Particle Model & PressureEdexcel GCSE Physics: Revision notes
Section 1
How do gas particles create pressure?
Gas particles move rapidly and randomly. Pressure in a gas is caused by particles colliding with the walls of their container — each collision exerts a tiny force on the wall, and the sum of all these forces over the area of the wall produces the gas pressure. The pressure of a gas acts as a net force at right angles (normal) to any surface it touches.
Section 2
How does temperature affect gas pressure at constant volume?
Increasing the temperature of a gas increases the average speed (kinetic energy) of its particles. At constant volume, faster-moving particles collide with the container walls:
- More frequently
- With greater force per collision
Both effects increase the pressure of a fixed mass of gas at constant volume as temperature rises.
Examiners want both reasons for a full-mark answer: more frequent collisions AND harder collisions, not just one.
Section 3
What is absolute zero and the kelvin scale?
Absolute zero is −273 °C — the temperature at which particles have the minimum possible kinetic energy (theoretically no movement). It is the zero point of the kelvin scale.
To convert Celsius to kelvin: temperature in K = temperature in °C + 273
The kelvin and Celsius scales have the same size degree, just a different zero point.
20 °C = 20 + 273 = 293 K.
Section 4
How does volume affect gas pressure at constant temperature?
Gases can be compressed (squeezed into a smaller volume) or expanded by pressure changes. At constant temperature, if the volume of a fixed mass of gas is decreased, the particles have less space, so they hit the container walls more frequently, increasing the pressure.
This relationship is described by:
P₁ × V₁ = P₂ × V₂
which lets you calculate a new pressure or volume when a fixed mass of gas at constant temperature changes state.
A gas at 100 kPa in a 2 m³ container is compressed to 1 m³ at constant temperature: P₂ = (P₁V₁)/V₂ = (100×2)/1 = 200 kPa.
Section 5
Why does doing work on a gas increase its temperature?
Doing work on a gas (e.g. compressing it rapidly, as with a bicycle pump) transfers energy to the gas particles, increasing their kinetic energy and therefore raising the gas's temperature. This is why a bicycle pump feels warm after use — the work done in compressing the air inside increases the internal energy and temperature of the gas.
Think of rapidly pushing the plunger of a bicycle pump with the outlet blocked — the effort you put in becomes heat in the trapped air.
Must Know
- Gas pressure is caused by particles colliding with container walls, producing a net force at right angles to any surface
- Increasing temperature at constant volume increases pressure (more frequent, harder collisions)
- Absolute zero = −273 °C; K = °C + 273
- Decreasing volume at constant temperature increases pressure (more frequent collisions): P₁V₁ = P₂V₂
- Doing work on a gas (e.g. compressing it) increases its temperature by increasing particle kinetic energy
That's the notes covered.
Carry on to the next subtopic.