Magnetic fields and field lines
A magnetic field is the region around a magnet where it can attract or repel another magnet or a magnetic material such as iron. We cannot see it, so we draw it using field lines.
- Magnetic field lines
- Lines that show the direction a free north pole would move: they always point from the north pole of a magnet to its south pole. Where the lines are closer together, the field is stronger.
Around a single bar magnet the lines loop out of the north pole, curve round, and return to the south pole. Two like poles brought together repel, and their field lines bend away from each other; two unlike poles attract, and the lines run straight across the gap between them.
Where this is assessed
Plotting field lines with a compass and explaining electromagnet behaviour is solid Criterion A and Criterion B work. Investigating how the number of turns changes an electromagnet's strength gives clean data for Criterion C.
Electromagnets
A current flowing through a wire creates its own magnetic field in circles around the wire. Wind that wire into a coil, called a solenoid, and the fields add together to give a field just like a bar magnet, with a north and a south end.
Put a piece of iron inside the coil and the field becomes much stronger: this is an electromagnet. Its great advantage is control. You can make it stronger, weaker or switch it off entirely.
| To make an electromagnet stronger | Why it works |
|---|---|
| Increase the current | A bigger current makes a stronger field |
| Add more turns to the coil | Each turn adds to the field |
| Add a soft iron core | Iron concentrates and boosts the field |
Because it can be switched on and off, an electromagnet is used in scrapyard cranes to pick up and drop cars, and in the circuit breakers and relays that protect wiring.
The motor effect and Fleming's left-hand rule
Here is the key idea that makes electric motors possible: when a current-carrying wire sits in a magnetic field, the two fields interact and the wire feels a force. This is the motor effect.
The direction of that force is found with Fleming's left-hand rule. Hold the thumb and first two fingers of your left hand at right angles to each other:
- First finger points along the magnetic Field (north to south).
- seCond finger points along the Current (conventional, positive to negative).
- Thumb then points along the Thrust, the force on the wire.
The force is largest when the wire is at right angles to the field, and it is reversed if you reverse either the current or the field. Reversing the current every half-turn is exactly what keeps a motor spinning.
Left hand for motors
Use the left hand for the motor effect (a force on a current). Later you meet the right-hand rule for the field around a wire. Mixing them up is common, so tie "left = motor" firmly in your memory.
Electromagnetic induction and transformers
The motor effect run in reverse gives us electricity. If you move a magnet into a coil, or move a wire through a magnetic field, a voltage is induced across the wire. If the circuit is complete, that voltage drives a current. This is electromagnetic induction, and it is how generators produce mains electricity.
- Electromagnetic induction
- A voltage is induced whenever a conductor experiences a changing magnetic field. The faster the change, the stronger the induced voltage. If the field does not change, no voltage is induced.
A transformer uses induction to change the size of an alternating voltage. It has two coils wound on an iron core: an alternating current in the primary coil makes a constantly changing field, which induces a voltage in the secondary coil. The ratio of turns sets the ratio of voltages.
A transformer has 200 turns on the primary coil and 50 turns on the secondary. The primary voltage is 240 V. What is the secondary voltage?
Transformers need alternating current
A steady direct current gives a steady field, and a steady field induces nothing. Transformers only work with a changing current, which is one reason the grid uses alternating current.
Check yourself
Work through each one before opening it.
1. State two ways to increase the strength of an electromagnet. +
Any two of: increase the current, add more turns to the coil, or add a soft iron core. Each one strengthens the magnetic field the coil produces.
2. Which hand and rule gives the direction of the force on a current-carrying wire in a magnetic field, and what do the fingers represent? +
Fleming's left-hand rule. First finger is the Field, second finger is the Current, and the thumb is the Thrust (force), all at right angles.
3. A transformer has 100 turns on the primary and 400 on the secondary, with 12 V across the primary. Find the secondary voltage and say what type it is. +
\( V_s = V_p \times \dfrac{N_s}{N_p} = 12 \times \dfrac{400}{100} = 12 \times 4 = \) 48 V. Since the voltage has increased, it is a step-up transformer.
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