# How the speed of particles is affected

1. Mar 30, 2014

### Cal124

Apologies if this isn't the right place but I saw a moderators post saying all questions should be posted in this section.
My physics knowledge is at a general A-level standard at the moment, I was curious about something and was just wondering if someone could help me find an answer.

My question: If a container of gas is traveling at a speed, would this affect the energy of the particles ?

I have a brief understanding of some Thermodynamics and I cant seem to find an answer everything I search it talks about the collisions with the surface of the container. I understand the relationship between volume, pressure and temperature with the three gas laws. I could be wrong but temperature is related to the energy of the particles? and the energy is related to the speed of the particles so if the particles (in a container) where traveling at a speed would this affect the speed/energy of the particles?
If I'm not completely wrong, does an equation exist?
I feel like this is a stupid question because I keep thinking about e=mc2 but that would be to work out the energy of the particles right? so I guess my question relates to: does the speed of the container affect the speed of the particles?

Sorry if my question isn't clear, first time on this forum. Thanks to anyone who takes the time to read.
Cal

Last edited: Mar 30, 2014
2. Mar 30, 2014

### haruspex

Yes, the particles would have more kinetic energy, but you need to distinguish between the kinetic energy of a coordinated motion, i.e. all the particles travelling together, and the random motion of the particles moving with respect to each other. The latter is the 'internal' energy, and its availability to do work is subject to the laws of thermodynamics. It is responsible for pressure on the container. The former exerts no pressure on the container and is entirely available to do work.
Admittedly the boundary can be fuzzy. Wind is a bulk movement on one scale, but on another there are eddies etc.

3. Mar 31, 2014

### Cal124

Thanks haruspex,
I have a little confusion over whether the increase in KE would affect the particle collisions?

4. Mar 31, 2014

### haruspex

No. Their relative velocities are not affected by their common bulk velocity.

5. Apr 1, 2014

### Cal124

Sorry last question, but even if their bulk velocity doesn't affect the relative. if the container is moving at a speed, when the particles collide with the surface of the container would this not affect the speed?
so would the KE gained not affect the energy needed for a successful collision? apologies if you did answer that,
thanks

6. Apr 1, 2014

### haruspex

Originally you asked about a container of gas travelling at speed. I took that to be in a steady state, i.e. the average speed of the particles equals the average speed of the container. If so, the relative speed of particle to container is the same as if the container were at rest.
Perhaps now you are considering the transition from container at rest to container moving. During that transition, there will be a difference between average particle velocity and container velocity, causing a net pressure on he trailing end of the container. But pretty soon that will bring the average velocity of the particles up to match that of the container.

7. Apr 1, 2014

### Cal124

That's great, thanks, its given me a better understanding. Sorry my question wasn't to clear, I guess I'm trying to understand the relationship between the speed of the container and the particles speed & energy.
Is there an equation to show the relationship? would the SUVAT equations be used?

8. Apr 1, 2014

### haruspex

SUVAT equations are most easily applied when there's a relatively small number of objects, each rigid. With a vast ensemble, such as in a gas body, you often need to take a more statistical approach. E.g., during acceleration of the container, how to determine the mass centre of the gas body?

9. Apr 4, 2014

### Cal124

So to see the relationship between the particles and the speed of the container, you could use the following?
3/2 RT (=KE of gas molecules) + 1/2 mv2 (container)
but this doesn't obey,
PV = nRT
or P = T
If there is not a change in pressure, this will = no change in temperature but how can there be an increase in KE but no change in temperature?
I'm not sure if external links are aloud on this site, but is there anyway you can point me in the right direction for information on Kinetic theory of gases with a moving container?

10. Apr 4, 2014

### haruspex

To an observer outside the moving container, there is an increase in temperature, but in everyday environments it is very small. What is the average speed of an air molecule at NTP? That's the sort of relative speed you need to see the change. Think of an object entering earth's atmosphere at great speed. To the object, the atmosphere seems extremely hot.