Section 1 of 7How a gas exerts pressure
1
Gas particles move randomly at high speed in every direction.
2
They collide with the walls of the container, and each collision exerts a small force at right angles to the wall.
3
Millions of collisions each second combine to give a steady overall force on the wall.
4
Pressure is that force spread over the area of the wall, so p = F ÷ A.
Section 2 of 7Pressure and area (Extended content)
| Quantity | Symbol | Unit |
|---|---|---|
| Pressure | p | pascal, Pa (1 Pa = 1 N/m2) |
| Force | F | newton, N |
| Area | A | square metre, m2 |
Section 3 of 7Raising the temperature at constant volume
1Gas is heated
- Gas is heated The particles gain thermal energy and their average kinetic energy increases.
2Faster particles
- Faster particles They travel at higher speeds, so they reach the walls more often.
3Harder impacts
- Harder impacts Each collision has a greater change of momentum, so the force per collision is larger.
4Pressure increases
- Pressure increases The total force on each unit area of wall grows.
- Cooling the gas reverses all of these changes.
Section 4 of 7Key conditions
Fixed mass of gas
- Fixed mass of gas No gas enters or leaves, so the number of particles stays the same.
- The comparison is only fair if volume is also kept constant.
Absolute zero
- Absolute zero Pressure would fall to zero at the temperature where the particles have no kinetic energy, which is –273 °C (0 K).
- This is why a graph of pressure against temperature in kelvin is a straight line through the origin.
Working in kelvin
- Working in kelvin Temperature in kelvin = temperature in °C + 273.
- The pressure of a gas is directly proportional to its kelvin temperature at constant volume.
Section 5 of 7Worked example
Calculating pressure
- Calculating pressure A force of 600 N acts on an area of 0.30 m2. Pressure = 600 ÷ 0.30 = 2000 Pa.
Explaining a change
- Explaining a change When a sealed tyre warms up during a long journey, its air particles move faster and collide with the inside wall more often and with more force,
- so the pressure rises.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Pressure | Force acting at right angles to a surface per unit area, measured in pascals. |
| Kinetic energy | Energy due to movement; the average for gas particles rises with temperature. |
| Absolute zero | The lowest possible temperature, –273 °C or 0 K, where particles have minimum kinetic energy. |
| Kelvin | The SI unit of temperature; 0 K is absolute zero. |
| Fixed volume | A container whose size does not change as the gas is heated. |
Section 7 of 7Exam technique
1
Saying the particles expand when heatedthey stay the same size and move faster.
2
Using Celsius in proportionality statementspressure is proportional to kelvin temperature, not °C.
3
Mentioning only more frequent collisionsthe collisions also become harder.
4
Forgetting the unit conversion cm2 to m2 when calculating pressure.
5
Thinking particles collide with each other to make the pressure: it is collisions with the container walls.