Electromagnets

IGCSE Edexcel Physics
6.8–6.14 Current-carrying conductors, solenoids and electromagnetic applications
Key Concepts: A current-carrying wire produces a magnetic field around it. In a solenoid, the field is like a bar magnet — strongest inside and along the axis. Field strength increases with greater current, more turns, or an iron core. Applications include doorbells, relays and circuit breakers.

Section A — Field Around a Current-Carrying Wire

1. Describe the shape of the magnetic field around a long, straight current-carrying wire. State the rule used to find the direction of the field. [3]
2. State two ways to increase the strength of the magnetic field around a straight wire. [2]

Section B — Solenoids

3. Describe the magnetic field inside and outside a solenoid carrying a current. [3]
4. State three ways to increase the strength of an electromagnet (solenoid). [3]
5. Explain why an iron core makes an electromagnet stronger than an air-core solenoid. [2]

Section C — Applications of Electromagnets

6. Describe how a relay works. Include what happens when current flows through the electromagnet coil. [3]
7. Explain one advantage of using a relay to switch a high-current circuit using a low-current control circuit. [2]
8. Describe how a circuit breaker uses an electromagnet to protect a circuit. [3]

Total marks: 21

Mark Scheme

1. Circular field lines centred on the wire [1]; right-hand grip rule — thumb points in direction of current, fingers curl in direction of field [2] [3]
2. Increase the current [1]; move closer to the wire [1] [2]
3. Inside: nearly uniform field running along the axis (like inside a bar magnet) [1]; field lines are parallel and evenly spaced inside [1]; outside: field pattern is like a bar magnet with N and S poles at the ends [1] [3]
4. Any three: increase the current [1]; increase the number of turns [1]; add an iron core [1]; reduce the length of the solenoid [1] [3]
5. Iron becomes magnetised inside the solenoid [1]; this greatly amplifies the total magnetic field [1] [2]
6. Current flows through the electromagnet coil, creating a magnetic field [1]; the iron core becomes magnetised and attracts a soft iron armature [1]; the armature pivots and closes (or opens) a switch in the output circuit [1] [3]
7. The control circuit carries only a small current [1]; the operator or low-power device is isolated from the high-current circuit, making it safer [1] [2]
8. When current exceeds safe level, the electromagnet becomes strong enough to attract a soft iron strip [1]; this releases a spring-loaded switch, breaking the circuit [1]; the switch can be reset once the fault is fixed [1] [3]