# Diferent kind of energy (heat and kinetic)

1. Jun 9, 2010

### AlexB2010

I have a hypothetical question:
Suppose a use a chemical source of energy to heat a 1Kg Iron ball, or, transform that same chemical energy in kinetic energy to accelerate an equal ball in space.
The heated ball in space will cool down, loosing energy.
The accelerated ball will maintain its velocity and the energy given to then will “stick” to the ball, manteining the energy level.
What is the difference in the energy that makes than stick or run away from the ball?
Best,
Alex

2. Jun 9, 2010

### Andrew Mason

There is another way to look at this in which heat "sticks" but the kinetic energy doesn't.

The heating of the ball will increase its temperature. That temperature will be the same as measured by observers in other reference frames (ie. moving with respect to the ball). So, in that sense, the thermal energy sticks to the heated ball.

The kinetic energy of the moving ball will not be the same for other moving observers. For some, the kinetic energy will decrease (ie. the impulse slows it down) and for others it will increase. So kinetic energy of the "moving" ball does not "stick" to it. It depends on who is doing the measuring.

Measurement of kinetic energy is relative to the frame of reference of the observer. Measurement of temperature (molecular kinetic energy) is always related to the frame of reference of the object whose temperature is being measured.

The reason the temperature of the ball drops is: radiation. The molecules of the ball are continually accelerating and decelerating randomly. This results in emission of electromagnetic radiation. The ball will radiate energy at a rate that depends on its temperature. The ball's temperature will drop until the rate of radiation emitted is equal to the rate of radiation absorbed from other sources.

The "moving" ball does not radiate because its motion is uniform.

AM

3. Jun 10, 2010

### AlexB2010

Hi Andrew,
I don’t think the changing of reference can be used here, because since Galileo, to perform a physics experiment you need to determine a reference frame were the measurements are made. On the same frame of reference the accelerated ball will maintain their velocity and the energy given to then will increase the “energy level” of the ball.
On heat, I know that they will be loose in space equilibrating in their environment, irradiating in space, but since you point out relativity: Heat is considered kinetic energy of individual molecules, why they will be the same on all frames of reference? Since they are movement like any other.
My question is, why the difference behavior of a energy that came from the same source? Why kinetic energy not irradiate in space? Why the molecular motion is different from acceleration on the whole ball?
Thanks,
Alex

4. Jun 10, 2010

### Andy Resnick

Well, you need to be a little careful here- the moving ball will continue to move as long as it doesn't interact with anything else, and the heated ball will radiate only if it is allowed to interact with a cold reservior.

But using common sense, you are basically correct; heat and kinetic energy are two different kinds of energy, and can be interconverted to some degree. Heat can be converted into work, and work can be converted into heat.... not with 100% efficiency, but that's entropy for you....

5. Jun 10, 2010

### AlexB2010

Hi Andy, thanks again.
Somehow kinetic energy becomes part of a moving body and heat goes away. Someone know how? What happens in a small particle defined by their momentum if they receive kinetic energy.
Best,
Alex

6. Jun 10, 2010

### Andrew Mason

This requires a bit of clarification. I would not consider space to be a cold reservoir. But the heated ball will radiate heat into space at a rate that is only dependent on its temperature. The ball will lose thermal energy unless and until the rate of absorption of energy from other sources equals or exceeds the rate at which it loses thermal energy. But it always radiates.

One might want to phrase this a little differently. Heat and mechanical energy derive from kinetic energy of matter. Heat is the kinetic energy of random molecular (non-uniform) motion. Mechanical energy is the kinetic energy of uniform, non-random motion of macroscopic pieces of matter.

AM

7. Jun 10, 2010

### Andy Resnick

The heated ball will not lose heat energy if it is at the same temperature as it's surroundings.

Heat does not have a mechanical origin.

8. Jun 11, 2010

### Andrew Mason

Can the surroundings not be empty space? The temperature of a vacuum is undefined, since there are no molecules in a vacuum. But a heated ball in space can still lose heat energy.
I would have to disagree. Heat can originate from mechanical motion. The flow of heat is essentially a mechanical flow - molecules transferring kinetic energy to other molecules through physical collisions.

AM

9. Jun 11, 2010

### Andy Resnick

This discussion has been had numerous times on other threads; I have no desire to cover that ground again unless you have something original to say. I'll simply mention the following:

1) You would agree (hopefully) that electromagnetic energy does not have a mechanical origin; why insist that thermal energy have a mechanical origin?

2) Your statement only has meaning in the context of equilibrium/partition functions/statistical considerations. Dissipative process (e.g. friction, viscous losses) are exempt from these considerations. There is no (AFAIK) mechanical microscopic model for friction. Do you know of one?

As to your first comment, blackbody radiation has a well-defined temperature and requires no atoms. Put the ball in a cavity held at the same temperature, and it cannot lose energy to the surroundings.

10. Jun 11, 2010

### AlexB2010

Coming to the basic question again:
Thermal energy given or taken to a body will equilibrate on their surroundings. Kinetic energy that accelerates the body will stay glued to de body.
Why this happens? Why differences, since both energy are movement?
Answers exist? or is this an unsolved question to physics?
Best,
Alex

11. Jun 11, 2010

### Andy Resnick

AlexB2010,

As I mentioned earleir, kinetic energy does not 'stay glued' to the body, if the body can interact with another body (e.g. friction). Heat will not 'stay glued' to the body either, if it can interact with other bodies (e.g. thermal equilibration).

12. Jun 11, 2010

### AlexB2010

Hi Andy,
I do a little thinking based in your explanation; correct me if I am wrong.
In my think experiment, in releasing the chemical bonds I liberate thermal energy that are used to heat a ball or run some kind of machinery that will convert thermal energy to motion that will be applied to the ball given acceleration and increasing the speed.
The ball traveling in open space in uniform motion will not interact with other solids and then will go forever.
The heated ball have the internal molecular motion increased, the molecules will be given kinetic energy. The internal collisions will transform the kinetic energy in thermal radiation that is capable of moving in free space, making the ball loose energy (In colder space).
Since the accelerated ball don’t have increased molecular motion, the kinetic energy given to then will be all in one direction, molecular interaction don’t occur and the kinetic energy will stay in the ball as long they are in uniform motion.
Thermal energy will be defined as wave of amplitude and frequency defined; can I define kinetic energy the same manner?
What happens if I give kinetic energy to a photon?
Best,
Alex

Last edited: Jun 11, 2010
13. Jun 11, 2010

### Andrew Mason

I am not insisting that thermal energy must have a mechanical origin. I am just disagreeing with your general statement that thermal energy (heat) does not have a mechanical origin.

I agree that friction is necessarily a macroscopic phenomenon. All collisions at the molecular level are elastic.

The black-body consists of matter in some form (plasma, atoms) and has a temperature. But the radiation does not really have a temperature. We refer to radiation temperature but what we are really referring to is the temperature of the blackbody matter that produces the observed electro-magnetic spectrum.

AM

14. Jun 11, 2010

### Andrew Mason

Kinetic energy is defined only for bodies of matter. You can't do work on a photon. You can't change its speed so you can't give it kinetic energy. It does have momentum, though.

AM

15. Jun 12, 2010

### AlexB2010

The ability of receive kinetic energy, is it exclusive to fermions ?

16. Jun 12, 2010

### Andy Resnick

Well.. this *is* an original statement. It's also at variance with Planck's law. No, blackbody radiation is the spectral distribution of a free field (with infinite degrees of freedom) at thermal equilibrium with a reservior. It is material independent, especially since there's no such thing as 'black body matter'.

17. Jun 12, 2010

### Andrew Mason

You are misinterpreting what I said. My point was that only matter can really have a temperature, at least in the classical sense of temperature. Radiation temperature is assigned to a spectrum of radiation that is emitted by a blackbody. The radiation temperature is the surface temperature of the blackbody that would produce that radiation distribution. That is all I am saying. If I am wrong on that, please correct me.
My reference to "the blackbody matter" is to the matter that makes up the blackbody. A blackbody has to be made up of matter. It doesn't matter what the matter is. Radiation can only be emitted from matter. If I am wrong on that, please correct me.

We seem to be getting a little far from the original question here.

AM

Last edited: Jun 12, 2010
18. Jun 12, 2010

### Andy Resnick

I'm just following where you are leading.

Edit: Let me try a different track, and describe blackbody radiation.

It is true that blackbody radiation can be defined as "the electromagnetic radiation emitted by a blackbody object at temperature 'T'". But since a blackbody is defined as a material for which the emissivity 'e'= 1 for all wavelengths, and since no real matter has that property, it's not a particular useful way to define blackbody radiation.

A better way to construct a blackbody is to instead realize that e = 1 also means the absorptivity 'a' = 1. That's Kirchoff's law. We *can* construct an object that absorbs all wavelengths equally (and perfectly): a cavity containing a tiny hole. In fact, making the hole not tiny doesn't really affect things too much:

http://physics.nist.gov/Divisions/Div844/facilities/pyro/pyro.html [Broken]
http://www.hgh.fr/corps-noir-infrarouge-infrared-source-blackbody-en.php
http://www.npl.co.uk/engineering-me...nd-services/npl-fixed-point-blackbody-sources

The important thing to realize is that, by making the blackbody a *cavity* instead of an object, we have explicity demonstrated that the radiation field *itself* has a temperature, independent of the material that comprises the walls of the cavity.

Blackbody radiation has nothing to do with the matter that 'created' it.

Matter is not needed for thermal energy to exist. Thermal energy is not mechanical in origin. We can, under some conditions, *model* thermal energy in terms of molecular motion.

Last edited by a moderator: May 4, 2017
19. Jun 13, 2010

### AlexB2010

Thermal energy can interact with matter generating kinetic molecular motion. How these interactions happen?
Best,
Alex

20. Jun 13, 2010

### Count Iblis

Perhaps it can help to think of this as follows. Picture the ball as consisting of atoms. Indeed, everything consists of elementary particles. These elementary particles interact with each other according to certain laws of physics. In principle this allows you to describe how the ball will move, cool down etc. etc.

But this is not a very practical way of doing computations. You don't want to account for what every atom in the ball is doing when you want to compute how fast it is moving. So, wat you do is you split the motion of the ball in two parts. One part is the center of mass motion of the ball, the other part is the motion of the atoms relative to the center of mass motion. And then you describe the latter in a statistical way.

Now, we can describe the motion of the ball using conservation of energy and momentum. But because momentum is linear in the velocity, the total momentum of the system equals the momentum of the center of mass of the ball. In case of energy this is not the case. First the atoms can have potential energy, but even the sum of the kinetic energies of the atoms is not the kinetic energy of the center of mass.

This means that the statistical description you get when you average out what the atoms are doing will have to take into account the energy of the atoms. Changes in this part of the energy is what we call heat.