Magnetism & Electromagnetism

Magnets and Magnetic Poles

A magnet is a material that produces a magnetic field. Every magnet has two ends called poles: a north pole (often shown red) and a south pole (often shown blue). The poles are where the magnetic effect is strongest.

The basic rule of magnetism:

Like poles repel (N–N or S–S push apart)
Unlike poles attract (N–S pull together)

A magnetic pole always comes as part of a pair. If you cut a bar magnet in half, you do not get a separate north and a separate south — you get two smaller magnets, each with its own N and S pole.

Key terms

Magnetic field — the region around a magnet where a magnetic force acts on another magnet or on a magnetic material.

Magnetic field lines — lines that show the direction and strength of a magnetic field. They run from N to S outside the magnet, and they never cross.

Magnetic Fields and Field Lines

We picture a field using field lines. The rules for drawing them:

1. Outside a magnet, field lines point from the north pole to the south pole.
2. The closer the lines, the stronger the field.
3. Arrows show the direction a free north pole would move.
4. Field lines never cross.

For a single bar magnet the lines loop out of the N pole and curve round into the S pole. Between two unlike poles (N facing S) the lines run straight across the gap — a roughly uniform field. Between two like poles the lines push away from each other, leaving a weak point (a neutral point) in the middle where the fields cancel.

Permanent and Induced Magnets

A permanent magnet keeps its magnetism on its own — for example a steel bar magnet. The four common magnetic materials are iron, steel, cobalt and nickel.

An induced magnet is a piece of magnetic material that becomes a magnet only while it is placed in a magnetic field. This is why a permanent magnet can pick up a steel paperclip: the paperclip is induced into a magnet, so its near end becomes an opposite pole and is attracted. When the magnet is removed, an induced magnet quickly loses most of its magnetism.

Watch out

A magnet always attracts an unmagnetised magnetic material — never repels it. Repulsion is the true test that something is itself a permanent magnet. Attraction alone is not enough.

The Field Around a Current-Carrying Wire

In 1820 it was discovered that an electric current produces a magnetic field. Around a straight wire the field forms concentric circles centred on the wire. The field gets weaker further from the wire.

You can find the direction with the right-hand grip rule: point your right thumb along the conventional current (+ to –) and your curled fingers show the direction the field circles the wire.

The Field Around a Solenoid

A solenoid is a coil of wire. When current flows, the circular fields from each turn add together to give a field exactly like that of a bar magnet — straight and strong inside the coil, looping out of one end (the N pole) and into the other (the S pole).

You can make this field stronger by:

increasing the current
adding more turns of wire
placing a soft iron core inside the coil

Key terms

Solenoid — a long coil of wire that produces a magnetic field like a bar magnet when current flows.

Electromagnet — a solenoid wound on a soft iron core; its magnetism can be switched on and off with the current.

Electromagnets and Their Uses

Because an electromagnet can be switched on and off and its strength controlled, it is far more useful than a permanent magnet in many devices:

Scrapyard crane — switch on to lift cars, switch off to drop them.
Electric bell — the electromagnet pulls an iron armature; a striker hits the gong and breaks the circuit, so it pulls back and the cycle repeats.
Relay — a small current energises an electromagnet that closes the contacts of a separate, larger circuit. This lets a low, safe current switch a high-power circuit.

The Motor Effect

When a current-carrying conductor sits in a magnetic field, the field of the magnet and the field of the wire interact and the wire experiences a force. This is the motor effect.

The force is larger when you increase the current, increase the strength of the magnetic field, or increase the length of wire in the field. The force is greatest when the wire is at 90° to the field, and zero when the wire is parallel to the field. Reversing the current or reversing the field reverses the direction of the force.

Fleming's Left-Hand Rule

To find the direction of the force, use Fleming's left-hand rule. Hold the thumb and first two fingers of your left hand at right angles:

First finger → Field (N to S)
seCond finger → Current (conventional, + to –)
thuMb → Motion (force)

The d.c. Motor

A d.c. motor uses the motor effect to spin a coil. A rectangular coil sits between the poles of a magnet. Current flows up one side and down the other, so by the left-hand rule one side feels an upward force and the other a downward force. This pair of forces (a turning effect) makes the coil rotate.

A split-ring commutator swaps the current direction in the coil every half turn. This keeps the force on each side pushing the same way round, so the coil keeps spinning. Carbon brushes press on the commutator to feed in the current. To make a motor turn faster: increase the current, increase the magnetic field strength, or add more turns to the coil.

Electromagnetic Induction

The motor effect run in reverse gives electromagnetic induction: moving a wire through a magnetic field (or moving a magnet near a coil) induces a voltage (an e.m.f.) across the wire. If the circuit is complete, an induced current flows.

A voltage is induced only while the wire cuts through field lines. Hold the wire still and nothing happens.

The induced voltage is bigger when you:

move the wire or magnet faster
use a stronger magnet
use more turns of wire on the coil

Reversing the direction of movement, or the poles of the magnet, reverses the direction of the induced current.

The a.c. Generator

An a.c. generator (alternator) is a coil spun inside a magnetic field. As the coil turns, its sides cut field lines and induce a voltage. Each half turn the sides swap over, so the voltage reverses direction — this gives alternating current (a.c.). The coil connects to the outside circuit through slip rings (not a commutator). The output voltage is greatest when the coil moves parallel to nothing — i.e. cutting lines fastest — and zero when it moves along the field lines.

Transformers

A transformer changes the size of an a.c. voltage. It has two coils wound on a soft iron core:

the primary coil (input), with $N_p$ turns
the secondary coil (output), with $N_s$ turns

A.c. in the primary makes a constantly changing magnetic field in the iron core. This changing field passes through the secondary and induces an alternating voltage in it. (Transformers only work with a.c. — d.c. gives a steady field that induces nothing.)

The voltages and turns are linked by the transformer equation:

$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$

Step-up transformer: more turns on secondary, so $V_s > V_p$.
Step-down transformer: fewer turns on secondary, so $V_s < V_p$.

Worked example

A transformer has 2000 turns on its primary coil and 50 turns on its secondary. The primary is connected to a 230 V a.c. mains supply. Find the secondary voltage.

Rearrange the transformer equation for $V_s$:

$V_s = V_p \times \dfrac{N_s}{N_p}$

Transformers and the National Grid

Power stations send electricity across the country through the National Grid. The power transferred is $P = VI$, and the energy wasted heating the cables depends on $P_{loss} = I^2R$.

To cut these losses, step-up transformers raise the voltage to very high values (e.g. 400 000 V) before transmission. For the same power, a higher voltage means a smaller current, and because losses depend on $I^2$, a smaller current means far less energy wasted as heat.

Near homes, step-down transformers bring the voltage back down to a safe 230 V for use.

Exam tip

Link your answer to the equation: high voltage → low current → less power lost as heat (since $P_{loss} = I^2R$). Always say transformers need a.c. because they rely on a changing magnetic field. State whether each transformer is step-up or step-down.