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.

Smoke particle

Smoke Particle

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.

Smoke particle

Air Particle

Smoke particle

Air Particle

Brownian Motion

Gas molecules exert force on the wall of the

container when they collide on it. The

average force per unit area is gas pressure.

Exerci se 10

A fixed mass of gas is heated at a constant pressure. Which of the following

statements is /are correct?

(1) The average speed of the gas molecules increases.

(2) The average spacing between the gas molecules increases.

(3) The number of gas molecules remains unchanged.

A. (2) only

B. (3) only

C. (1) and (2) only

D. (1) and (3) only

E. (1), (2) and (3) only

Ans: E

Exerci se 11

Energy is supplied to a fixed mass of gas which is kept at a constant volume. Which

of the following statements is INCORRECT?

A. The average speed of the gas molecules increases.

B. The average spacing between the gas molecules increases.

C. The gas molecules hit the container wall more frequently.

D. The temperature of the gas increases.

E. The pressure exerted by the gas increases.

Ans: B

Exerci se 12

Which of the following descriptions concerning the gas molecules is correct when a

fixed mass of gas is compressed and also heated at the same time?

Average spacing between

the gas molecules

Average speed of

the gas molecules

A. remain unchanged increases

B. remain unchanged remain unchanged

C. decreases decreases

D. decreases remain unchanged

E. decreases increases

Ans: E

Kinetic theory Model (3 Dimensional model)

Gas molecules

Volume of GasVolume of Gas

Pressure of Gas

Temperature of Gas

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.