Work and energy

1. Work is done whenever a force moves an object through a distance in the direction of the force.
2. Doing work is one way of transferring energy, and the energy transferred is equal to the work done.
3. Work done is measured in joules, $J$, the same unit as energy.
4. Work is done on a gas when a force pushes a piston or plunger into it and makes its volume smaller.
5. The energy transferred by this work goes into the internal energy of the gas.
6. In a gas, the internal energy is almost entirely the kinetic energy of its fast-moving molecules.

Why the temperature rises

1. As the piston moves inwards, molecules keep colliding with its moving face.
2. A molecule that hits a piston moving towards it rebounds faster than it arrived, in the same way that a tennis ball leaves a racket faster when the racket is swung into it.
3. Each of these collisions transfers energy from the piston to a molecule, so the molecules gain kinetic energy.
4. The average kinetic energy of the molecules therefore increases.
5. Temperature depends on the average kinetic energy of the molecules, so the temperature of the gas rises.
6. The energy comes from whatever pushes the piston, such as the chemical energy store of your muscles when you work a pump.
7. The faster the gas is compressed, the larger the rise in temperature, because there is less time for energy to be transferred to the surroundings by heating.
8. If the gas is compressed very slowly, energy leaves through the walls almost as fast as it arrives, so the temperature hardly changes.
9. The reverse also happens: a gas that expands and pushes a piston outwards does work on its surroundings, so its molecules lose kinetic energy and the gas cools.

Examples of doing work on a gas

1. Bicycle pump: pushing the handle down quickly compresses the air in the barrel, so after a few strokes the barrel feels warm, especially near the valve end where the air is squeezed most.
2. Some of that warmth comes from friction between the plunger seal and the barrel, but the work done compressing the air adds to it.
3. Fire piston: slamming the plunger of a sealed, clear tube down very quickly raises the temperature of the trapped air by several hundred degrees, enough to set a small piece of cotton wool alight.
4. Diesel engine: the air in each cylinder is compressed so strongly and so quickly that it becomes hot enough to ignite the fuel sprayed into it, so no spark plug is needed.