F.4 Physics Lectures

Gas Laws

Kinetic Theory

Boyle’s LawThe 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(kpa)

Volume(cm3)

1/volume(cm-3)

100120140160180200220

514338332925.523

0.0200.0230.0260.0300.0350.0390.043

pressure/kPa

1/Volume /cm-3

0.020

220

200

180

160

140

120

0.030 0.040 0.050

Graph Plotting

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 aircolumn/cm

Temperature/K

6.06.37.07.47.78.18.9

273289308324338352373

Graph Plotting

length of air column/cm

temperature T/K

10

8

6

4

2

100 200 300 0

400

Conclusion

V T 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/K94 273

100 293110 315114 333124 358128 373

Graph Plotting

pressure p/kPa

temperature T/K

10

8

6

4

2

100 200 300 0

400

Conclusion

P T 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.