P7.1 Permanent and Induced Magnetism, Magnetic Forces and Fields
Magnets and magnetic poles
Every magnet has a north pole and a south pole:
- Like poles repel (N–N or S–S)
- Unlike poles attract (N–S)
This is the fundamental rule of magnetism.
Permanent magnets vs induced magnets
Permanent magnets: Produce their own magnetic field at all times. Made from hard magnetic materials (iron, nickel, cobalt alloys; also steel and alnico/neodymium). The magnetic domains are permanently aligned.
Induced magnets: Become magnetised when placed in an external magnetic field (domains align temporarily). They are made from soft magnetic materials (e.g. soft iron). When the external field is removed, they quickly lose their magnetism (domains randomise).
| Permanent magnet | Induced magnet | |
|---|---|---|
| Material | Hard (steel, cobalt alloy) | Soft (iron) |
| Magnetism | Permanent | Temporary |
| Example | Fridge magnet | Electromagnet core |
Magnetic field
A magnetic field is a region around a magnet where a magnetic force is experienced. Magnetic field lines:
- Travel from North to South outside the magnet (by convention)
- Are closer together where the field is stronger (near the poles)
- Never cross each other
- Form closed loops (inside the magnet: S to N)
Drawing magnetic field patterns
Bar magnet:
- Lines emerge from N, curve around, enter at S
- Strongest at the poles (lines densest)
- Uniform field in centre if two magnets face each other
Uniform field between two opposite poles: Parallel, equally-spaced lines.
Compasses align with field lines: the N of the compass points in the direction of the field line (i.e. away from the magnet's N pole).
Attraction and repulsion — force on magnets
- Two magnets can attract iron/steel objects — by inducing temporary magnetism.
- Induced magnets always attract permanent magnets (because the near pole is always opposite).
Magnetic field lines and compasses
A plotting compass placed in a magnetic field aligns with the field at that point — the north needle points in the direction the field line points. This allows mapping of the field.
Earth's magnetic field
Earth acts as a giant magnet with field lines entering near the geographic North Pole (the magnetic south pole of Earth's core) and emerging near the geographic South Pole. A compass N needle points towards geographic north because Earth's magnetic north is actually a magnetic south pole.
Common exam errors
- Drawing field lines that cross — they never cross.
- Getting the direction of field lines wrong — N to S outside the magnet.
- Confusing permanent and induced magnets — soft iron is induced (temporary).
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