Pressure in Gases (GCSE Physics)

Pressure in Gases

Motion related to Pressure and Temperature

Movement related to Pressure

• Gas particles constantly collide. The particles in the gas will move around in different speeds and directions, and they will have lots of collisions. These gas particles are inside a container. The collisions will occur between two particles, or between a particle and the walls of the container.
• Colliding particles exert a pressure on the walls of a container. As the particles collide with the sides of the container, they will exert forces onto the walls of the container. All the individual forces from each collision will combine together. This combined force will exert a pressure on the walls of the container (since pressure exerted over an area will result in a force).

Effect of Temperature on Pressure

Increased temperature leads to an increase in pressure at a constant volume. The mechanism for this is shown in the flow chat below:

The Pressure Law

The pressure of a gas is proportional to the temperature of a gas, when measured in Kelvin (K). This is assuming the volume is kept constant.

We can express this relationship using the following equation:

Where:

• pressure, P, in pascals, Pa
• temperature, T, in Kelvin, K

Pressure

• Pressure is affected by volume. Changing the volume of a container will affect the pressure of the gas inside it. When the volume of the container increases, the pressure of the gas will decrease.

Calculating Pressure and Volume

We can calculate pressure and volume using the following equation. The constant does not change (assuming the temperature doesn’t change), which means that if the volume rises, pressure has to fall, and vice versa.

Another way of expressing this is through Boyle’s Law:

Where:

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

Question: The volume of a sample of gas in a sealed container is changed with the temperature remaining the same. Which statement is correct?

If the pressure is doubled, the volume also doubles.
If the pressure is halved, the volume doubles.
If the pressure is halved, the volume is also halved.
If the pressure is doubled, the volume reduces to a quarter.

Answer: Using our equation, pressure x volume = constant, we can work out that B must be the correct answer.

Work

• Work is the transfer of energy. Work done is simply another way of saying that energy is transferred. The energy is usually transferred by a force. You may wish to re-cap our previous work in 4.1 Energy.
• Doing work will increase internal energy. By doing work on a gas (for example increasing its pressure), we can increase the internal energy of the gas. Previously, we discussed that the internal energy is the combination of kinetic energy and potential energy of the particles.
• Internal energy affects temperature. As the internal energy of the particles increases, the temperature of the gas also increases. The kinetic energy has been increased and therefore the gas becomes warmer.

Example: A Bicycle Pump

Question: Yasmin is using a bike pump. As she is inflating her bike tyre, she feels the pump handle getting warmer. Why is this happening?

More particles are being forced into the tyre. As Yasmin is pumping up the bike tyre, she is making more air particles move into the tyre. She is doing work against a force for this to happen.

Kinetic energy of the particles increases. By doing work, Yasmin is transferring energy to the particles inside the tyre. This energy is converted into kinetic energy.

Increased KE leads to increased temperature. The increase in kinetic energy will result in the temperature of the tyre increasing.

Tyre is connected to the pump. The tyre is connected to the bike pump, therefore as the tyre gets warmer, Yasmin will feel the bike pump also getting warmer.