F.4 Physics Lectures
Gas Laws
Kinetic Theory
Boyle’s Law
The following apparatus is used to investigate
how the pressure of a fixed mass of air varies
with its volume when the temperature is kept
constant.
Procedure
•Pressure is increased by applying the
foot-pump
•Volume of air is read from the scale
•Pressure is measured by a Bourdon
gauge
•Several pairs of readings of pressure
and volume are taken
Result
Pressure Volume 1/volume
(kpa) (cm3) (cm-3)
100 51 0.020
120 43 0.023
140 38 0.026
160 33 0.030
180 29 0.035
200 25.5 0.039
220 23 0.043
Graph Plotting
pressure/kPa
220
200
180
160
140
120
1/Volume /cm-3
0.020 0.030 0.040 0.050
Conclusion
P 1/V PV = constant
P1V1 = P2V2
Boyle’s Law
Pressure of a fixed mass of gas at
constant temperature is inversely
proportional to its volume
Charle’s Law
Procedure
• An air column is trapped in a capillary tube.
• It is heated up in a water bath in stages.
• The thermometer reading is taken.
• The length of the air column is measured.
• The reading are taken only after they have
remained steady.
Result
Length of air Temperature/K
column/cm
6.0 273
6.3 289
7.0 308
7.4 324
7.7 338
8.1 352
8.9 373
Graph Plotting
length of air column/cm
10
8
6
4
2 temperature T/K
0
100 200 300 400
Conclusion
VT V/T = constant
V1 /T1 = V2/T2
Charle’s Law
Volume of a fixed mass of gas at
constant pressure is directly
proportional to its Kelvin temperature.
Pressure Law
Procedure:
• Air trapped in a flask is heated in a water bath.
• Use the thermometer and Bourdon gauge to
measure the temperature and the pressure of
the air.
• Take about several readings with different
temperature.
Result
Pressure P/kPa Temperature T/K
94 273
100 293
110 315
114 333
124 358
128 373
Graph Plotting
pressure p/kPa
10
8
6
4
2 temperature T/K
0
100 200 300 400
Conclusion
PT P/T = constant
P1 /T1 = P2/T2
Pressure Law
Pressure of a fixed mass of gas at
constant volume is directly proportional
to its Kelvin temperature.
General Gas Equation
Boyle’s Law: PV = constant
Charle’s Law: V/T = constant
Pressure Law: P/T = constant
PV/T = constant
Kinetic Theory
• All matter is made up of particles, called
molecules.
• They are constantly in motion.
• When they are close together, the molecules
attract each other strongly.
• When they are far apart, they hardly attract
each other.
Solids
• The particles are close
together.
• They are held together
by strong forces.
• They vibrate to and fro,
but cannot change
positions.
• They have a fixed
volume and shape.
Liquid
• The particles are close
together.
• They vibrate so
vigorously that the
forces can no longer
hold them in fixed
position.
• They have a fixed
volume but no fixed
shape.
Gas
• The particles are very
far apart and hardly
attract each other.
• They move at random
at very high speeds.
• They have no fixed
volume and shape.
Brownian Motion
• A smoke particle is bombarded by millions
of air molecules around it.
• The bombardment comes from all sides but
not in equal number.
• Brownian motion provides evidence for
particle motion.
• This can be explained by using the kinetic
theory.
Kinetic theory and Boyle’s Law
• When a gas is
compressed,
• the molecules have
less volume to move
in.
• They hit the walls
more often and so
produce a greater
pressure.
Kinetic theory and Charles’ Law
• As temperature rises,
the molecules move
faster and hit the walls
more often.
• If the pressure is to
remain constant, the
volume must increase.
Kinetic theory and Pressure Law
• As temperature rises,
the molecules move
faster.
• When the volume is
fixed, the molecules
hit the walls more
often.
• This increases the
pressure.