Generators & Transformers

Generators & Transformers ⚡

You already know a current makes a magnetic field. The reverse is just as powerful: **move a magnet near a coil and you generate electricity**. This one idea — the **generator effect** — runs power stations, microphones and the whole National Grid. (Higher tier.)

Inducing a potential difference 🧲

When the **magnetic field through a coil changes** — by moving a magnet in or out, or moving a wire through a field — a **potential difference (p.d.) is induced** across the coil. If the circuit is complete, an **induced current** flows. Crucially, the field must be **changing**: reverse the movement and the induced p.d. reverses too.

A bigger induced p.d.

Select ALL THREE changes that increase the size of the induced potential difference.

• Moving the magnet faster
• Using a stronger magnet
• Using more turns on the coil
• Holding the magnet still inside the coil
• Using fewer turns on the coil
• Using a weaker magnet

Magnet at rest

A bar magnet is held completely still inside a coil of wire. What potential difference is induced?

• None — a p.d. is only induced when the field through the coil is changing
• A large, steady potential difference
• An alternating potential difference
• A potential difference that grows over time

Alternators and dynamos 🔁

Spin a coil in a magnetic field and you have a **generator**. There are two kinds: • An **alternator** uses **slip rings** and generates **alternating current (a.c.)**. • A **dynamo** uses a **split-ring commutator** and generates **direct current (d.c.)** — the same part as a motor, but here it flips the output to keep it one-way.

Match each part to what it does

Inside a microphone 🎤

A **moving-coil microphone** uses the generator effect in reverse of a loudspeaker. Sound waves hit a **diaphragm** attached to a **coil** that sits in a magnetic field. As the sound makes the coil **vibrate**, the changing field induces a **varying p.d.** that matches the pattern of the sound wave — turning sound into an electrical signal.

Sound into signal

How does a moving-coil microphone turn sound into an electrical signal?

• Sound vibrates a diaphragm and coil in a magnetic field, inducing a varying p.d. that matches the sound
• Sound heats a wire, changing its resistance
• Sound charges a small battery inside the microphone
• Sound is reflected off a magnet to make a current

Transformers 🔌

A **transformer** changes the size of an **alternating** voltage. It has two coils — a **primary** and a **secondary** — wound on an **iron core**. The alternating current in the primary makes a **changing magnetic field** in the core, which induces a p.d. in the secondary. It only works on **a.c.** — a steady d.c. gives no changing field.

Label the transformer

An interactive activity.

The transformer equations 📐

Two given equations describe an ideal transformer: **Vp / Vs = np / ns** — the voltages are in the same ratio as the numbers of turns. **Vp Ip = Vs Is** — power in = power out (no losses). More turns on the secondary (**step-up**) raises the voltage; fewer (**step-down**) lowers it.

Find the secondary voltage

An interactive activity.

Find the secondary current

An interactive activity.

Across the National Grid

An interactive activity.

Grid summary

Moving a magnet near a coil, or changing the magnetic field through it, _____ a potential difference — the generator effect. A transformer uses two coils on an iron _____ to change an alternating voltage; a _____-up transformer has more turns on the secondary. The National Grid transmits power at high voltage so the current is _____, reducing energy wasted as heat in the cables.