A technician tests a small electrode in an electrostatic sorting machine. The electrode is small enough to be treated as a point charge.
The electrode is given a positive charge. A small positively charged bead and a neutral metal foil strip are tested separately at different distances from the electrode. At each distance, the force on the bead is away from the electrode and the foil bends towards the electrode.
| Distance from electrode (cm) | Force on positive bead (mN) | Displacement of end of foil towards electrode (mm) |
|---|---|---|
| 2 | 18 | 12 |
| 4 | 5.0 | 4 |
| 6 | 2.0 | 1 |
The electrode is then given the same amount of negative charge.
Jaya predicts: "At 4 cm, the force on the positive bead will still be 5.0 mN and away from the electrode. The foil will still be attracted because its electrons always shift towards a charged object."
Kian predicts: "At 4 cm, the force on the positive bead will be 5.0 mN towards the electrode. The foil will be repelled because its electrons shift away from the negative electrode."
Evaluate the two predictions using the data and the electric-field model. Include what will happen to the foil when the electrode is discharged.
80 exam-style questions on Edexcel GCSE Physics 11.2 Uses, dangers and electric fields, covering 11.2.1 Uses of electrostatic charge, 11.2.2 Dangers of sparking, 11.2.3 Electric fields, and 11.2.4 Electric fields and static electricity. Each one has a worked solution and a mark scheme showing where the marks go.