1. A-level Physics
Unit G484:
The Newtonian
World
Gas pressure - 1
Thermal physics

2. Brownian motion recall LOs
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.
Thermal physics

3. Lesson focus
• Pressure in gases
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.
Thermal physics

4. 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 physics

5. Pressure – AS recall LOs
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)?
Thermal physics LO 1: define the term pressure

6. Pressure – AS recall LOs
Thermal physics

7. Pressure of a gas: practical investigation LOs
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.
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases

8. Gas pressure LOs
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 3 rd
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.
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases

9. Pressure of a gas: practical investigation LOs
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.
V
p
Extension
Repeat for a ‘higher
temperature’.
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases