Explain why lightning occurs and describe what happens to the charges during a lightning strike.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Thunderclouds can build up very large amounts of static charge. This can lead to a lightning strike between the cloud and the Earth.
Model answer (4 marks)
Thunderclouds become charged by friction between ice particles, which separates positive and negative charges.
The negative charge accumulates at the cloud base, creating a large potential difference with the positively charged Earth.
When the electric field exceeds the breakdown strength of air, electrons jump from the cloud to the Earth, forming a conductive channel.
The discharge neutralises the charge difference, equalising the cloud and Earth charges.
The negative charge accumulates at the cloud base, creating a large potential difference with the positively charged Earth.
When the electric field exceeds the breakdown strength of air, electrons jump from the cloud to the Earth, forming a conductive channel.
The discharge neutralises the charge difference, equalising the cloud and Earth charges.
Examiner tips
- Use the word ‘friction’ and ‘charge separation’ to show understanding of the source of charge.
- Explain the build‑up of potential difference and the breakdown of air explicitly.
- Describe the electron transfer correctly – electrons move from negative to positive.
- Show that the discharge balances the charges between cloud and Earth.
Common mistakes
- Confusing the direction of electron flow; students say electrons move from Earth to cloud.
- Omitting the role of ice particle friction in charge separation.
- Failing to mention that the discharge neutralises the charge difference.
Mark scheme (4 marks)
- Static charge builds up in the clouds (due to friction / movement of ice particles / charge separation)
- The charge difference between the clouds and the Earth becomes very large / a large potential difference builds up
- The discharge occurs when electrons jump through the air from the negatively charged region to the positively charged region (or equivalent transfer described correctly)
- The lightning strike balances / neutralises the charges (between the cloud and the Earth)
Key terms in this question
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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