Course: Combined Science + Separate Physics (volume changes and doing work on a gas are S only) | Tier: Foundation + Higher | Linked equation: pV = constant
Notes
✏️ Copy into your book
- The molecules of a gas are in constant random motion.
- The temperature of a gas is related to the average kinetic energy of its molecules.
- Gas pressure is caused by molecules colliding with the walls of their container. The collisions produce a force at right angles to the wall.
- At constant volume, heating a gas makes the molecules move faster. They hit the walls more often and harder, so the pressure increases.
Changing the volume (S only)
For a fixed mass of gas at constant temperature, pV = constant. Squashing the gas into a smaller volume means the molecules hit the walls more often, so the pressure increases. Halve the volume and the pressure doubles.
Doing work on a gas (S only, HT)
✏️ Copy into your book (HT)
Work is the transfer of energy by a force. Doing work on a gas (for example, pushing in the piston of a bicycle pump) increases the internal energy of the gas, which can cause an increase in its temperature.
Questions
Q1 (F) Describe the motion of gas molecules.
Show answer
They move constantly, randomly and quickly in all directions.
Q2 (F) What causes the pressure of a gas in a container?
Show answer
Gas molecules colliding with the walls of the container, which produces a force on the walls.
Q3 (F) What happens to the average kinetic energy of gas molecules when the gas is heated?
Show answer
It increases (the molecules move faster).
Q4 (F/H) A sealed can of gas is heated. Explain why the pressure inside it increases.
Show answer
The molecules gain kinetic energy and move faster. They collide with the walls more often and with more force, so the pressure increases (the volume stays the same).
Q5 (S only) A gas has a pressure of 100 kPa in a volume of 2.0 m³. It is compressed to 0.50 m³ at constant temperature. Calculate the new pressure.
Show answer
F: p₁V₁ = p₂V₂
I: 100 × 2.0 = p₂ × 0.50
F: p₂ = 200 ÷ 0.50
A: 400 kPa
Q6 (S only) Explain, in terms of particles, why reducing the volume of a gas at constant temperature increases its pressure.
Show answer
The molecules have less space, so they collide with the walls more often. The molecules’ speed is unchanged, but the more frequent collisions produce a bigger force per unit area.
Q7 (S only, HT) Explain why a bicycle pump gets warm when you pump up a tyre.
Show answer
Pushing the piston does work on the gas. This transfers energy to the gas, increasing its internal energy, so its temperature rises and it warms the pump.
