Magnetism and Electromagnetism
Magnetic fields, electromagnets, motors, and generators.
In this topic
Magnetic Fields
A magnetic field is a region where a magnetic material experiences a force. Magnetic field lines go from North to South.
Permanent magnets: always produce a magnetic field (e.g., bar magnet, horseshoe magnet). Induced magnets: become magnetic when placed in a magnetic field, lose magnetism when removed.
Electromagnets: a coil of wire (solenoid) with current flowing through it produces a magnetic field. The field can be made stronger by: - Increasing the current - Adding more turns to the coil - Adding an iron core
The motor effect: a current-carrying wire in a magnetic field experiences a force. Force = magnetic flux density × current × length (F = BIl) Direction given by Fleming's left-hand rule.
Key Points
- Like poles repel, opposite poles attract
- Field lines are closer together where the field is stronger
- Electromagnets can be switched on and off
- Fleming's left-hand rule: thumb=motion, first finger=field, second finger=current
Example Questions
2A wire carrying a current of 3 A has a length of 0.2 m in a magnetic field of flux density 0.5 T. Calculate the force on the wire.
F = BIl = 0.5 × 3 × 0.2 = 0.3 N
[2 marks]
3Describe three ways to increase the strength of an electromagnet.
1) Increase the current flowing through the coil. 2) Increase the number of turns/coils of wire. 3) Add a soft iron core inside the coil.
[3 marks]