A weather balloon is filled with a fixed mass of gas at ground level, where the temperature is 17 °C and the pressure is high. As the balloon rises into the upper atmosphere, the surrounding pressure decreases and the temperature drops to −33 °C. Explain what happens to the volume of the gas inside the balloon as it rises, considering the effects of both the pressure change and the temperature change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Weather balloons are used to carry scientific instruments high into the atmosphere. They are only partially inflated at ground level before release.
Model answer (5 marks)
The volume of the gas increases as the balloon rises.
1. The surrounding pressure falls with altitude.
2. For a fixed mass of gas, Boyle’s law (P V = constant at constant T) means V increases when P decreases.
3. The temperature also falls from 17 °C to –33 °C.
4. From Charles’ law (V ∝ T at constant P) the lower temperature would tend to reduce the volume.
5. The pressure effect is much larger than the temperature effect, so the net result is an increase in volume – this is why balloons are only partially inflated at launch.
1. The surrounding pressure falls with altitude.
2. For a fixed mass of gas, Boyle’s law (P V = constant at constant T) means V increases when P decreases.
3. The temperature also falls from 17 °C to –33 °C.
4. From Charles’ law (V ∝ T at constant P) the lower temperature would tend to reduce the volume.
5. The pressure effect is much larger than the temperature effect, so the net result is an increase in volume – this is why balloons are only partially inflated at launch.
Examiner tips
- Use the command word ‘Explain’ – describe the two opposing effects and state which dominates.
- Show the reasoning for each effect (Boyle’s law, Charles’ law).
- Mention the net result clearly.
- Use correct UK spelling and terminology (e.g. ‘Boyle’s law’, ‘Charles’ law’).
Common mistakes
- Confusing the direction of change for pressure and volume – writing that volume decreases when pressure decreases.
- Forgetting to mention that the temperature effect is smaller and does not outweigh the pressure effect.
- Using the wrong law (e.g. saying temperature increase causes volume increase without reference to pressure).
Mark scheme (5 marks)
- As pressure decreases (at higher altitude), the volume of the gas increases
- This is because pressure and volume are inversely related (for a fixed mass of gas at constant temperature)
- As temperature decreases, the volume of the gas decreases
- This is because at lower temperatures the gas particles have less kinetic energy / move more slowly, so they exert less pressure on the walls, reducing the volume
- Overall the volume increases because the effect of the large pressure decrease outweighs the effect of the temperature decrease (which is why balloons expand / are only partially inflated at launch)
Key terms in this question
pressure · volume · temperature · fixed mass of gas
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Thermal physics questions
- A bicycle pump contains a fixed mass of air. A cyclist places their thumb firmly…
- A student fills a rigid metal container with air and seals it. The container is …
- A sealed syringe contains a fixed mass of gas at room temperature. The plunger i…
- A sealed metal can contains a fixed amount of gas. The can is placed in a freeze…