Describe magnetic fields from magnets and currents and represent them with field lines.

What this dot point is asking

This dot point describes where magnetic fields come from and how to represent them, the groundwork for the motor effect and induction.

Poles and field lines

Every magnet has two poles, north and south. Like poles repel and unlike poles attract. A magnetic field is represented by field lines that, by convention, point from the north pole to the south pole outside the magnet and continue from south to north inside it, forming closed loops. The lines never cross, and where they bunch together the field is stronger.

A compass needle is a small magnet that aligns with the field, so the direction a compass north points defines the field direction at that spot.

Magnetic materials

At a deeper level, magnetism comes from the motion of charges, specifically the spin and orbital motion of electrons. In most materials these effects cancel, but in ferromagnetic materials like iron, regions called domains can align to give a net field.

• A permanent magnet keeps its alignment, producing a steady field.
• A temporary magnet, such as soft iron, becomes magnetised only while in another field and loses it afterward.
• An electromagnet is a coil of wire whose magnetism can be switched on and off with the current, and strengthened with an iron core.

Fields from currents

A straight current-carrying wire produces a circular magnetic field around it. The right-hand grip rule gives the direction: point your right thumb along the conventional current and your curled fingers show the field looping around the wire.

A coil or solenoid carrying current produces a field very like a bar magnet, with a north and south pole at its ends. A second right-hand rule applies: curl your fingers in the direction of the current around the loops and your thumb points to the solenoid's north pole. Adding an iron core concentrates the field, making a strong electromagnet used in motors, relays and lifting magnets.

Earth's magnetic field

Earth behaves like a giant bar magnet tilted slightly from its spin axis. A compass aligns horizontally with this field, but a needle free to rotate vertically also tilts downward; the angle below the horizontal is the angle of dip. The dip is near zero at the equator and close to vertical near the magnetic poles, which tells us the field has a vertical component that varies with latitude.

In the exam, use the right-hand grip rule for the field around a wire and the coil version for a solenoid. When describing Earth's field, mention both the horizontal compass direction and the angle of dip, which shows the field is genuinely three dimensional.