A student rubs a plastic rod with a woollen cloth. The plastic rod becomes negatively charged. Explain why the rod becomes negatively charged and describe what charge is left on the woollen cloth. Include in your answer what would happen if a charged plastic rod were brought close to, but not touching, a second negatively charged rod.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Electrons are transferred from the woollen cloth to the plastic rod by friction when they are rubbed together.
The plastic rod therefore gains electrons and becomes negatively charged.
The woollen cloth loses electrons, leaving it with a positive static charge.
If a second negatively charged rod is brought close to the charged plastic rod, the two rods have like charges and will repel each other.
The repulsive force is a non‑contact (electrostatic) force, felt even though the rods are not touching.
The plastic rod therefore gains electrons and becomes negatively charged.
The woollen cloth loses electrons, leaving it with a positive static charge.
If a second negatively charged rod is brought close to the charged plastic rod, the two rods have like charges and will repel each other.
The repulsive force is a non‑contact (electrostatic) force, felt even though the rods are not touching.
Examiner tips
- Use the word ‘electrons’ and ‘friction’ to show the transfer mechanism. State the charge on each object explicitly. Explain the repulsion using the concept of like charges. Mention that the force is non‑contact to cover the last point.
Common mistakes
- Saying the plastic rod becomes positively charged. Forgetting to mention the woollen cloth becomes positively charged. Not recognising that the force is non‑contact (electrostatic).
Mark scheme (5 marks)
- Electrons are transferred from the woollen cloth to the plastic rod (by friction / during rubbing)
- The plastic rod gains electrons, giving it a negative static charge
- The woollen cloth loses electrons, leaving it with a positive static charge
- The two negatively charged rods have like charges, so they repel each other
- The repulsive force is a non-contact (electrostatic) force, felt even though the rods are not touching
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →