RealistValued Senior Member

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Assuming it is a state of water molecules that are farther apart than in their liquid phase, what is between those molecules? Air molecules? Or a vacuum?

21. ### DaveC426913Valued Senior Member

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Vacuum. Like in any other gas. They are bouncing off each other with such vigor that the bubble expands. Like popcorn popping ,and blowing the top off the bag/kettle.

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22. ### James RJust this guy, you know?Staff Member

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Just to emphasise: they aren't air bubbles (mostly). They are bubbles of gaseous water (water vapour).

Classically, the explanation would be that accelerating charged particles create electromagetic waves.
Quantum mechanically, molecules have vibrational energy levels. Transitions between vibrational levels result in the emission of photons.

I haven't seen that either, but I'd expect aluminium to conduct heat considerably faster than that.
Water vapour is gaseous water. Once the steam bubble reaches the surface of the liquid water, then you start to get mixing of air with the water vapour.

Yes, they are.

A light bulb filament (like lots of hot objects) produces a range of light frequencies, including some infrared and some visible light. With an incandescent bulb, obviously the goal is to have the peak of the output in the visible range, which means a very hot filament (typically about half the temperature at the surface of the Sun).

A human body, in contrast, is at 37 degrees C (or a bit less at the surface of the skin), and so its peak of emission is in the infrared.

23. ### birchValued Senior Member

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counter, you are making sense. good for you. no counter-type op's like 'are fire-ants hot?'

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