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