Two kinds of magnet

Every magnet has a north and a south pole, and exerts a non-contact force on other magnetic materials. The rule for poles:

• Like poles repel (N–N or S–S push apart).
• Opposite poles attract (N–S pull together).

The two kinds you must distinguish:

• A permanent magnet is always magnetic and always has its own poles (e.g. a steel bar magnet).
• An induced magnet is a magnetic material that becomes a magnet only while it sits in a magnetic field. Its domains line up in the field; remove the field and a soft material loses most of its magnetism. Iron, nickel and cobalt are the common magnetic materials.

Key fact: an induced magnet always attracts the magnet inducing it — induction never produces repulsion. That is the test that tells a magnet apart from an unmagnetised lump of iron.

Permanent or induced?

The field around a bar magnet

A magnetic field is the region where a magnetic force acts. We draw it with field lines. The crucial rule:

Outside the magnet, field lines run from North to South.

You can reveal the pattern with iron filings (they line up along the lines) or trace the direction at each point with a small plotting compass. Where field lines are evenly spaced and parallel, the field is uniform.

Watch out: a very common error is drawing the arrows pointing S → N or with no arrows at all. Outside the magnet the direction is always N → S.

Reading a field

A current makes a magnetic field

When a current flows through a wire it creates a magnetic field of concentric circles around the wire. Reverse the current and the field reverses; increase the current (or move closer) and the field gets stronger.

Wind the wire into a coil (a solenoid) and the circular fields add together to give a strong, almost uniform field inside — just like a bar magnet, with a N pole at one end and a S pole at the other.

An electromagnet (a solenoid with a soft-iron core) can be switched on and off and its strength varied — used in scrapyard cranes, relays, electric bells and circuit breakers.

Which field shape?

Force on a current in a field

Put a current-carrying wire across a magnetic field and the two fields interact, pushing the wire. This is the motor effect. The size of the force is:

The force is biggest when the wire is at 90° to the field, and zero when the wire lies along (parallel to) the field. Reverse either the current or the field and the force flips direction.

Watch out: if the wire runs parallel to the field there is no force at all — the two fields do not cut across each other. Maximum force needs the wire perpendicular to the field.

Your turn — the motor effect

Finding the direction of the force

To find which way the wire is pushed, use Fleming's left-hand rule. Hold the thumb and first two fingers of your left hand at right angles to each other:

• ThuMb → Motion (the force / push on the wire)
• First finger → Field (points from N → S)
• seCond finger → Current (conventional current, + to −)

Using the rule

Turning the force into spin

A d.c. motor uses the motor effect to make a coil spin. Current flows round a coil sitting between the poles of a magnet. By Fleming's left-hand rule, one side of the coil is pushed up and the other side is pushed down — so the coil turns.

To make the motor turn faster / with more force: increase the current, use a stronger magnet, or add more turns to the coil. To reverse the direction, reverse the current or swap the magnet's poles.

Stronger turning force?

Tap the change that would make the motor's turning effect bigger in each pair.

Moving a wire generates a voltage

The motor effect run in reverse is the generator effect. If a conductor cuts through magnetic field lines — or the field through a coil changes — a potential difference is induced across it. Connect a circuit and an induced current flows.

Watch out: induction needs a changing field or relative motion. A magnet sitting still inside a coil — no matter how strong — induces no voltage. The faster the change, the bigger the induced p.d.

The size of the induced p.d. is increased by: moving faster, using a stronger magnet, and adding more turns to the coil. Reversing the motion reverses the induced p.d.

When is a voltage induced?

The a.c. generator

An alternator generates alternating current (a.c.). A coil is rotated in a magnetic field; as it turns, each side cuts field lines, inducing a p.d. Every half-turn each side swaps from moving up to moving down, so the induced p.d. reverses direction — giving a smoothly alternating output.

The induced p.d. is increased by: spinning the coil faster, using a stronger magnet, adding more turns, or adding an iron core. (A d.c. generator / dynamo uses the same idea but a split-ring commutator to give one-way d.c.)

Generator essentials

Tap a term on the left, then its correct match on the right.

Changing the size of an a.c. voltage

A transformer changes the size of an alternating voltage. Two coils are wound on a soft-iron core. The a.c. in the primary makes a changing magnetic field; the core carries it to the secondary, where a changing field induces an a.c. voltage.

The turns ratio sets the voltage ratio:

If a transformer is 100% efficient, the power in equals the power out (P = VI), so:

Watch out: a transformer only works with alternating current. A steady d.c. current makes a steady field that does not change, so nothing is induced in the secondary. Stepping the voltage up steps the current down by the same factor.

Your turn — transformer voltage

Your turn — transformer current

Why we transmit at high voltage

The National Grid carries electricity from power stations to homes. Transformers make this efficient. A step-up transformer raises the voltage to hundreds of thousands of volts for transmission; a step-down transformer lowers it again to a safe ~230 V for homes.

Key idea: for a fixed power, raising the voltage lowers the current (P = VI). The heat lost in the lines is I²R — so halving the current cuts the loss to a quarter. That is the whole reason the Grid runs at very high voltage.

Why high voltage?

Sound and the motor / generator effect

These devices are just the motor and generator effects applied to sound:

• A loudspeaker uses the motor effect. An oscillating (a.c.) current flows through a coil attached to a cone, sitting in a magnetic field. The varying current makes a varying force that pushes the cone in and out, squashing the air to make sound waves.
• A microphone uses the generator effect — the reverse. Sound waves make a diaphragm (and the coil attached to it) vibrate in a magnetic field, inducing a varying current that matches the sound. The current's frequency matches the sound's frequency.

Speaker or microphone?

The facts & equations to know

You've covered all four parts of Eduqas Topic 8 — magnets & fields, the magnetic effect of a current & the motor effect, induced potential & transformers, and microphones, speakers & oscillating currents. Press Finish to see your score.