Gas pressure
A gas exerts pressure because its particles are constantly moving and colliding with the container walls.
Origin of pressure
Each collision exerts a tiny force on the wall. The total force per unit area is the pressure:
P = F / A, units N/m^2 = pascals (Pa).
Pressure depends on:
- Number of particles per second hitting the wall.
- Speed of the particles (which depends on temperature).
- Mass of the particles (which is fixed for a given gas).
Pressure and temperature (constant volume)
Heating a fixed-volume gas raises the average kinetic energy of the particles. They:
- Move faster.
- Hit the walls more often.
- Hit harder.
So pressure rises. P is proportional to T (in kelvin), provided V is constant.
Pressure and volume (constant temperature) — Higher tier
If the temperature is fixed and you reduce the volume of a gas:
- Particles still move at the same average speed.
- They hit the walls more often (less distance to travel).
- Pressure rises.
This is Boyle's Law: PV = constant, or P_1 V_1 = P_2 V_2 at constant T.
✦Worked example— Worked example (Higher)
A gas at 100 kPa occupies 0.50 m^3. It is compressed at constant temperature to 0.20 m^3. Find the new pressure.
P_1 V_1 = P_2 V_2 100 x 0.50 = P_2 x 0.20 P_2 = 50 / 0.20 = 250 kPa.
Kelvin scale
T (K) = theta (degrees C) + 273.
Use kelvin in any pressure-temperature calculation. 0 K = absolute zero — particles have minimum kinetic energy and no gas pressure.
WJEC exam tip
For "explain why heating a sealed gas increases pressure" you need to mention three things: faster particles, more frequent collisions, harder collisions. Just saying "particles move faster" alone is one mark out of three.
AI-generated · claude-opus-4-7 · v3-wjec-combined-science-leaves