# The Naked Scientists Forum

### Author Topic: What is Heat?  (Read 4989 times)

#### thebrain13

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##### What is Heat?
« on: 03/07/2009 21:26:44 »
What exactly is heat? I know the definition is the average kinetic energy of the object. But I don't understand what that means.

What do scientists mean when they say that? What is the difference between say, turning a blender on so the water is sloshing around (adding kinetic energy) and the water just being hot, thus adding kinetic energy?

#### Dr.IC

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##### What is Heat?
« Reply #1 on: 10/07/2009 08:48:36 »
A related term is thermal energy, loosely defined as the energy of a body that increases with its temperature. Heat is also loosely referred to as thermal energy, although many definitions require this thermal energy to actually be in the process of movement between one body and another to be technically called heat ...

Heat, in physics, is energy which is spontaneously flowing from an object with a high temperature to an object with a lower temperature.

Heat and cold are just a notation to represent the temperature levels. It is nothing but thermal energy in a body.

#### lyner

• Guest
##### What is Heat?
« Reply #2 on: 10/07/2009 18:31:56 »
If you take an object and give it a velocity by doing work / expending energy on it then it's easy  to see where your energy has gone -  straight into the Kinetic Energy of the moving object. If you give it the same amount of energy in the form of heat, the molecules will all start to move about, vibrating (in a solid) or jostling about (if a gas), increasing their average speed and raising its temperature. The total increase in energy in both cases will be the same, if you put in the same amount of energy.
If you look at the total 'internal energy' (which is what we call heat, these days) of a ikg mass of air, for instance, at room temperature, the average velocity of the molecules will be about 500m/s (around 1500mph!). To take that 1kg mass of air (put it a strong enough balloon and send it off through empty space at 500m/s that would need exactly the same amount of energy but you could 'see' the effect clearly.
In the first case, the motion is all in one direction but, in the second case, the motion is in all random directions and with a range of speeds. All of the energy of the coherent motion in the first case is readily available to do useful work  but the energy available to do work in the second case will depend on letting some of that energy flow from the air into somewhere colder. This is called 'a heat engine'- steam engines and internal combustion engines all work this way and are pretty inefficient because they need  a big temperature difference  to get  much useful energy out of the random internal energy.
« Last Edit: 10/07/2009 18:34:05 by sophiecentaur »

#### Soul Surfer

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##### What is Heat?
« Reply #3 on: 10/07/2009 23:51:28 »
To go back to the original question there is no difference.  Just stirring up water makes it hotter that is precisely how Joule first measured the mechanical equivalent of heat.

#### lyner

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##### What is Heat?
« Reply #4 on: 11/07/2009 01:18:07 »
The bulk movement is not 'heat' until it has dispersed as random motion of the molecules.

#### thebrain13

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##### What is Heat?
« Reply #5 on: 11/07/2009 04:11:21 »
What I don't get about that definition of heat is, if heat is only motions of random particles, then why wouldn't a solid object eliminate most of the heat almost immediately? All the particles have to move in the same direction in a solid object, or at least very close or it will break the bond. Also a piece of steel can vibrate, but even a vibrating particle is generally traveling at the same velocity as the particles in its immediate vicinity. Why is heat conserved so well?

#### Soul Surfer

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##### What is Heat?
« Reply #6 on: 11/07/2009 09:09:38 »
Yes it is heat sophie, you are not thinking clearly (unusual for you).  In his experiment Joule made sure that the movements were as turbulent as possible to get the energy distributed between the molecules quickly.

Brain you are forgetting that even in a normal solid the atoms and molecules are all vibrating it is just that they are vibrating within controlled limits.  We live in a dynamic universe every particle in it is always moving (even if this movement is constrained) at all times.  The quarks inside the protons and neutrons inside the nuclei of atoms are moving at relativistic speeds but they can't travel far before they bounce back.

You can start to appreciate this by looking at "Brownian motion" of tiny particles in suspension being buffeted continually by water molecules.  This continues inside solids as well as liquids and becomes a limit of what can be visualised even with an atomic force microscope.

