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Velocity changes with temperature?

by MathewsMD
Tags: temperature, velocity
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MathewsMD
#1
Dec6-13, 12:10 AM
P: 295
I am just discussing gases and how substances change when they undergo temperature changes. We know that temperature is a measure of the average speed of a substance. Okay, but when we look at velocity, it has both direction and a magnitude. So, when we heat a substance (ex. water), I understand that its molecules' average speed increases, but does its net velocity? Since all the molecules are pointing in random directions, wouldn't there be a net cancelling effect on the velocities of the individual molecules in this sample since it is a vector quantity we're assessing? For a cup of quiescent water, there is no net velocity, right? So if we were to either cool or heat this sample, it still wouldn't have a net velocity change, right?

Any clarification on the above matters would be great since I'm having trouble finding discussions regarding velocity changes instead of just speed changes. Thank you!
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Simon Bridge
#2
Dec6-13, 04:14 AM
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I understand that its molecules' average speed increases,
technically true - but the temperature is a manifestation of the average kinetic energy of the molecules, and it is the average of all the molecules, not the average kinetic energy of each individual molecule.
http://hyperphysics.phy-astr.gsu.edu...ic/kintem.html

... but does [the molecule]'s net velocity?
The instantaneous speed of the molecule has changed - yes.
As a result of a collision with another molecule, it will have a different direction and speed.

Since all the molecules are pointing in random directions, wouldn't there be a net cancelling effect on the velocities of the individual molecules in this sample since it is a vector quantity we're assessing?
Yes - this is why a container of gas sits still instead of gently shuffling across the floor.
The result of all the cancelling out is a net outwards pressure at the walls of the container - outwards in all directions.

For a cup of quiescent water, there is no net velocity, right? So if we were to either cool or heat this sample, it still wouldn't have a net velocity change, right?
That is correct.

That's why the talk is usually about speed rather than direction changes ... the directions tend to cancel out, and it's the energy we care about anyway.


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