Increasing the pressure of a gas (physics only) (HT only) — AQA GCSE Physics
Test yourself on Increasing the pressure of a gas (physics only) (HT only) with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Increasing the pressure of a gas (physics only) (HT only) explained
Work done is energy transferred by a force moving its point of application through a distance, W = F s, with W in joules, F in newtons and s in metres measured along the direction of the force.
Read the full explanation
One joule is the energy transferred when a force of one newton moves an object one metre. Naming a force is not enough for an explanation: energy only moves when that force acts through a distance. Pushing a syringe plunger in does work on the gas inside it. Braking does work against friction and raises the internal energy of the brakes, which is why examiners refuse answers that simply say friction and stop there.
Doing work on a gas increases the internal energy of the gas and can cause an increase in the temperature of the gas.
Internal energy is the total kinetic energy plus the total potential energy of all the particles in a system. When an external force pushes a piston in, it moves through a distance, so work is done on the gas and energy is transferred into that internal energy store. Molecules rebounding from an inward-moving piston come away faster, so the mean kinetic energy of the molecules rises, and since temperature is a measure of mean kinetic energy, the gas gets hotter. Nothing was heated: the energy came in mechanically. If the compression is slow and the walls conduct well, much of that energy escapes to the surroundings and the temperature rise is small, which is why fast compressions feel hottest.
Students should be able to explain how, in a given situation eg a bicycle pump, doing work on an enclosed gas leads to an increase in the temperature of the gas.
In a bicycle pump the air is enclosed by the valve and the piston. Pushing the handle applies a force to the piston that moves through a distance, so work is done on the air. That energy is transferred to the internal energy store of the gas, the mean kinetic energy of its molecules rises, and its temperature goes up, which is why the barrel becomes noticeably warm after a few strokes. The same physics drives a fire piston, where air compressed rapidly gets hot enough to ignite tinder, and a diesel engine, which needs no spark plug. Friction between piston and barrel adds a little warming, but it is not the main effect and on its own it does not earn the marks.
Your focus
- State the equation linking work done, force and distance, and give the unit of each quantity.
- Calculate the work done when a stated force moves through a stated distance.
- Explain why the temperature of a vehicle's brakes rises during braking, using the idea of work done against friction.
Show all 9 objectives
- Define internal energy as the total kinetic and potential energy of the particles of a system.
- Explain the chain of reasoning from work done on a gas to a rise in its temperature.
- Predict whether a fast or a slow compression of the same gas produces the larger temperature rise, and justify the prediction.
- Explain why a bicycle pump barrel becomes warm when it is used, in terms of work done on the enclosed air.
- Apply the same explanation to an unfamiliar situation such as a fire piston or a diesel engine cylinder.
- Distinguish the contribution of work done on the gas from the contribution of friction between the piston and the barrel.
Increasing the pressure of a gas (physics only) (HT only) exam tips
Marking Points
- one mark for work done = force × distance moved along the line of action of the force
- one mark for stating that one joule is transferred when a force of one newton moves through one metre
- one mark for identifying the force that does the work in the situation described, such as the piston pushing on the gas
- one mark for stating that work done against friction increases the internal energy of the objects, rather than simply naming friction
- one mark for stating that the force compressing the gas moves through a distance, so work is done on the gas
- one mark for stating that the internal energy of the gas increases
- one mark for stating that the mean kinetic energy of the molecules increases
- one mark for linking the increase in mean kinetic energy to a rise in the temperature of the gas
- one mark for identifying a force applied to the piston that moves through a distance, so work is done on the enclosed gas
- one mark for stating that the energy transferred increases the internal energy of the gas
- one mark for stating that the mean kinetic energy of the gas molecules increases
- one mark for concluding that the temperature of the gas, and so of the pump, rises
Examiner Tips
- 💡Use the phrase work is done in any explanation about heating by compression or by friction; it is often the marking point.
- 💡Quote W = F s and name the force and the distance separately when you are asked to explain rather than calculate.
- 💡Remember that energy transferred and work done are the same quantity, so the unit is always the joule.
- 💡Define internal energy as the total kinetic and potential energy of the particles if a question asks what it is.
- 💡Set your answer out as a chain: work done, internal energy up, mean kinetic energy up, temperature up.
- 💡Only the kinetic part of the internal energy changes here, because the gas stays a gas.
- 💡Name the force and say it moves through a distance; that is what turns a description into an explanation of work.
- 💡Mention the gas is enclosed, so the energy transferred stays in the gas rather than escaping with it.
- 💡Use the same four-step chain for any context the examiner invents, from a fire piston to a diesel engine.
Common Mistakes
- writing that friction causes heating without saying that work is done
- using the total path length rather than the distance moved in the direction of the force
- treating work and power as the same quantity
- giving work done in watts instead of joules
- saying the gas gains heat from the pump, when the energy is transferred by doing work
- attributing the whole temperature rise to friction between the piston and the barrel
- treating internal energy and temperature as the same quantity
- claiming energy is created by squashing the gas
- explaining the warm pump entirely by friction between the piston and the barrel
- saying the gas gets hot because the molecules are squashed closer together
- treating the rise in pressure as though it were itself the rise in temperature
- omitting the phrase work is done, which is usually the first marking point