Does Thermal Energy Have Momentum?

In summary: If you heat up a gas (like air), the molecules start to move around more and their energies (J) start to increase. But at some point the molecules reach a temperature where their energy is equal to their mass (K). And from that point on, the molecules keep oscillating around that equilibrium point.Thermal energy is momentum right?Thermal energy is momentum right? Is this a correct interpretation:If I slide an object across a frictional surface, the momentum will still be conserved, just not visibly (because it's now atomic momentum, called "thermal energy").The increased agitation of heated molecules being randomly orientated, it would seem that the nett
  • #1
Nathanael
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Thermal energy is momentum right? Is this a correct interpretation:
If I slide an object across a frictional surface, the momentum will still be conserved, just not visibly (because it's now atomic momentum, called "thermal energy").

If yes, then, in all of these conservation of momentum problems I'm doing, why is it safe to assume that some of the momentum is not converted into thermal energy? If two objects collide, surely you would expect an increase in thermal energy?

Is it just that even large amounts of thermal energy have negligable momentum? Or is there another reason? Is thermal energy even momentum?
 
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  • #2
No, momentum and energy are completely different things. Any kind of energy has units of "Joules= kg meters squared per second squared" and momentum has units of "kg meters per second".
 
  • #3
HallsofIvy said:
No, momentum and energy are completely different things. Any kind of energy has units of "Joules= kg meters squared per second squared" and momentum has units of "kg meters per second".

Yes, sorry I was careless with my terminology.

But my question remains, does thermal energy "contain" momentum (in the sense that kinetic energy "contains" momentum)

Is momentum implied in thermal energy?
 
  • #4
No. Energy and momentum are two different things. They are related in special relativity, in the way that time and space are related, but they aren't the same.

All the random microscopic momenta of the particles that make up a hot object cancel out. Maybe there is some total momentum of the center of mass, but we don't call this thermal energy.
 
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  • #5
Khashishi said:
All the random microscopic momenta of the particles that make up a hot object cancel out.

Ah, I see. Good point, I didn't think about it that it would all cancel out.


But then what happens to the momentum of an object sliding across a frictional surface? If momentum is always conserved, then it must go somewhere?

Is it that the momentum is transferred to the momentum of the Earth?


That makes sense actually, because if you had a box in space and had something sliding across the ground of it (with friction) then you probably would expect the box to gain momentum as the object loses momentum.



Thank you, this was a good point:
Khashishi said:
All the random microscopic momenta of the particles that make up a hot object cancel out.
 
  • #6
Nathanael said:
Is it that the momentum is transferred to the momentum of the Earth?

Yes, that's it. The mass of the Earth is so much larger than the mass of the object that it's impossible to observe the resulting change in the Earth's velocity.

Unless you have a really really really massive object, of course...
 
  • #7
Nathanael said:
Thermal energy is momentum right? Is this a correct interpretation:
If I slide an object across a frictional surface, the momentum will still be conserved, just not visibly (because it's now atomic momentum, called "thermal energy").
The increased agitation of heated molecules being randomly orientated, it would seem that the nett momentum of a bunch of them remains steadfastly zero---with vibrations going in all directions the particles' momenta would be cancelling with each other, overall, and this regardless of the temperature. That would be my take on the momentum idea.

Now, the molecules' energy on the other hand ...
 

1. Is thermal energy the same as momentum?

No, thermal energy and momentum are two different concepts. Thermal energy refers to the total energy of particles in a system, while momentum is a measure of the motion of an object.

2. How is thermal energy related to momentum?

Thermal energy can indirectly affect momentum through collisions between particles. When particles in a system collide, they can transfer thermal energy to each other, which can change their momentum.

3. Can thermal energy be converted into momentum?

Yes, thermal energy can be converted into momentum through collisions between particles. When particles collide, they can transfer thermal energy to each other, which can change their momentum.

4. Does thermal energy have direction like momentum?

No, thermal energy does not have direction like momentum. Thermal energy is a scalar quantity, meaning it only has magnitude, while momentum is a vector quantity, meaning it has both magnitude and direction.

5. How does temperature affect thermal energy and momentum?

As temperature increases, the thermal energy of a system also increases. This can lead to more collisions between particles, causing an increase in momentum transfer. However, the relationship between temperature and momentum is complex and depends on various factors such as the type of particles and their interactions.

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