A van de Graaff generator builds up a large static charge on its metal dome. A small piece of tissue paper placed near the dome is first attracted towards it, then repelled away from it after making contact. Explain why the tissue paper is first attracted and then repelled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The dome is charged and the tissue paper is initially neutral.
The charged dome induces a charge of the opposite sign on the surface of the tissue paper nearest the dome, so opposite charges attract and the paper is drawn towards the dome.
When the paper touches the dome, electrons are transferred between them. The paper now carries the same type of charge as the dome.
Like charges repel, so the paper is repelled away from the dome.
The charged dome induces a charge of the opposite sign on the surface of the tissue paper nearest the dome, so opposite charges attract and the paper is drawn towards the dome.
When the paper touches the dome, electrons are transferred between them. The paper now carries the same type of charge as the dome.
Like charges repel, so the paper is repelled away from the dome.
Examiner tips
- Use the word ‘induce’ to show understanding of charge induction.
- Show the sequence: neutral → induced opposite charge → attraction → charge transfer → like charge → repulsion.
- Include the sign of the charge transfer (electrons moving) to demonstrate depth.
Common mistakes
- Failing to mention that the paper is initially neutral.
- Confusing the direction of electron transfer or saying the paper becomes opposite to the dome.
- Omitting the step that like charges repel after contact.
Mark scheme (4 marks)
- The dome is charged and the tissue paper is initially uncharged / neutral
- Opposite charges attract, so the charged dome attracts the neutral tissue paper (accept: the charged dome induces / causes a charge on the tissue paper surface closest to it, resulting in attraction)
- On contact, electrons are transferred from the dome to the tissue paper (or from the tissue paper to the dome, depending on sign of charge), so the tissue paper gains the same type of charge as the dome
- Like charges repel, so the tissue paper is now repelled away from the dome
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Static electricity (Phys only) questions
- Explain why a plastic rod becomes negatively charged when rubbed with a cloth.
- A student rubs a perspex rod with a silk cloth. Explain why the perspex rod beco…
- Explain why a balloon that has been rubbed against a woollen jumper sticks to a …
- Explain why a spark is produced when a person who has built up a static charge r…