Knowledge Organiser: Magnetic Fields and Electromagnets
Key Terms
- Magnetic field: region where magnetic material feels a force
- Permanent magnet: always magnetic (steel)
- Induced magnet: temporarily magnetic (iron)
- Electromagnet: current in coil creates magnetic field
- Magnetic flux density (B): field strength in Tesla (T)
Key Facts
- Field lines: N to S outside, arrows, closer = stronger, never cross
- Like poles repel; unlike poles attract
- Induced magnets ALWAYS attract to permanent magnets
- Soft iron core: loses magnetism when current stops
- Steel core: retains magnetism (hard magnetic material)
Electromagnet Strength
- Increase current (I)
- Increase number of turns/coils
- Add soft iron core
- Uses: cranes, MRI, relays, bells, loudspeakers
Exam Tips
- Always add arrows to field lines (N to S outside)
- Iron core NOT steel — must be able to switch off
- Induced magnets cannot be repelled
- Cutting a magnet makes two smaller magnets (no monopoles)
Key Equations
- Closer field lines = stronger magnetic field
- Electromagnet strength ∝ current × number of turns
- Field direction: right-hand rule for a solenoid (thumb = N pole direction)
- Magnetic force on a conductor: F = B × I × L (Higher tier)
Common Mistakes
- Drawing field lines without arrows: Magnetic field lines must have arrows showing direction from North to South pole outside the magnet — missing arrows loses marks
- Using steel instead of iron for electromagnet cores: Iron is used because it is a temporary (soft) magnetic material — it magnetises and demagnetises easily; steel retains magnetism and cannot be switched off
- Saying induced magnets can be repelled: Induced magnets always have the opposite pole facing the permanent magnet — they are always attracted, never repelled
- Confusing permanent and induced magnets: Permanent magnets retain their magnetism; induced magnets only become magnetic when near a magnet and lose it when removed
- Thinking cutting a magnet removes a pole: Cutting a bar magnet creates two complete magnets each with a North and South pole — isolated magnetic monopoles do not exist
Memory Aid
- North to South (direction outside)
- Arrows show N-pole movement
- Closer = stronger
- Can't cross
- Current (increase it)
- Iron core (add one)
- Turns (add more)