Is a single atom hot? Will it emit radiation?

[SamRoss]

Trying to understand the concept of heat. As I understand it, heat is really just kinetic energy. In Newtonian mechanics, it is 1/2mv^2. Here are my questions...

(1) On a microscopic level, are conduction and convection simply atoms bumping into each other and passing along some of their kinetic energies until equilibrium is reached?

(2) When I put my hand on a hot surface, are the changes that happen in my hand identical to those that would happen if I were to put my hand in the air and feel the radiation from the sun? In other words, will conduction and radiation have the same effect on biology? Certain radiation such as microwave would obviously be harmful so let's stick with simple infrared radiation.

(3) A hot body, meaning a body made up of atoms in motion, radiates heat away. This apparently results in a loss of kinetic energy. Over time, then, the atoms would slow down. What are the implications for a single atom? In particular...
(a) Would a single atom also radiate energy away and therefore slow down on its own? This does not seem likely to me.
(b) Can a single atom be considered hot due to its kinetic energy? If so, wouldn't that same atom be considered cool in a reference frame traveling with it? Even absolute zero? And if it has no kinetic energy in a reference frame moving with it, how could it radiate energy away?
(c) Some of the above problems would go away if radiation only occurs due to an atom's collision with something else. Is this the case?

 

[A.T.]

Would a single atom also radiate energy away and therefore slow down on its own?

An atom is neutral and on its own it would move inertially. So no radiation.

 

[Heikki Tuuri]

A single atom is like a single stone you throw. Its energy is translational kinetic energy. Conservation of energy and momentum guarantees that the stone does not give up its energy unless it collides with another one.

The concept of heat is a statistical concept which applies to a large number of stones flying into random directions. If you have a box full of stones and you shake it, then you can define a temperature for the stones in the box.

If you have a (still) box of stones and you throw the whole box to some direction, then the kinetic energy is again translational, and the concept of temperature does not apply.

 

[Dale]

Summary: Questions regarding heat, radiation, and a change of reference frame

As I understand it, heat is really just kinetic energy.

This is unfortunately a very common misunderstanding. Thermal energy is really just kinetic energy ONLY for an ideal monoatomic gas. Unfortunately, the ideal gas law is so well taught that most people mistakenly over-generalize and believe that thermal energy is kinetic energy for all substances, not just for the special case of ideal monoatomic gasses.

Thermal energy is any energy in microscopic internal degrees of freedom. Many molecules have degrees of freedom such as bending and stretching and also degrees of freedom like electronic excitation and ionization. Any microscopic internal degree of freedom can store thermal energy, not just those that represent KE.

Directly related to your question, the overall system KE is not an internal degree of freedom, so it does not contain thermal energy.

 

[SamRoss]

An atom is neutral and on its own it would move inertially. So no radiation.

Conservation of energy and momentum guarantees that the stone does not give up its energy unless it collides with another one.

So is it correct to say that a hot body will only radiate energy when the atoms inside collide and also that when they do so the total kinetic energy is actually less than the total kinetic energy before the collision due to the radiation?

 

[Heikki Tuuri]

So is it correct to say that a hot body will only radiate energy when the atoms inside collide and also that when they do so the total kinetic energy is actually less than the total kinetic energy before the collision due to the radiation?

Yes.

 

[SamRoss]

Yes.

Thank you.