Why the temperature of a moving jar of gas doesn't increase

[RubinLicht]

Homework Statement

Why doesn't the temperature of a moving jar of gas increase?

Homework Equations

Average KE is proportional to temperature

The Attempt at a Solution

This is just an extension of a problem my friend sent me, and I'm stuck on why even though average velocity is increasing, the temperature does not increase.

 

[Doc Al]

I'm stuck on why even though average velocity is increasing, the temperature does not increase

Think about the reference frame that makes sense to use when talking about the random KE of the molecules of an object.

And realize that the macroscopic velocity of an object is always relative to something. Relative to your chair, you are at rest. Relative to a moving train, you might be moving at 70 mph. Relative to the sun... and so on.

 

[RubinLicht]

Think about the reference frame that makes sense to use when talking about the random KE of the molecules of an object.

And realize that the macroscopic velocity of an object is always relative to something. Relative to your chair, you are at rest. Relative to a moving train, you might be moving at 70 mph. Relative to the sun... and so on.

The frame in which the average velocity is zero, so the jar.

 

[Doc Al]

The frame in which the average velocity is zero?

Right. (Also called the center of mass frame.)

 

[RubinLicht]

Right. (Also called the center of mass frame.)

Ah thanks, makes much more sense than my friends explanation haha

 

[haruspex]

Think about the reference frame that makes sense to use when talking about the random KE of the molecules of an object.

And realize that the macroscopic velocity of an object is always relative to something. Relative to your chair, you are at rest. Relative to a moving train, you might be moving at 70 mph. Relative to the sun... and so on.

So shouldn't the question state, just to be clear, that the thermometer is moving with the jar?

 

[Doc Al]

So shouldn't the question state, just to be clear, that the thermometer is moving with the jar?

Good point.

 

[lychette]

So shouldn't the question state, just to be clear, that the thermometer is moving with the jar?

the question makes no reference to thermometers!

 

[haruspex]

the question makes no reference to thermometers!

Quite so, but how is the temperature to be measured/defined? My point is that it affects the answer.
If the gas is heteropolar, you would be able to measure its temperature externally by infrared radiation. This would show an elevated temperature if you stand in front of it but a lower one if measured from behind, because of the Doppler effect. Measuring all around and averaging, you would still see a slightly higher temperature because of the increased total energy (KE).

 

[lychette]

Quite so, but how is the temperature to be measured/defined? My point is that it affects the answer.
If the gas is heteropolar, you would be able to measure its temperature externally by infrared radiation. This would show an elevated temperature if you stand in front of it but a lower one if measured from behind, because of the Doppler effect. Measuring all around and averaging, you would still see a slightly higher temperature because of the increased total energy (KE).

I am viewing temperature (of a gas) as the average translational KE of molecules which has nothing to do with how temperature is measured. It is a mathematical concept from kinetic theory. What thermometers 'measure' is a different matter.

 

[haruspex]

I am viewing temperature (of a gas) as the average translational KE of molecules which has nothing to do with how temperature is measured. It is a mathematical concept from kinetic theory. What thermometers 'measure' is a different matter.

The usual view is that temperature is a measure of the thermal energy. Bulk motion does not contribute to that.
The question setter clearly agrees with that. Your view makes the question wrong, and makes it hard for observers to agree on temperature.

 

[lychette]

The usual view is that temperature is a measure of the thermal energy. Bulk motion does not contribute to that.
The question setter clearly agrees with that. Your view makes the question wrong, and makes it hard for observers to agree on temperature.

thankyou for pointing out how wrong I have been.