Two low-voltage power units, P and Q, are tested before one is used to charge a rechargeable cell.
For each unit, a data logger samples the potential difference of terminal X relative to terminal Y throughout a test. Each output pattern repeats during the test. The table gives the range of all the readings.
| Power unit | Lowest potential difference in V\mathrm{V}V | Highest potential difference in V\mathrm{V}V |
|---|---|---|
| P | +3.5 | +4.3 |
| Q | -4.1 | +4.1 |
Explain which power unit should be used to charge the cell.
The chosen power unit is connected correctly to the cell. Terminals X and Y are then swapped over.
State the effect this has on the direction of the conventional current in the cell.
81 exam-style questions on Edexcel GCSE Physics 10.4 Mains electricity, covering 10.4.1 Direct and alternating current, 10.4.2 Live, neutral and earth wires; safety, 10.4.3 Mains wire potential differences and dangers, and 10.4.4 Power ratings and energy use. Each one has a worked solution and a mark scheme showing where the marks go.