3.3.3 Increasing the pressure of a gas
Work: transferring energy with a force
Definition
Work
Work is the transfer of energy by a force.
- Work is done when a force makes an object, or part of a system, move through a distance.
- Energy is transferred mechanically from the thing applying the force to the system the force acts on.
- For example, pushing down a bicycle pump handle exerts a force that moves the piston, doing work on the gas inside.
Key Idea
Work is not an energy store; it is a way of transferring energy from one store to another.
Doing work on a gas raises its internal energy
Definition
Internal energy
The internal energy of a system is the total kinetic energy and potential energy of all the particles in it.
- Doing work on an enclosed gas transfers energy to it, increasing its internal energy and often its temperature.
- A force moves a piston and compresses the enclosed gas.
- The moving piston does work on the gas, transferring energy to it mechanically.
- The internal energy of the gas increases.
- The gas particles gain kinetic energy and move faster on average.
- So the temperature of the gas increases.
- The gas must be enclosed, so the piston compresses it rather than simply pushing it out of the container.
Applying it to a bicycle pump
- Pushing the pump handle down moves the piston and compresses the enclosed air, so the person exerts a force over a distance and does work on the air.
- Energy is transferred mechanically to the air, raising its internal energy.
- The temperature of the air rises, which is why a bicycle pump can become warm during rapid pumping.
Common Mistake
- Do not say that work is “stored” in the gas; work is an energy transfer, while internal energy is the store.
- Do not explain the temperature rise only by friction in the pump; the key point is that the moving piston does work on the enclosed gas.
Exam technique
- For an “Explain” question, give the full chain: work done on the gas →\rightarrow→ energy transferred →\rightarrow→ internal energy increases →\rightarrow→ temperature increases.
- Do not jump straight from “the gas is compressed” to “the temperature rises”; state how the energy is transferred and what happens to the internal energy.
Self review
- What is the definition of work?
- What happens to the internal energy of a gas when work is done on it?
- Why can doing work on a gas increase its temperature?
- In a bicycle pump, which object does work on the enclosed air?
- Why is it wrong to say that work is stored in the gas?
