A helium weather balloon is released from the Earth's surface and rises high into the atmosphere. Explain why the balloon expands as it rises to a greater altitude.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Weather balloons are used by meteorologists to collect data about the atmosphere. They are filled with helium at the Earth's surface and released to rise through the atmosphere.
Model answer (4 marks)
Atmospheric pressure falls with altitude, so the weight of the air above the balloon decreases.
The helium inside the balloon is at a higher pressure than the surrounding air.
As the external pressure drops, the pressure difference between inside and outside reduces.
The balloon therefore expands until the internal pressure equals the external pressure, reaching equilibrium.
The helium inside the balloon is at a higher pressure than the surrounding air.
As the external pressure drops, the pressure difference between inside and outside reduces.
The balloon therefore expands until the internal pressure equals the external pressure, reaching equilibrium.
Examiner tips
- State that atmospheric pressure decreases with altitude. Explain the pressure difference between helium and air. Show the balloon expands until pressures equalise. Use correct terminology: atmospheric pressure, equilibrium, expansion.
Common mistakes
- Confusing the cause of expansion with the effect (e.g. saying the balloon expands because it is lighter). Using vague terms like "more space" instead of pressure. Forgetting to mention that equilibrium is reached when internal and external pressures are equal.
Mark scheme (4 marks)
- Atmospheric pressure decreases as the balloon rises to a greater altitude
- There are fewer air molecules above at greater altitude, so the weight of air above is less
- The pressure of the helium inside the balloon is greater than the atmospheric pressure outside
- The balloon expands until the pressure inside equals the pressure outside / until equilibrium is reached
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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