A weather balloon is filled with a fixed mass of gas at ground level. As the balloon rises to a higher altitude, the surrounding air pressure decreases and the balloon expands. The temperature of the gas remains constant throughout. Explain why the pressure of the gas inside the balloon decreases as it expands.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The gas molecules are in constant random motion and collide with the walls of the balloon.
The pressure is caused by the force exerted by these collisions on the walls (per unit area).
When the volume increases, the molecules are more spread out, so the number of collisions per unit area of the wall decreases.
Fewer collisions per unit area means less force per unit area, so the pressure is lower.
The pressure is caused by the force exerted by these collisions on the walls (per unit area).
When the volume increases, the molecules are more spread out, so the number of collisions per unit area of the wall decreases.
Fewer collisions per unit area means less force per unit area, so the pressure is lower.
Examiner tips
- Use the word ‘collide’ to show understanding of kinetic theory.
- Mention that temperature is constant, so kinetic energy per molecule is unchanged.
- Show the link between volume, collision frequency and pressure.
- Use the term ‘per unit area’ to demonstrate knowledge of pressure definition.
Common mistakes
- Confusing pressure with force; forgetting to divide by area.
- Saying the pressure increases instead of decreases. Mentioning temperature change when it is constant.
Mark scheme (4 marks)
- The gas molecules are in constant random motion and collide with the walls of the balloon
- The pressure is caused by the force exerted by these collisions on the walls (per unit area)
- When the volume increases, the molecules are more spread out / the number of collisions per unit area of the wall decreases
- Fewer collisions per unit area means less force per unit area, so the pressure is lower
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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