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Doing work on a gas, such as compressing it, increases its internal energy rather than its potential energy. Free GCSE Physics notes and 13 FAQs.
Work, Energy and Gas Pressure is part of Particle model and pressure in GCSE AQA Higher Physics.
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Doing work on a gas, such as compressing it, increases its internal energy rather than its potential energy. Free GCSE Physics notes and 13 FAQs.
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Use this lesson to stay inside Particle model and pressure while connecting the detail back to the wider Particle model of matter topic.
Read the core explanations from Work, Energy and Gas Pressure before testing yourself from memory.
Picture pushing a heavy box across the floor. Your muscles are doing something that physicists call 'work' - but it's not what you might think.
In physics, work has a very specific meaning. It's not about effort or how tired you get. Work only happens when a force makes something move in the direction of that force.
Here's the key insight: work is energy in motion. When you do work on something, you're transferring energy to it.
Think of a hammer hitting a nail. The moving hammer has kinetic energy. When it hits the nail, it does work on the nail, transferring energy. The nail gets hot and moves into the wood.
Worked example: Calculating Work Done
A crane lifts a 500 kg concrete block vertically upward by 12 m. Calculate the work done against gravity. (g = 9.8 N/kg)
Answer: 58,800 J (or 58.8 kJ)
Notice how the crane transferred 58,800 J of energy to the concrete block. This energy is now stored as gravitational potential energy.
Now here's where it gets interesting. What happens when you do work on a gas instead of a solid object?
When you compress a gas - like pushing down on a syringe or pumping up a bicycle tyre - you're doing work on the gas particles. But unlike lifting a block, the energy doesn't become potential energy. Instead, it becomes internal energy.
When you do work on a gas, you're essentially giving the gas particles more kinetic energy. They move faster and collide more frequently with the container walls.
You've probably noticed that a bicycle pump gets warm when you use it. This isn't friction - it's a direct result of doing work on the gas inside.
Here's what happens step by step: You push the handle down, doing work on the air inside. This work increases the internal energy of the air particles. Higher internal energy means faster-moving particles. Faster particles mean higher temperature.
Worked example: Explaining the Bicycle Pump
Explain why a bicycle pump gets hot when you compress air inside it.
Answer: Work done compressing the gas increases particle kinetic energy, raising temperature and making the pump hot.
To understand why doing work on gases matters, we need to think about pressure. Gas pressure comes from particles constantly hitting the container walls.
When you compress a gas, two things happen: the particles get squashed into a smaller space AND they move faster (higher kinetic energy). Both effects increase pressure dramatically.
This explains why it gets harder to pump a bicycle tyre as it fills up. You're not just fighting the existing pressure - you're also increasing the internal energy of the gas with every pump stroke.
Work and energy questions are mark-winners if you know the technique. Here's how to approach them systematically.
The most common mistake is confusing work with effort. Remember: work has a precise definition in physics. No movement in the direction of force means zero work done.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Work done is the transfer of energy by a force when an object moves in the direction of the force, measured in joules (J). It is not about effort: with no movement in the direction of the force, the work done is zero.
Gas pressure is measured in pascals (Pa). It is the force per unit area exerted by gas particles colliding with the container walls. When a gas is compressed, its particles collide with the walls more often and its pressure rises.
A bicycle pump gets hot because pushing the handle down does work on the air inside, which increases the internal energy of the air particles. The particles move faster, so the temperature rises and the gas transfers thermal energy to the pump walls; it is not friction.
Doing work on a gas, such as compressing it, increases its internal energy rather than its potential energy. The gas particles gain kinetic energy, move faster and collide more frequently with the container walls, so the temperature of the gas increases.
Find the weight with F = mg, then use W = F × d. A crane lifting a 500 kg block 12 m upward (g = 9.8 N/kg) applies 4900 N, so the work done is 4900 × 12 = 58,800 J, which is stored as gravitational potential energy.
It gets harder to pump a tyre as it fills up because compressing the gas squashes the particles into a smaller space and makes them move faster, and both effects increase pressure. Each pump stroke also increases the internal energy of the gas.
The formula for work done is W = F × d, where W is work done in joules, F is force in N and d is the distance moved in the direction of the force in metres. A crane lifting a 4900 N block 12 m does 58,800 J of work.
Work in physics is not the same as effort or how tired a person gets. Work only happens when a force makes something move in the direction of that force, so without movement in that direction the work done is zero, however hard the push.
When a hammer hits a nail, the moving hammer's kinetic energy is transferred as it does work on the nail. The nail gets hot and moves into the wood, showing that work is a transfer of energy.
Compressing a gas increases its temperature because the work done on the gas increases its internal energy, giving the particles more kinetic energy. Faster particles mean a higher temperature, since temperature is a measure of average kinetic energy.
When a crane lifts a block, the work done is stored as gravitational potential energy. A crane lifting a 500 kg block 12 m upward transfers 58,800 J, using W = F × d with a weight of 4900 N.
Compressing a gas increases its pressure in two ways: the particles are squashed into a smaller space, and they move faster because work done on the gas raises their kinetic energy. Both effects increase pressure dramatically.
Unlike lifting a block, doing work on a gas does not turn the energy into gravitational potential energy; instead it increases the gas's internal energy. The particles move faster and collide more frequently with the container walls.