You must remember that even at the absolute zero of temperature some of these motions will cntinue to take place.
« Last Edit: 11/07/2009 09:13:57 by Soul Surfer »

#### lyner

• Guest
##### What is Heat?
« Reply #7 on: 11/07/2009 09:31:41 »
SS . Even Joule's experiment started off with the water having bulk KE.  The temperature is only established when all bulk movement ceases.  He didn't get it all right - a nightmare for his wife on honeymoon, reputedly.
In the old days they seemed to regard heat as a separate form of energy: hence "the mechanical equivalent of heat". I did that in School, when the Joule and Calorie were somehow different things. But today, the Watt and the Joule apply everywhere but on the sides of Mars Bars.

The definition of temperature also applies in solids. Average KE whilst vibrating. Because the molecules  in many solids are massive, they may vibrate at lower average speeds than those in air because they will still have the same mean KE at a given temperature.

#### lrvv

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##### What is Heat?
« Reply #8 on: 11/07/2009 10:27:24 »
To go back to the original question there is no difference.  Just stirring up water makes it hotter that is precisely how Joule first measured the mechanical equivalent of heat............

#### thebrain13

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##### What is Heat?
« Reply #9 on: 11/07/2009 10:34:12 »
The thing I don't understand is, why would atoms in a solid vibrate? Why wouldn't they just transfer their motion elsewhere, until all the atoms were not vibrating(or barely vibrating)?

#### lightarrow

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##### What is Heat?
« Reply #10 on: 11/07/2009 15:09:15 »
The thing I don't understand is, why would atoms in a solid vibrate? Why wouldn't they just transfer their motion elsewhere, until all the atoms were not vibrating(or barely vibrating)?
To transfer their internal energy as heat, the atoms have to be in a surrounding with a lower temperature than them. To transfer *all* of that energy as heat, the surrounding should be at 0K (and that's impossible). If you put them in deep space, they can go down to 2.7K (CMBR's temperature) but not lower than that. For even lower temperatures, you need a suitable apparatus.

Just to precise: heat is neither motion of particles, nor energy that a body can have; heat is the *flux of energy* from a body to another one, because of a temperature difference only, as Dr.IC wrote .
« Last Edit: 12/07/2009 12:12:45 by lightarrow »

#### lyner

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##### What is Heat?
« Reply #11 on: 12/07/2009 00:37:04 »
To go back to the original question there is no difference.  Just stirring up water makes it hotter that is precisely how Joule first measured the mechanical equivalent of heat............
To be accurate, he called it the mechanical equivalent of heat. The amount of work you need to put in to produce a detectable rise in temperature  is usually quite a lot. To raise the temperature of 1 kg of water by 1 degreeC is the equivalent to raising  4.2 kg by 100m in height.

#### Soul Surfer

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##### What is Heat?
« Reply #12 on: 12/07/2009 10:20:10 »
Brain that's prcisely what does happen all theatoms and molecures Are vibrating and when we feel something as being hot it is these vibrations that are being transferred to us or cold when we are transferring vibrations to the object.

#### thebrain13

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• Posts: 442
##### What is Heat?
« Reply #13 on: 16/07/2009 05:37:18 »
Lets say an atom entered a very small system that was very near 0 kelvin. Would it be possible that this atom could contain different amounts of heat, and warm up the system more than an atom without heat? Or would the heat of the system only rely on the motion of the new atom relative to the new systems.

Can heat be carried by individual particles?

#### lyner

• Guest
##### What is Heat?
« Reply #14 on: 16/07/2009 10:28:48 »
Quote
Would it be possible that this atom could contain different amounts of heat
That is not a valid description. An atom cannot 'contain' heat.  An atom may have more Kinetic Energy or it may be in an electronically excited state - either of those may add to the total average KE of the system and, so, raise its temperature.

#### The Naked Scientists Forum

##### What is Heat?
« Reply #14 on: 16/07/2009 10:28:48 »