GCSE · Physics · AQA · Spec 8463

Pressure in gases (Boyle's law, p V = constant) (physics only)

Give the gas in a balloon more room and its push on each bit of skin gets weaker, yet not one particle has been added or lost. What changed?

Give the gas more room

give it more room →Squeezed: 0.2 m³120000 PaPressure of the gas

Volume of the gas: 0.2 m³. State: Squeezed: 0.2 m³. Pressure of the gas: 120000 Pa. 10 hits a second on the moving wall. animating

10 hits a second on the moving wallp × V = 24000 Pa m³
Volume of the gas0.2 m³

Squeezed into less space, the particles are packed more tightly. They are always moving quickly in random directions, and with less room every square millimetre of wall is hit more often each second. More pushes on each bit of wall add up to a bigger pressure.

One fixed mass of gas, held at constant temperature. The outline is the most room the gas can have. Drag the volume up and down and watch how the particles fill the space.

Watch out: No particles have escaped and none have been added: it is the same gas. What changes is how often each square millimetre of wall gets hit.

Which way does the gas push?

The box holds a gas. Its particles hit every wall from random directions. Tap each arrow to see which way the net force on that wall points.

Sealed box of gas

Tap a force to see what it does.

p V = constant: working it through

Problem

A fixed mass of gas at constant temperature has a pressure of 100 000 Pa in a volume of 0.060 m³. It is squeezed into a volume of 0.024 m³. What is its new pressure?

Now you supply the steps

Find the new volume

A fixed mass of gas at constant temperature has a pressure of 160 000 Pa in a volume of 0.025 m³. The pressure on it is lowered to 50 000 Pa. What volume does the gas now fill?

  1. The mass and temperature do not change, so p × V is the same before and after.This is what lets you link the two situations.
  2. missing step
Which line is step 2?

Your turn to explain

Write it, then check it

A fixed mass of gas at constant temperature is allowed to expand into a larger volume. Use the particle model to explain why the pressure of the gas decreases. [4 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

p V = constant

For a fixed mass of gas at constant temperature: more room means each bit of wall is hit less often, so the pressure is lower. Pressure times volume never changes.

What you need to know

  • Gas particles are always moving quickly, in random directions.
  • Pressure comes from particles hitting surfaces: each hit is a tiny push, and the pushes added together and spread over the area make the pressure.
  • Gas pressure produces a net force at right angles to the wall of its container, or to any surface.
  • For a fixed mass of gas at constant temperature, a larger volume means each unit area of wall is hit less often, so the pressure is lower.
  • p V = constant for a fixed mass of gas at constant temperature, with p in pascals (Pa) and V in metres cubed (m³).

The big picture

Gas pressure is the total of countless tiny pushes from particles hitting a surface, spread over its area, and it acts at right angles to that surface. Give a fixed mass of gas more room at constant temperature and each bit of wall is hit less often, so the pressure falls; squeeze it into less space and the pressure rises. In numbers, p V = constant.

Key points

1A gas can be compressed or expanded by changing the pressure on it.
2More hits on the walls each second means a bigger total outward force, so a greater pressure.
3Squeezing a gas into less space packs its particles more tightly, and its pressure rises.
4To find a new pressure or volume, set p × V before the change equal to p × V after it, then divide by the quantity you know.
5Check the direction of your answer: for a fixed mass of gas at constant temperature, when one of p and V goes up, the other comes down.

Worked example

Problem

A fixed mass of gas at constant temperature has a pressure of 90 000 Pa when its volume is 0.040 m³. It expands until its volume is 0.120 m³. Calculate its new pressure.

⚠ Watch out

Letting pressure and volume move the same way. With p × V staying the same, a bigger volume must mean a smaller pressure. If your new pressure rose when the gas was given more room, you have put the volumes the wrong way up.

🧠

Memory hook

More room, fewer knocks, lower pressure. Same gas, same temperature: p times V never changes, so when one goes up the other comes down.

✓

Check yourself

A fixed mass of gas at constant temperature has its volume halved. What happens to its pressure, and why, in terms of particles? (It doubles: each bit of wall is hit more often.)

Flashcards

(12)
How do the particles in a gas move?
They are always moving quickly, in random directions.
Where does the pressure of a gas come from?
Particles hitting surfaces. Each hit is a tiny push; all the pushes added together and spread over the area make the pressure.
In which direction does gas pressure push on a wall?
The net force acts at right angles to the wall, or to any surface the gas touches.
Particles hit the container walls more times each second. What happens to the pressure?
The total outward force grows, so the pressure is greater.
A balloon of gas stretches and grows larger. How often is each square millimetre of its inner surface hit?
Less often than before.
Why does giving a fixed mass of gas a larger volume at constant temperature lower its pressure?
Each unit area of wall is hit less often, so the pushes on each bit of wall add up to less.
What happens to the pressure when a gas is squeezed into less space?
Its particles are packed more tightly and its pressure rises.
What can compress or expand a gas?
A change in pressure.
Write the equation linking the pressure and volume of a gas.
p V = constant (pressure × volume = constant).
Which two conditions must hold for p V = constant?
A fixed mass of gas, held at a constant temperature.
What units do p and V take in p V = constant?
p in pascals (Pa); V in metres cubed (m³).
How do you find a new pressure or volume with p V = constant?
Work out p × V before the change, set it equal to p × V after, then divide by the quantity you know.

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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How this lesson was checked. This AQA GCSE Physics (specification 8463)lesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 24 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.

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