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3.3.3Particle Model of Matter

Work on a Gas

5 min read 5 parts Lesson 14 of 14 Unit quiz

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Section 1 of 5Work Done in Compression

Pushing a piston into a cylinder transfers energy to the gas inside, and this energy is stored in the particles.

Work done
  • Work done Work is done when a force moves an object through a distance, and the energy transferred is the force multiplied by the distance.
Piston
  • Piston When a piston is pushed into a cylinder of gas, the force on the piston does work on the gas.
Internal energy
  • Internal energy Work done on a gas increases the kinetic energy of its particles, so the internal energy of the gas increases.
Temperature
  • Temperature If the gas is well insulated from its surroundings, its temperature rises because its internal energy has increased.
Expansion
  • Expansion When a gas expands and pushes a piston outwards, the gas does work on its surroundings and its internal energy falls.

Section 2 of 5Energy in a Bicycle Pump

A bicycle pump gives a clear everyday example of work being done on a gas.

1
The person pushes the handle, applying a force that moves the piston down into the barrel of the pump.
2
Energy is transferred from the store in the person's body to the air in the barrel as work done.
3
Air particles collide with the moving piston and gain kinetic energy, so the air becomes hotter.
4
The pressure of the air rises as its volume decreases, and the barrel of the pump becomes warm to the touch.
5
When the person stops pushing, the energy remains in the air until it is lost to the surroundings as thermal energy.

Section 3 of 5Key Quantities

Three quantities link the force on the piston, the pressure of the gas and the work done.

Key Quantities
Force
  • Force The force needed depends on the pressure of the gas and the area of the piston that it pushes against.
Distance
  • Distance The distance moved by the piston is the change in length of the gas column in the cylinder.
Work
  • Work Work done W = F × d, measured in joules when force is in newtons and distance is in metres.
Pressure link
  • Pressure link Since pressure = force ÷ area, the work can also be written as W = p × ΔV, where ΔV is the change in volume.

Section 4 of 5Worked Example

Question

  • A gas is compressed by a piston at a steady pressure of 1.0 × 10⁵ Pa, and its volume falls by 2.0 × 10⁻³ m³.
  • Calculate the work done on the gas.

  1. Work done = pressure × change in volume, so W = p × ΔV.
  2. Change in volume ΔV = 2.0 × 10⁻³ m³ (the size of the change, ignoring its direction).
  3. W = 1.0 × 10⁵ Pa × 2.0 × 10⁻³ m³ = 2.0 × 10² J.

Answer 200 J of work is done on the gas.

Section 5 of 5Exam Checklist & Tips

3.3.3 – Key Points for the Exam

  • Work done The energy transferred when a force moves an object through a distance, measured in joules.
  • Internal energy The total kinetic and potential energy of the particles in a substance.
  • Piston A sliding part that compresses or is pushed by a gas inside a cylinder.
  • Pressure The force acting per unit area, measured in pascals (Pa).
  • Joule The unit of energy and work, equal to one newton metre.

Common Mistakes

  • Saying that compressing a gas always lowers its temperature;
    • work done on a gas increases its internal energy and usually its temperature.
  • Forgetting to convert cm³ into m³ before calculating the work done, which gives an answer a million times too large.
  • Mixing up the direction of energy transfer: work is done on the gas when it is compressed, and by the gas when it expands.
  • Using pressure in pascals and volume in cm³ without conversion, then giving an answer in newtons rather than joules.
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