Module 3
Electricity & magnetism
Quick-reference revision notes for parents.
3.1 Charging up (static electricity)
- Rubbing two insulators transfers electrons from one to the other.
- The object that gains electrons becomes negatively charged; the one that loses electrons becomes positively charged.
- Like charges repel; unlike charges attract.
3.2 Circuits and current
- Current = the flow of electrons around a circuit. Measured in amperes (A) with an ammeter connected in series.
- A circuit must be complete for current to flow.
3.3 Potential difference (voltage)
- Potential difference (p.d.) = the energy given to each unit of charge by the cell. Measured in volts (V) with a voltmeter connected in parallel across a component.
3.4 Resistance
How hard it is for current to flow. Measured in ohms (Ω).
resistance = voltage ÷ current (R = V ÷ I)
Worked example
A bulb has 6V across it and a current of 2A. Find its resistance.
R = 6 ÷ 2 = 3 Ω
3.5 Changing the subject
Rearrange the formula depending on what's missing:
- V = I × R
- I = V ÷ R
- R = V ÷ I
3.6 Series and parallel
| Series | Parallel | |
|---|---|---|
| Components arranged | One after another in a single loop | On separate branches |
| Current | Same everywhere | Splits between branches; total = sum of branch currents |
| Voltage | Shared between components (sums to battery V) | Same across each branch (= battery V) |
| If one bulb breaks | All go out | Other branches keep working |
Why house wiring is parallel
Each appliance gets the full mains voltage, and switching one off doesn't kill the others.
3.7 Magnets and magnetic field
- Magnets have a north and a south pole. Like poles repel, unlike attract.
- A magnetic field is the region around a magnet where its force acts. Drawn with field lines from N to S.
- Magnetic materials: iron, nickel, cobalt and steel. Aluminium and copper are not magnetic.
3.8 Electromagnets
A current flowing through a coil of wire produces a magnetic field. Wrap the coil around an iron core for a stronger magnet — a solenoid.
To make a stronger electromagnet:
- More turns on the coil
- Bigger current
- Use a soft iron core
3.9 Using electromagnets
- Scrap-yard cranes: pick up cars (switch off to release).
- Electric bells, relays, motors, loudspeakers all use electromagnets.
- Advantage over ordinary magnets: can be turned on and off, and the strength can be controlled.
Quick reference
- V = I × R (volts = amps × ohms)
- Ammeter in series; voltmeter in parallel
- Series: current same everywhere, voltage shared
- Parallel: voltage same on each branch, current shared
- Magnetic materials: iron, nickel, cobalt, steel
- Electromagnet stronger with more turns, more current, iron core