## Gases & Pressure (Triple Physics Only)

Pressure (measured in Pascals, Pa) can be calculate using:

pressure = force / area

Force exerted by person: weight = mass x gravitational field strength

weight = 70 x 9.8 = 686 Newtons

pressure = force / area

pressure = 686 / 0.05

The pressure is 13720 Pascal

The molecules of a gas are in constant random motion, with a range of speeds.

The temperature of the gas is related to the average kinetic energy of the molecules (high temperature means high average kinetic energy).

## Changing the Gas Temperature (Triple Physics Only)

Increasing the temperature of a gas, held at constant volume, increases the pressure exerted by the gas. Decreasing the temperature of a gas, held at constant volume, decreases the pressure exerted by the gas.

Motion of the molecules in a gas is related to:

• temperature: Higher temperature means faster speed, because the kinetic energy is higher
• pressure: Increased pressure will increase the frequency of collisions between particles

For a gas in a container of fixed volume: if the temperature increases then the pressure will increase. This is because the increased temperature speeds up the particles, so they collide with the container walls with more frequency and force.

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).

• particles would have a higher (mean) kinetic energy
• so increased number of collisions with the walls of the balloon per second (or increased frequency)
• so greater forces exerted in collisions (between particles and balloon walls)
• so greater force exerted on same area

For a gas which is kept at a constant temperature: if the volume of the container is increased, the pressure will decrease. This is because the particles will take longer to collide with a container wall, decreasing the number of collisions with the walls.

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

pressure × volume = constant

initial pressure × initial volume = final pressure × final volume

We use this fact 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 changed.

initial pressure × initial volume = final pressure × final volume

100000 x 0.03 = final pressure x 0.025

final pressure = (100000x 0.03) / 0.025

The final pressure is 120000 Pascals

## Doing Work on a Gas (Triple Physics Only)

Work is the transfer of energy by a force.

Doing work on a gas increases the internal energy of the gas. So the kinetic energy of the particles increases which can cause an increase in the temperature of the gas.

• work is done on the air (in the tyre)
• so the temperature (of the air) increases