Section 1 of 4Work and Energy Transfer
Work is done whenever a force moves something, and it transfers energy.
Work done
- Work is the energy transferred when a force moves an object through a distance in the direction of the force.
Compressing
- A piston pushed into a gas does work on the gas, transferring energy from the person or machine to the gas.
Internal energy
- The internal energy of a gas is the total kinetic energy of its particles, plus any energy stored in bonds.
Temperature
- For a gas the average kinetic energy of the particles determines its temperature, so more energy means a higher temperature.
Section 2 of 4Why Compression Heats a Gas
The piston moves, and the particles that rebound from it gain speed.
1
A force pushes the piston inwards, and the piston does work on the gas as the volume decreases.
2
Gas particles that collide with the moving piston rebound with greater speed than they had before the collision.
3
The average kinetic energy of the particles increases, so the internal energy of the gas goes up.
4
Because the temperature depends on the average kinetic energy, the gas becomes hotter.
5
If the compression is rapid, there is little time for energy to escape, so the temperature rise is noticeable.
Section 3 of 4Examples and the Reverse Process
Examples and the Reverse Process
Bicycle pump
- Pushing the handle quickly compresses the air, and the barrel near the outlet becomes warm.
Diesel engine
- Air in the cylinder is compressed so strongly that it becomes hot enough to ignite the fuel sprayed into it.
Expansion
- A gas that pushes a piston outwards does work on its surroundings, so it loses internal energy and cools.
Aerosol can
- Gas escaping from a can expands rapidly and cools, which is why the can feels cold when sprayed for long.
| Process | Work done | Effect on gas temperature |
|---|---|---|
| Compression | On the gas | Increases |
| Expansion | By the gas | Decreases |
Section 4 of 4Exam Checklist & Tips
1.3.3 – Key Points for the Exam
- Work done The energy transferred when a force moves an object through a distance.
- Internal energy The total energy stored in the particles of a system, including their kinetic energy.
- Compression A decrease in the volume of a gas caused by an applied force.
- Expansion An increase in the volume of a gas, during which it can do work.
Common Mistakes
- Saying the gas heats up because of friction between particles; the cause is work done by the moving piston.
- Forgetting that a gas which expands and does work loses energy and cools.
- Confusing temperature with heat; the energy transferred by work is not the same as thermal transfer.
- Giving the pump example without explaining that the rebounding particles gain kinetic energy.