# Is a single atom hot? Will it emit radiation?

• SamRoss
In summary, heat is just kinetic energy. It is the energy that is in microscopic internal degrees of freedom. Thermal energy is energy that is stored in molecules, and it is related to the temperature of a substance. Heat is the result of collisions between molecules, and it radiates away to lose energy.
SamRoss
Gold Member
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?

SamRoss said:
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.

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.

SamRoss said:
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
A.T. said:
An atom is neutral and on its own it would move inertially. So no radiation.
Heikki Tuuri said:
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?

SamRoss said:
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.

Heikki Tuuri said:
Yes.
Thank you.

Heikki Tuuri

## 1. Is a single atom considered hot?

No, a single atom is not considered hot. Temperature is a measure of the average kinetic energy of a group of atoms, so it does not apply to a single atom.

## 2. Can a single atom emit radiation?

Yes, a single atom can emit radiation. This is known as spontaneous emission, where an unstable atom releases energy in the form of electromagnetic radiation.

## 3. What determines if a single atom will emit radiation?

The likelihood of a single atom emitting radiation depends on its energy state. If the atom is in an excited state, it is more likely to emit radiation.

## 4. Is the radiation emitted by a single atom harmful?

The radiation emitted by a single atom is generally not harmful. Atoms emit low levels of radiation that are not strong enough to cause harm to living organisms.

## 5. How does the temperature of a group of atoms affect the radiation emitted by a single atom?

The temperature of a group of atoms does not directly affect the radiation emitted by a single atom. However, a higher temperature can increase the likelihood of a group of atoms having more excited atoms, which could lead to more radiation being emitted.

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