# Question about the derivation of the Ideal Gas Law

1. Nov 5, 2016

### jackferry

I'm an undergraduate taking a physical chemistry course, and I got to a part in my reading about the derivation of the ideal gas law. The passage is linked below.

http://chem.libretexts.org/Textbook...f_Gases/16.01:_Gases_Behave_Ideally_if_Dilute

My question has to do with the time interval selected. My understanding of calculating the force of a collision makes me think that the time interval delta t should be the time the particle is in contact with the wall, however they use it as the time it takes the particle to come back and collide with the wall again. Why do they use this? Is it some way to average out the force over time?

2. Nov 6, 2016

### Let'sthink

Good question They are not using it to calculate the change of momentum which is taken as 2mu. They use time to calculate the frequency of collisions to calculate the rate of change. Momentum is a quantity which can be given or taken where as the force is some thing which happens between molecules and walls. In each collision very large force acts between the wall and each molecule but that does not contribute to pressure it just helps in retaining the speed of molecule and doing almost nothing to the box rigidly attached to the earth. are not calculating the force acting on wall by each molecule but the average force exerted by all molecules averaged over a time which is much larger than the collision time which is ideally taken as zero.

Last edited: Nov 6, 2016