Work and energy - Higher

Pressure can be increased by:

  • increasing the temperature - this increases the force of each collision
  • decreasing the volume - this increases the number of collisions per second

Boyle concluded that volume could only be inversely proportional to pressure for a fixed mass of gas and at a constant temperature if gases contained particles that were moving around. This and subsequent discoveries triggered the Industrial Revolution in the 18th century.

A piston closes the end of a cylinder with some gas molecules inside. The pressure applied to the piston is doubled and the volume inside the cylinder halfs.

Forces applied to the particles in a gas result in a transfer of energy.

When a person presses a piston down on a column of gas, they apply a force that moves the piston a certain distance. They have done work on the gas by compressing it.

work done = force × distance

curriculum-key-fact
Mechanical work transfers energy from the person or machine’s store of chemical energy to the internal energy store of the gas.

Since the volume of the gas has decreased, the pressure increases because the particles are moving in less space and collide more often. The temperature of the gas is not fixed in most applications and the increased pressure leads to an increase in temperature. This is because the temperature is a measure of the average kinetic energy of particles - the increase in the internal energy of the gas (as kinetic energy of the particles) means that the temperature must increase.

This process explains why a bicycle pump gets warm when it is used to inflate a tyre.

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