A thundercloud has a large negative charge on its lower surface. Explain why this causes the ground directly below the cloud to become positively charged, and describe the type of force that then acts between the cloud and the ground.

Eduqas GCSE Physics — 7.3 Static electricity – forces and electric fields · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

During a thunderstorm, charge builds up on clouds. This can cause the ground beneath to become charged in the opposite way.

Model answer (4 marks)

The negative charge on the cloud pushes electrons in the ground away from the surface, leaving the surface with a deficit of electrons and therefore a net positive charge. The cloud is negatively charged and the ground surface is positively charged, so opposite charges attract. The force between them is an attractive electrostatic force acting at a distance without contact.

Examiner tips

  • Use the word ‘repels’ for electrons and ‘deficit’ for positive charge
  • Mention ‘opposite charges attract’ to link charges to force
  • State the force is electrostatic and acts at a distance

Common mistakes

  • Saying the ground becomes negatively charged instead of positively
  • Confusing the direction of electron movement
  • Forgetting to specify that the force is electrostatic and acts without contact

Mark scheme (4 marks)

  1. The negative charge on the cloud repels electrons in the ground downwards / away from the surface
  2. This leaves the surface of the ground with fewer electrons / a deficit of electrons, making it positively charged
  3. The cloud is negatively charged and the ground surface is positively charged — opposite charges attract
  4. The force between the cloud and the ground is an attractive electrostatic force (acting at a distance / without contact)

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