Particle motion in gases
Gas pressure
The particles in a gas are in constant . When they hit the walls of their container, each collision pushes on the wall. Together, these collisions make a . The force on each square metre of wall is the .
of a gas in a container with a fixed volume:
- The particles gain kinetic energy and move faster.
- They hit the walls more often and with more force.
- So the .
Common mistake
The particles don't move further apart or get bigger: the volume is fixed. They move faster, so they hit the walls more often and harder.
Key terms
Pressure and volume: pV = constant (Separate Physics only)
For a fixed mass of gas at a , squashing it into a smaller volume makes the particles hit the walls more often, so the . This equation is on the equation sheet:
pressure × volume = constant, or pV = constant
| Quantity | Symbol | Unit |
|---|---|---|
| Pressure | p | pascals (Pa) |
| Volume | V | cubic metres (m³) |
So p₁V₁ = p₂V₂. Halve the volume and the pressure doubles.
Example: 0.02 m³ of air at 100 000 Pa is squashed to 0.005 m³, at the same temperature. Find the new pressure.
- p₁V₁ = p₂V₂
- 100 000 × 0.02 = p₂ × 0.005
- p₂ = 2000 ÷ 0.005 = 400 000 Pa
A graph of pressure against volume is a curve that falls steeply then levels off.
Key terms
Doing work on a gas (Higher tier only) (Separate Physics only)
is energy transferred by a . When you do work on a gas, you increase its , which can .
A bike pump is a good example. Each push of the handle forces the piston in, squashing the air. The force does work on the air, so its particles gain kinetic energy and its temperature rises. That's why a pump gets warm when you pump up a tyre quickly.