Magnetism & ElectromagnetismOxford AQA IGCSE Physics: Revision notes

Magnetic forces are non-contact forces:

• Like poles repel (N-N or S-S)
• Unlike poles attract (N-S)

A magnetic field is the region around a magnet where a force acts on a magnetic material (iron, steel, cobalt, nickel) or another magnet.

• Field lines run from north to south outside the magnet
• Lines are closer together where the field is stronger, e.g. near the poles
• In a uniform field, the field lines are parallel and evenly spaced

When a magnetic material (such as iron) is placed in a magnetic field, it becomes a magnet itself — this is induced magnetism.

• Induced magnetism always causes attraction towards the inducing magnet, never repulsion
• When the material is removed from the field, it loses most or all of its induced magnetism (soft iron loses it almost completely; steel keeps more)

The Earth behaves as if it has a giant bar magnet inside it, with a magnetic field that is most concentrated at the magnetic north and south poles.

A plotting compass contains a small pivoted magnet that aligns itself with the Earth's magnetic field, pointing towards magnetic north — this is how compasses assist navigation.

A wire carrying a current produces a magnetic field around it:

• The field forms concentric circles perpendicular to the wire
• The field is stronger closer to the wire
• Increasing the current strengthens the field
• Reversing the current reverses the direction of the field

Winding a wire into a coil — a solenoid — makes the magnetic fields from each loop add together, producing a much stronger field than a straight wire, with a field pattern similar to a bar magnet.

Adding an iron core inside the solenoid concentrates and strengthens the field further, forming an electromagnet. Unlike a permanent magnet, an electromagnet can be switched on and off, and its strength can be varied by changing the current or the number of turns.