Thermal physicsThermal physics

A-level Physics

Unit G484: The Newtonian World

Gas pressure - 1

A-level Physics

Unit G484: The Newtonian World

Gas pressure - 1

Thermal physicsThermal physics

1. You have seen, through a microscope, the ‘Brownian motion’ of smoke particles in a smoke cell.

a. Describe the motion of the smoke particles had and illustrate, with a sketch, the movements of one particle.

b. Explain why the motion of the smoke particle suggests that air molecules are also in motion, with much greater speeds.

c. Particles larger than smoke particles would be easier to see.

d. What two disadvantages would there be in using larger particles for this experiment?

2. A table tennis ball is suspended on a fine thread in a closed, and entirely draught-free, room. Would the ball be completely at rest? Explain your answer.

Brownian motion recall LOs

Thermal physicsThermal physics

Learning objectives

At the end of the lesson you will be able to:

• define the term pressure;

• use the kinetic model to explain the pressure exerted by gases.

Lesson focus• Pressure in gases

Thermal physicsThermal physics

Learning outcomes

All of you should be able to

• recall and use the equation for pressure;

• describe in simple terms how moving gas particles cause gas pressure;

• use a model to investigate gas pressure and explain how it simulates the real thing;

• use the data you collect to describe the link between the pressure and volume of a gas.

Most of you will be able to

• describe the distribution of energy between the particles of a gas at different temperatures.

Some of you will be able to

• explain how gas from the Earth’s atmosphere is able to escape into space.

Thermal physicsThermal physics

To answer

1. Define pressure in words and write the defining equation.

2. Give two alternative SI units for pressure.

3. What is normal atmospheric pressure (in atmospheres, Pa, bar, and mm of mercury)?

LO 1: define the term pressure

Pressure – AS recall LOs

Thermal physicsThermal physics

Pressure – AS recall LOs

Thermal physicsThermal physics

You are going to model the behaviour of a gas using this apparatus.

Part 1: a qualitative description of gas pressure

1. Describe the distribution of particle energies for a particular ‘temperature’.

2. What evidence do you have for this?3. Explain how the energy distribution

changes at higher ‘temperature’. Sketch and label a graph that shows the two distributions.

4. Describe what you feel when you lightly touch the end of the wooden rod.

5. Explain this observation.

LO 2: use the kinetic model to explain the pressure exerted by gases

Pressure of a gas: practical investigation LOs

Thermal physicsThermal physics

The pressure of a gas results from the collisions between its particles

(atoms or molecules) and the walls of its container. When a particle

collides with the wall it exerts a force on the wall and the wall, in turn,

exerts the same-sized force on it, in the opposite direction (Newton’s 3rd

law of motion). The gas pressure is the sum of many forces, due to a vast

number of particles, divided by the area over which these forces are

exerted (P = F/ A). Although the pressure appears to be constant

(because the number of particles is so large) it varies on a microscopic

scale.

LO 2: use the kinetic model to explain the pressure exerted by gases

Gas pressure LOs

Thermal physicsThermal physics

Part 2: a quantitative description of the effect of pressure on the volume of a ‘gas’.

You are going to investigate the relationship between the pressure of a gas and its volume.

LO 2: use the kinetic model to explain the pressure exerted by gases

V

p

Pressure of a gas: practical investigation LOs

ExtensionRepeat for a ‘higher temperature’.