Particle motion in gases

The molecules of a gas are in constant random motion. The temperature of the gas is related to the average kinetic energy of the molecules.

Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas.

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Students should be able to:
- explain how the motion of the molecules in a gas is related to both its temperature and its pressure
- explain qualitatively the relation between the temperature of a gas and its pressure at constant volume.

Pressure in gases

A gas can be compressed or expanded by pressure changes. The pressure produces a net force at right angles to the wall of the gas container (or any surface).

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Students should be able to use the particle model to explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure.

For a fixed mass of gas held at a constant temperature:

\(\text{pressure} × \text{volume} = \text{constant}\)

\(pV = \text{constant}\)

pressure, p, in pascals, Pa
volume, V, in metres cubed, m³

Students should be able to calculate the change in the pressure of a gas or the volume of a gas (a fixed mass held at constant temperature) when either the pressure or volume is increased or decreased.

Increasing the pressure of a gas

Work is the transfer of energy by a force.

Doing work on a gas increases the internal energy of the gas and can cause an increase in the temperature of the gas.

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Students should be able to explain how, in a given situation eg a bicycle pump, doing work on an enclosed gas leads to an increase in the temperature of the gas.