# I still don't understand heat and energy

bobsmith76
Ok, I still don't understand heat. As I see it, heat is the measure of how fast atoms are moving. And energy is also a measure of how fast atoms are moving, but I don't see the difference. Is that right? When an electron moves from an excited state to a ground state I don't see why that should give off heat?

Second, I don't understand how you cause atoms to move faster, if that is the definition of heat. When a hot system moves into contact with a cold system, why does the hot system make the cold system's atoms move faster, the cold system's atoms make the hot system's atoms move slower? Maybe it's because the fast atoms when they move toward the slow atoms their attractive force causes them to move faster and likewise the slow atoms also slow down the fast atoms due to their attractive force but I'm not sure.

Third, when Na and Cl bond, that is supposed to cause heat. I don't see why. I can sort of understand the reverse, that you need to produce heat in order to separate them, because they are tightly bonded, so you need to cause them to move faster so that their probability of finding the "way out" of the bond increases because you increase the probabilistic resources. Maybe the answer, but I'm still not sure, is that when the electrons of the Na come near the Cl, that attractive force forces them to move faster and because they're moving faster, that causes heat, but I still don't know if that's right.

facenian
Ok, I still don't understand heat. As I see it, heat is the measure of how fast atoms are moving. And energy is also a measure of how fast atoms are moving, but I don't see the difference. Is that right?
Temperature is a measure of how fast atoms are moving(kinectic energy only), however heat is measure of energy. Of course there is a relation between temperature and heat but they are not the same thing, for instance, with the same amount of heat you can raise twice as much the temperature of one body than another.

Dr Lots-o'watts
Ok, I still don't understand heat. As I see it, heat is the measure of how fast atoms are moving. And energy is also a measure of how fast atoms are moving, but I don't see the difference. Is that right? When an electron moves from an excited state to a ground state I don't see why that should give off heat?

Second, I don't understand how you cause atoms to move faster, if that is the definition of heat. When a hot system moves into contact with a cold system, why does the hot system make the cold system's atoms move faster, the cold system's atoms make the hot system's atoms move slower? Maybe it's because the fast atoms when they move toward the slow atoms their attractive force causes them to move faster and likewise the slow atoms also slow down the fast atoms due to their attractive force but I'm not sure.

Third, when Na and Cl bond, that is supposed to cause heat. I don't see why. I can sort of understand the reverse, that you need to produce heat in order to separate them, because they are tightly bonded, so you need to cause them to move faster so that their probability of finding the "way out" of the bond increases because you increase the probabilistic resources. Maybe the answer, but I'm still not sure, is that when the electrons of the Na come near the Cl, that attractive force forces them to move faster and because they're moving faster, that causes heat, but I still don't know if that's right.

Here's an analogy:

Electricity (conduction of) refers to the flow of charges from a higher voltage electrode to a lower voltage electrode. It is like a domino effect between electrons. In space, electricity is a spark or a cathode ray. If the charged mass containing the electrons is moving, we have a moving, charged mass.

Heat (conduction of) refers to the flow of (atomic or molecular) vibrations from a high temperature region to a low temperature region. It is like a domino effect between vibrating molecules (or atoms). In space, it is radiation. If the hot mass containing the heat is moving, we have convection.

Homework Helper
Ok, I still don't understand heat. As I see it, heat is the measure of how fast atoms are moving. And energy is also a measure of how fast atoms are moving, but I don't see the difference. Is that right? When an electron moves from an excited state to a ground state I don't see why that should give off heat?
Just to add to what the others have said:

We sometimes use the term "heat" to mean something that a body contains ie: a feeling of warmth. This is not how the term is used in thermodynamics.

The term "heat flow" is used. Heat flow is energy that is transferred between bodies that are in thermal contact with each other due to temperature difference. Heat flow is $\Delta Q$, the sign being positive when the heat flow is into the body and negative when the flow is out.

Heat flow can cause temperature change (change in internal energy $\Delta U$). The relationship between heat flow and temperature change is derived from the first law of thermodynamics: $\Delta Q = \Delta U(T) + W$

When an atom emits a photon, it does not create heat flow. Thermodynamics is not really applicable to single atoms or single photons since temperature is not defined for single atoms or photons. Thermodynamics deals with large macroscopic bodies for which temperature is defined.

When a macroscopic quantity of atoms in thermodynamic equilibrium at temperature T emit radiation according to a Maxwell-Boltzmann (temperature) distribution, this energy emitted would be considered heat flow.

Second, I don't understand how you cause atoms to move faster, if that is the definition of heat. When a hot system moves into contact with a cold system, why does the hot system make the cold system's atoms move faster, the cold system's atoms make the hot system's atoms move slower? Maybe it's because the fast atoms when they move toward the slow atoms their attractive force causes them to move faster and likewise the slow atoms also slow down the fast atoms due to their attractive force but I'm not sure.
The second law of thermodynamics deals with this. Heat flow can not occur spontaneously from a colder to a hotter body. It is much more probable that a faster moving atom will transfer energy to slower moving ones rather than the other way around. So with large numbers of atoms, the probability that there will be a net total transfer of energy from the slower to the faster ones becomes effectively 0. The spontaneous heat flow will always be toward thermal equilibrium rather than away from it.

Third, when Na and Cl bond, that is supposed to cause heat. I don't see why. I can sort of understand the reverse, that you need to produce heat in order to separate them, because they are tightly bonded, so you need to cause them to move faster so that their probability of finding the "way out" of the bond increases because you increase the probabilistic resources. Maybe the answer, but I'm still not sure, is that when the electrons of the Na come near the Cl, that attractive force forces them to move faster and because they're moving faster, that causes heat, but I still don't know if that's right.
When Na+ and Cl- ions bond, they enter a lower energy state: ie it requires energy to break that ionic bond and return them to their original state. The forming of a single bond does not produce heat flow. It causes the two bonded ions to come together forcefully and that causes the two bonded ions to increase their kinetic energy. The forming of many such bonds in a macroscopic quantity of matter, when that matter reaches stable thermal equilibrium, results in an increase in temperature of that matter. That can then cause heat flow to another colder body that is in thermal contact with it.

AM

bobsmith76
Ok, I think I'm understanding more but not entirely. I still don't know what causes a system to increase in energy. I know you have to do work to the system but that doesn't help me because i don't understand how the atoms physically gain energy. But here's my try: I imagine a closed room in outerspace with tennis balls flying around in it and there velocity would remain constant due to lack of gravity. If I shot maybe 500 very fast tennis balls into that closed system then the tennis balls that were already there in the closed system would start to move faster but, here's the critical part, only because the new fast tennis balls would bounce into them and speed them up. Likewise the slow tennis balls I suppose would also somewhat slow the fast tennis balls so that a new average constant speed would be reached.

Is this a good analogy for energy?

I still don't see the difference between heat and energy unless heat is just a form of energy.

Dr Lots-o'watts
Is this a good analogy for energy?...heat is just a form of energy.

1. It is a good analogy for the spreading of heat to reach temperature equilibrium.
2. Some people see heat as energy, some see it as a flow of energy.

By the way, heat is function of the random motion of atoms. If you were to pick up a ball and throw it at a very high speed, you would have increases its energy (relative to yourself- energy is alway relative) by its increased kinetic energy, $(1/2)mv^2$ but not its heat since every atom in the ball would have the same velocity vector added to its velocity, not changing the velocity of individual atoms relative to the others in the ball.