Particle Motion in GasesAQA GCSE Physics: Revision notes
Section 1
Random Motion and Temperature
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 - the higher the temperature, the faster the molecules move on average.
Section 2
Temperature and Pressure at Constant Volume
Gas pressure is caused by gas molecules colliding with the walls of their container. At constant volume, increasing the temperature increases the average kinetic energy of the molecules, so they collide with the walls more often and with greater force - increasing the pressure.
Section 3
Volume, Pressure and Boyle's Law
For a fixed mass of gas at constant temperature, increasing the volume causes a decrease in pressure (molecules collide with the walls less often because there is more space). This relationship is described by pV = constant (pressure in Pa, volume in m³). Use this to calculate the change in pressure or volume when one of them changes, provided temperature and mass stay constant.
A gas at 100 kPa in a 2 m³ container is compressed to 1 m³ at constant temperature. New pressure = (100 × 2)/1 = 200 kPa.
Section 4
(Physics Only) Doing Work on a Gas
(Physics only) Doing work on a gas - for example, pushing down on a bicycle pump - increases its internal energy, which can cause an increase in its temperature. This is because the work done is transferred to the kinetic energy of the gas molecules.
Compressing a gas quickly (doing work on it) can raise its temperature even without any external heating - don't assume temperature change always requires a heat source.
Must Know
- Gas molecules move in constant random motion; temperature relates to their average kinetic energy
- At constant volume, higher temperature means higher pressure (more frequent, harder collisions)
- At constant temperature, pV = constant for a fixed mass of gas (Boyle's Law)
- Increasing volume at constant temperature decreases pressure, and vice versa
- (Physics only) Doing work on a gas increases its internal energy and can raise its temperature
That's the notes covered.
Carry on to the next subtopic.