Undergrad Why the temperature of a car does not increase with velocity?

[Mohammad Sakib]

We know temperature is a measure of average kinetic energy of molecules/particles of a system. Now if a car starts to move, its velocity increases so does its kinetic energy. Therefore all the molecules are gaining velocity too. Shouldn't this increase the temperatre as average kinetic energy of molecules seems to be increasing?

 

[Vanadium 50]

We know temperature is a measure of average kinetic energy of molecules/particles of a system.

Not at all. Where did you get that?

 

[Mohammad Sakib]

I have noticed this in some forums. Many of them also told that they are proportional

Not at all. Where did you get that?

 

[russ_watters]

Even if it were true (it is a little true for gases), it matters that the motion of the gas molecules is random vs the car's motion being in one direction.

 

[Mohammad Sakib]

It would be a great help if anyone of you could explain me why in cases where average kinetic energy of particles is related to temperature needs to be relative to the centre of mass of a body? I know that on the basis of reference frame temperature of a body would differ a lot. But a body might have a very high velocity with respect to something actually static, however with respect to centre of mass the particles won't seem to move that fast. Why doesn't this factor affect temperatre?

 

[Dale]

We know temperature is a measure of average kinetic energy of molecules/particles of a system.

This is an unfortunately common misunderstanding. Temperature (actually 1/T) is the rate of change of entropy with respect to a change in energy. At low temperature a small increase in energy gives a large increase in entropy, and vice versa at high temperature. Energy is involved, but it is about the relationship between energy and entropy.

Now, for an ideal monoatomic gas it is true that the average KE is directly proportional to the temperature. That is a specific feature of ideal monoatomic gasses, not a general rule. It doesn’t hold for many real gasses at high temperatures or pressures, and it really doesn’t hold for solids at all. It is very unfortunate that a specific rule for ideal gasses has been so over generalized that people think it applies where it doesn’t.

Summary:: Why does temperature of a car does not depend on its state of motion?

Now if a car starts to move, its velocity increases so does its kinetic energy. Therefore all the molecules are gaining velocity too. Shouldn't this increase the temperatre as average kinetic energy of molecules seems to be increasing?

The rate of change of entropy with respect to energy has not decreased, so the temperature has not increased.

But a body might have a very high velocity with respect to something actually static, however with respect to centre of mass the particles won't seem to move that fast. Why doesn't this factor affect temperatre?

Because it doesn’t affect the rate of change of entropy with respect to energy.

 

[Drakkith]

It would be a great help if anyone of you could explain me why in cases where average kinetic energy of particles is related to temperature needs to be relative to the centre of mass of a body?

Imagine you put your hand up against something hot. The atoms and molecules of your hand and the object are now touching, but the hot object's atoms and molecules are moving much more vigorously than your hand's. This difference causes energy to be transferred from the hot object to your hand through all of the collisions and various interactions taking place.

The key here is that the movement of the atoms and molecules is disordered. If the movement was ordered, well, you've simply accelerated the object as a whole and instead of transferring heat, it slams into your hand at some velocity.

 

[anorlunda]

Think of a bottle of gas. The bottle moves at high speed. The molecules in the bottle don't collide with each other or collide with the bottle walls any faster.

Besides, relative to the other bottles in the box, it's not moving at all. Remember, all motion is relative.

But heat the gas inside the bottle and both kinds of collisions happen more often.

 

[Mohammad Sakib]

So basically kinetic energy, thermal energy,internal energy all these are relative but entropy isn't relative and temperature is defined based on this entropy. Isn't it?

 

[russ_watters]

Imagine you put your hand up against something hot. The atoms and molecules of your hand and the object are now touching, but the hot object's atoms and molecules are moving much more vigorously than your hand's. This difference causes energy to be transferred from the hot object to your hand through all of the collisions and various interactions taking place.

The key here is that the movement of the atoms and molecules is disordered. If the movement was ordered, well, you've simply accelerated the object as a whole and instead of transferring heat, it slams into your hand at some velocity.

I'm not a fan of that last bit, because if you slam a car into something it will dissipate a lot of that energy as heat. IMO it is better to simply point out that if you are a passenger in the car the kinetic energy with respect to you is zero, so ordered kinetic energy can't be related to temperature because it is frame dependent (even for situations where kinetic energy works for describing temperature).

 

[Dale]

So basically kinetic energy, thermal energy,internal energy all these are relative but entropy isn't relative and temperature is defined based on this entropy. Isn't it?

Speaking of Galilean relativity rather than special relativity then of those only kinetic energy is relative. All the others are not.