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### Course: Thermal physics (Essentials) - Class 11th>Unit 3

Lesson 2: Why is there so little hydrogen in the Earth's atmosphere?

# Kinetic molecular theory of gases

Explore the kinetic-molecular theory of gases and its implications for macroscopic properties like pressure, volume, and temperature. Uncover the relationship between microscopic properties and macroscopic properties, and delve into the concept of average kinetic energy in a gas. Created by David SantoPietro.

## Want to join the conversation?

• how delta t=2l/v?
delta t is the time taken during collison or time taken during impact, whereas 2l/v is time taken to cover 2l distance.
• This question has been stuck in my mind for so long, and I stressed out for that. (because the textbook just simply saying t=2l/v(I'll write v to substitute vx))

Here's my opinion...

During a long period of time,(Since n is large compared with the time it takes to go back and forth) we assume that the particle has gone through n whole number of collisions and has travelled a distance of 2nl in the direction of x.

F=the total change in momentum by this side of the wall/the change in time=-2nmv/(2nl/v)=-mv^2/l

We got the same result but I feel that this looks much better than the original explanation.
• how does an ideal gas not have potential energy but have kinetic energy?
• As far I know. their is no attraction between molecules of ideal gases, means no (inter-molecular) bond. And as we know potential energy is stored energy between bonds, that's why ideal gases have no potential energy.
• I have a question....this question has already been asked in the comment section but not clearly answered in any of the comments....

Why are you using the time which takes for the particle to travel back and forth instead of the time that takes for the actual collision? I mean, the change in momentum is not happening gradually over the time 2l/v, it is happening almost instantly at the time of collision. Can someone please explain it to me?
• So with this theory it is possible to calculate the internal energy of a given gas system at a specific Temp, Pressure, or volume, but how would you calculate a change in energy if one of those values change? Thanks
• DU (change in internal energy or kinetic energy)= Q (heat)+/- W (work done on the system/work dome by the system).........
• at
3/2PV=thermal energy
Why only for mono-atomic gases? does khan mean ideal gases?
• I think David says only monoatomic, because if we had diatomic or polyatomic - we would also have to consider rotation of the molecules.. In monoatomic the molecules don't have anything to rotate about, but in diatomic- the molecules can rotate about another. So, that would change time between collisions(because translatory- motion in a straight line- velocity is lesser, as the energy is also distributed into rotatory motion..), so the overall relation must change.
• David takes the time for the change in momentum of the particle as the time in between collisions.. but generally if you want to know the time for change in momentum or velocity, you would take the time in which the velocity or momentum actually changes , but here the change must be instantaneous (since it' a collision), so why do we take time like this?
• This video is dizzying, and I don't see the connection to MCAT content.