
1)
2)
3)
4)
The behavior of a gas should be described by the molecules.
The gas consists of a large number of identical particles of mass m .
The particles have negligible size and no internal structure.
The particles are moving in random dirctions with speeds independent of direction
Collisions between particles and with the walls are perfectly elastic.

L
F x m v


Follow a collision with a wall perpendicular to x .
Particle has mass m and a velocity v x
.
• Strike the wall:
D t = 2 L / v x
.
• Impulse:
D p = 2 mv x
.
 The force from one particle:
F

D p
D t

2
2
L mv
/ v x x
 mv
L
2 x

L
A
 The pressure on the wall comes from all the particles.
P

F x
A


A
F xi  m

AL v x
2
 The volume is V = AL .

P

Find the value for N particles.
m
 v x
2
 mN
 v x
2
AL V N
 mN v x
2
V

V
P
N
 The pressure come from all three dimensions, and is equal in all three.
v
2  v x
2  v
2 y
 v z
2 
3 v x
2
 Relate the pressure to the average speed.
P
 mN v
2
V 3

2 N
3 V
( 1
2 m v
2
)

V
P
N
 The expression from a particle level relates the average kinetic energy.
PV

N
2
3
( 1
2 m v
2
)
 This almost matches the ideal gas law.
PV

N ( kT )

 For the particle-level theory to match the experimental law we equate them.
PV

N
2
3
( 1
2 m v
2
)

N ( kT )
1
2 m v
2 
3
2 kT
 This is kinetic theory .
 Temperature measures the average kinetic energy.

 What is the average energy of an air molecule at room temperature (293 K)?
 Energy directly relates to temperature.
 (3/2) kT = 6.07 x 10 -21 J .
 What is average speed for a nitrogen molecule (28 g/mol)?
 To get the speed requires the mass.
 m = (0.028 kg/mol) / (6.022 x 10 23 /mol) = 4.65x10
-26 kg v

3 kT

511 m/s m

 Kinetic theory used the average speed.
 Actual atoms fall into a range of speeds.
 The Maxwell-Boltzmann distribution describes the probability of a molecule having a particular speed.

 The assumptions for kinetic theory are approximately true.
• Non-zero molecular size
• Non-zero force between molucules
 For the Van der Waals force in air, the effect is about
1% difference from an ideal gas.
next