GCSE · Physics · AQA · Spec 8463
Doing work on a gas (physics only, HT only)
Pump up a bike tyre, then grab the pump's barrel. It often feels warm. Nothing was plugged in, so where did that energy come from? From you.
Do work on the gas. Watch what the particles do.
The piston hasn't moved. The gas particles are already zipping about, because the gas already has some energy. Now slide to the right and do some work on it.
The slider runs from 0 (no work done yet) to 4 (lots of work done) on the same trapped gas: a scale of more and less, not joules. The picture zooms in on the particles, so the piston itself isn't drawn.
Following the energy
Reason it through
Why does pushing a piston into trapped air make the air warmer?
First link · your turn
You push the handle and the piston moves in against the air. What is your force doing to the air?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Doing work on a gas
What you need to know
- Work is the transfer of energy by a force.
- Doing work on a gas increases the internal energy of the gas.
- That increase in internal energy can cause the temperature of the gas to rise.
- You should be able to explain this for a real situation, such as a bicycle pump.
The big picture
Work is the transfer of energy by a force. When you push a piston into a trapped gas, your force does work on the gas, so energy is transferred to it. The gas particles bounce off the moving piston faster, so the internal energy of the gas increases, and that can cause the temperature of the gas to rise. That is why the air in a bicycle pump warms up as you use it.
Key points
Worked example
Problem
The outlet of a pump is sealed, trapping the air inside. Someone pushes the handle in, and their force does 12 J of work on the trapped air. Assume none of that energy leaves the air while the handle moves. By how much does the internal energy of the air change, and what would you expect to happen to its temperature?
⚠ Watch out
Stopping the explanation too early, with something like "the piston squashes the air, so it gets hot". Squashing is what you see happening; the reason is energy. Say that work is done on the air, that this increases its internal energy, and that this can raise its temperature.
Memory hook
Push it in, speed them up, warm it up: work done, faster particles, more internal energy, higher temperature.
Check yourself
Cover the page and answer out loud: what is work, and what does doing work on a trapped gas do to its particles, its internal energy and its temperature?
Flashcards
(8)What is work?
What does doing work on a gas do to its internal energy?
What can an increase in a gas's internal energy cause?
What changes about the particles when work is done on a trapped gas?
How does a piston moving in speed up gas particles?
How is the amount of work done linked to the energy transferred?
What is the internal energy of a gas?
Why isn't 'friction' a full explanation of a warm bicycle pump?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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