Magnetism & Electromagnetism

Magnets

A magnet has two poles: north (N) and south (S). Unlike poles attract each other, whilst like poles repel.

Only magnetic materials such as iron, steel, nickel and cobalt can be magnetised or attracted to a magnet.

Magnetic field lines

A magnetic field is the region around a magnet where a magnetic force is felt. We represent it using field lines.

• Field lines go from north to south.
• The direction of the lines shows the direction of the force on a north pole placed in the field.
• Where the lines are closer together, the field is stronger.

Magnetic fields from electric currents

A current-carrying wire produces a magnetic field made up of concentric circles around the wire.

The right-hand grip rule

Use the right-hand grip rule to find the direction of the field around a straight wire:

• Point your thumb in the direction of the current.
• Your curled fingers show the direction of the magnetic field.

Solenoids

A solenoid is a coil of wire carrying a current. The magnetic field around a solenoid is similar to the field around a bar magnet, with a distinct north and south pole at each end.

Electromagnets

An electromagnet is a solenoid with a soft iron core inside. It is a temporary magnet that can be switched on and off by controlling the current.

An electromagnet becomes stronger when you:

• increase the number of turns of wire on the coil
• increase the current
• place an iron core inside the coil

The motor effect

When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is called the motor effect.

The size of the force depends on the magnetic flux density ($B$), the current ($I$) and the length of wire ($l$) in the field:

$F=BIl$

where $F$ is force in newtons (N), $B$ is magnetic flux density in tesla (T), $I$ is current in amperes (A), and $l$ is length in metres (m).

Fleming's left-hand rule

Use Fleming's left-hand rule to find the direction of the force on a wire in a magnetic field:

• First finger – magnetic field, from north to south
• Second finger – current
• Thumb – force (motion)

The three fingers must be held at right angles to one another.

Electric motors

An electric motor uses the motor effect to produce rotation.

• Current flows through a coil placed in a magnetic field.
• Forces on opposite sides of the coil act in opposite directions, creating a turning effect.
• A split-ring commutator reverses the direction of the current in the coil every half turn, so the coil keeps rotating in the same direction.

Electromagnetic induction

Electromagnetic induction is the process of creating a potential difference (voltage) across a conductor when there is a change in magnetic field through a coil.

A voltage is induced when you:

• move a magnet into or out of a coil
• move a coil relative to a magnetic field
• change the current in a nearby coil (for example, in a transformer)

The induced voltage increases when the magnetic field changes more quickly.

Generators

A generator uses electromagnetic induction to produce electricity. It works as the opposite of a motor.

• A coil rotates in a magnetic field.
• The changing magnetic field through the coil induces a voltage.
• In a simple dynamo, the output is alternating current (AC) – the voltage repeatedly changes direction.

Transformers (Higher tier)

A transformer changes the size of an alternating voltage using electromagnetic induction. It has two coils wrapped around a laminated iron core:

• the primary coil – connected to the input voltage
• the secondary coil – connected to the output voltage

Transformers only work with AC, because a changing current in the primary coil produces a changing magnetic field in the core, which induces a voltage in the secondary coil.

The voltage ratio equals the turns ratio:

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

where $V_p$ and $V_s$ are the primary and secondary voltages, and $N_p$ and $N_s$ are the number of turns on each coil.

• A step-up transformer has more turns on the secondary coil, so the output voltage is higher.
• A step-down transformer has fewer turns on the secondary coil, so the output voltage is lower.

Putting it together

• Motor effect – magnetic fields and current produce a force.
• Electromagnetic induction – a changing magnetic field produces a voltage.
• Motors – electrical energy to motion.
• Generators – motion to electrical energy.