1 12/10/10 - umd
TRANSCRIPT
12/8/10 11 Physics 121
Average force of gas on the wall
= (# of molecules hitting the wall in the time t)
x (force each molecule exerts on the wall)
Fwall molecule = mvx
t= Fmolecule wall
Only the x-component matters.
All we need to figure this out is our three
basic equations, and a way to count
the number of molecules hitting the wall.
vx =x
t
A
vx t
12/8/10 12 Physics 121
N = n Volume( ) = nA vx t( )
px = 2m vx
n = number density
= # / unit volume
12/8/10 14 Physics 121
F = Np
t=
12 (nAvx t)
2mvx
t= nmvx
2A
Interpret
F = pA n =N
Vvx
2= 1
3 v 2
pA = 13
N
Vmv 2A
pV = N( 13 mv 2) = N 2
312 mv 2( )
12/8/10 18 Physics 121
pV = N 23 ( 1
2 mv2 ) pV = nRT
Make the N parts look alike.
n = N / NA
pV = NR
NA
T
Define: kB =R
NA
so pV = NkBT
12/10/10 20
pV = nmolesRT
pV = NkBT
Chemist’s form
Physicist’s form
nmoles =N
NA
R = kBNA
p = nmvx2 3
2 kBT =12 mv 2