Magnets and Magnetic Fields

Permanent Magnets

• Produce their own magnetic field
• Always have a north pole and a south pole
• Made from magnetic materials (iron, steel, cobalt, nickel) or alloys (neodymium, alnico)

Induced Magnets

• Become magnetic only when placed in a magnetic field
• Lose their magnetism when removed from the field (temporary)
• Always attracted to the permanent magnet (regardless of pole nearby)

Magnetic Materials

• Iron, steel, cobalt, nickel — attracted to magnets
• Iron is a soft magnetic material (easy to magnetise and demagnetise — used in electromagnets)
• Steel is a hard magnetic material (hard to magnetise but keeps magnetism — used in permanent magnets)

Like poles repel: N-N or S-S → push apart Unlike poles attract: N-S → pull together

This is similar to electric charges: like repels, unlike attracts.

You cannot have a magnetic monopole — if you cut a magnet in half, you get two smaller magnets, each with a N and S pole.

A magnetic field is the region around a magnet where a magnetic material or another magnet experiences a force.

Representing Magnetic Fields

Magnetic field lines:

• Go from north to south (outside the magnet)
• Never cross
• Closer together = stronger field
• Further apart = weaker field

Field of a Bar Magnet

• Lines emerge from the north pole and curve around to the south pole
• Strongest at the poles (lines closest together)
• Inside the magnet, lines go from south to north

Plotting Magnetic Field Lines

1. Place a bar magnet on paper
2. Place a small compass near one pole
3. Mark where the compass needle points
4. Move the compass to the new mark and repeat
5. Join the marks to form field lines

Uniform Magnetic Field

Between two flat magnets facing each other (opposite poles), the field lines are parallel and evenly spaced — this is a uniform field.

• The Earth behaves like a giant bar magnet
• The geographic North Pole is near a magnetic south pole (which is why the north-seeking pole of a compass points towards geographic north)
• The Earth's magnetic field is used for navigation (compasses)
• It also protects us from charged particles from the Sun (solar wind)

An electromagnet is a magnet created by passing electric current through a coil of wire (solenoid), usually wrapped around an iron core.

Advantages over Permanent Magnets

• Can be switched on and off
• Strength can be varied (by changing current)
• Polarity can be reversed (by reversing current direction)

Increasing Electromagnet Strength

• Increase the current
• Increase the number of turns on the coil
• Use an iron core (soft iron is easily magnetised)

Applications

• Scrapyard cranes (lift and release metal)
• Electric bells and buzzers
• Relay switches
• MRI machines
• Magnetic locks

• Magnets have N and S poles; like repels, unlike attracts
• Magnetic field lines go from N to S; closer = stronger
• Induced magnets are temporary; permanent magnets retain magnetism
• Electromagnets: current through a coil; controllable, switchable
• Strength: more current, more turns, iron core
• Earth's magnetic field enables compass navigation