Electromagnets and Motors: KS3 Physics Guide

An electromagnet is a magnet created by passing electric current through a coil of wire. Unlike a permanent magnet, an electromagnet can be switched on and off, and its strength can be controlled. This makes electromagnets enormously useful in everyday devices, including electric motors.

What is an electromagnet?

When an electric current flows through a wire, it creates a magnetic field around the wire. If the wire is wound into a coil (called a solenoid), the magnetic fields from each loop reinforce each other to create a much stronger overall field.

Adding a soft-iron core inside the coil concentrates the field further, creating a powerful electromagnet:

Components of a simple electromagnet:

A coil of insulated wire (the solenoid)

A soft-iron core inside the coil

A power source (battery or power supply)

A switch (to turn it on and off)

When the switch is open (no current), the iron core loses its magnetism almost immediately — soft iron is easily magnetised and demagnetised. This is the crucial difference from a permanent magnet (like a steel bar magnet), which remains magnetised.

The magnetic field of a solenoid

The magnetic field pattern of a solenoid is similar to that of a bar magnet:

Field lines run from a south pole (S) at one end of the coil to a north pole (N) at the other end, outside the coil.

Inside the coil, field lines run from S to N — in the opposite direction.

The field is strongest and most uniform inside the coil.

Which end is north? Use the right-hand rule: wrap your right hand around the coil with your fingers pointing in the direction the current flows (conventional current = positive terminal to negative). Your thumb points toward the north pole.

Factors that affect the strength of an electromagnet

Factor | How it affects strength
Number of coil turns | More turns → stronger field (each loop adds to the total)
Current size | Larger current → stronger field
Core material | Soft iron core much stronger than air core; air core stronger than no core
Core length and area | Shorter, thicker core concentrates the field more effectively

Worked example: investigation

A student tests how the number of turns affects the strength of an electromagnet by counting how many steel paperclips it can hold.

Number of turns | Paperclips held
10 | 3
20 | 6
30 | 9
40 | 12

Conclusion: Doubling the number of turns doubles the number of paperclips held — the relationship is directly proportional (as long as current and core remain constant).

Control variables: current (kept constant using a fixed resistor or fixed battery), core material (same iron bolt), wire gauge (same wire throughout).

Uses of electromagnets

Electromagnets are preferred over permanent magnets whenever:

The magnetic field needs to be switched on and off (e.g. MRI scanners switch field pulses; scrap-yard cranes release steel by cutting the current).

The strength needs to be varied (e.g. loudspeakers adjust field for different volumes).

Device | How the electromagnet is used
Scrap-yard crane | Lifts steel; releases it when current is off
Electric bell | Electromagnet pulls the hammer, breaks the circuit, releases the hammer, circuit closes again — rapid ringing
Loudspeaker | Varying current creates varying force on a coil attached to a cone
MRI scanner | Very powerful electromagnets align hydrogen nuclei in the body
Maglev trains | Electromagnets in the track and train repel each other for levitation

The motor effect

When a current-carrying conductor is placed inside a magnetic field, it experiences a force. This is the motor effect (also called the electromagnetic force or Lorentz force).

The direction of the force depends on:

The direction of the current

The direction of the magnetic field

These three directions — current, field, and force — are always mutually perpendicular (at 90° to each other).

Fleming's left-hand rule helps predict the direction of the force:

First finger = direction of the magnetic Field (N to S)

sEcond finger = direction of the Electric current (conventional, + to −)

thuMb = direction of the Motion (force on the conductor)

Hold the left hand with thumb, first finger, and second finger at right angles. Point each in the given direction and the third gives the unknown.

How a simple DC electric motor works

A DC motor converts electrical energy into rotational kinetic energy using the motor effect.

Key components:

A rectangular coil (armature) that can rotate freely on an axle

A permanent magnet (or electromagnets) providing the external magnetic field

A commutator — a split-ring device that reverses the current direction every half turn

Brushes — carbon contacts that maintain electrical connection with the spinning commutator

How it rotates

Current flows through the coil in the external magnetic field.

The motor effect creates forces on the two sides of the coil that carry current in opposite directions (one side carries current left-to-right, the other right-to-left).

These opposite forces create a turning effect (torque) that spins the coil.

After half a turn, without the commutator, the forces would reverse and stop the coil. The commutator reverses the current every half turn, keeping the forces in the same rotational direction.

The coil spins continuously.

Increasing the speed and force of a DC motor:

Increase the current

Increase the number of coil turns

Use stronger magnets

Add an iron core inside the coil

Electromagnetic induction (link to generators)

The motor effect can be reversed: moving a conductor through a magnetic field (or changing the magnetic field near a conductor) induces a voltage and, if the circuit is complete, a current. This is electromagnetic induction, and it is the principle behind generators and transformers — the opposite process to a motor.

According to the Department for Education's KS3 Science Programme of Study, pupils should be taught about magnets and the magnetic effect of a current, including electromagnets and their uses, and the relationship between electricity and magnetism. BBC Bitesize KS3 Physics covers electromagnets, factors affecting strength, and simple motor construction as core Year 8 and Year 9 content.

Frequently asked questions

Why is soft iron used as the core, not steel?

Soft iron is easily magnetised when current flows through the coil and loses its magnetism almost immediately when the current is switched off. Steel, by contrast, retains its magnetism (it becomes a permanent magnet), which would prevent the electromagnet from being reliably turned off. Soft iron is the ideal core material for any electromagnet that needs to be switched.

How does reversing the current change an electromagnet?

Reversing the direction of the current reverses which end of the solenoid is the north pole and which is the south pole. The field lines reverse direction. This is the principle behind AC-powered electromagnets and is important in motors (where the commutator effectively reverses current direction to maintain rotation) and in generators.

What is the difference between a motor and a generator?

A motor converts electrical energy into kinetic energy — a current-carrying coil in a magnetic field spins. A generator converts kinetic energy into electrical energy — a coil rotated in a magnetic field has a current induced in it. Both devices use the same physical relationship between electricity and magnetism; they just transfer energy in opposite directions.

Where do I encounter electromagnets in daily life?

Electromagnets are inside almost every electrical appliance that moves, makes sound, or stores data. Electric motors in hairdryers, washing machines, and electric vehicles use the motor effect. Speakers and headphones use small electromagnets to push air and create sound. Hard drives use tiny electromagnets to read and write data. MRI scanners use superconducting electromagnets to image soft tissue without X-rays.

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