In simple terms: a changing magnetic field can create electricity. It does not require a battery directly connected to the wire. What matters is that the magnetic conditions around the wire change over time.

The Basic Idea: Magnetism Meets Electricity

Faraday's discovery showed that electricity and magnetism are closely connected. A current flowing through a wire creates a magnetic field. Faraday found that the reverse is also possible: a changing magnetic field can induce a voltage in a wire.

Imagine moving a magnet toward a coil of wire. As the magnet approaches, the magnetic field passing through the coil changes. This change induces a voltage in the coil. If the coil is connected in a complete circuit, current flows.

If you move the magnet away, the magnetic field changes again, and current is induced in the opposite direction. If the magnet stays still, the magnetic field through the coil does not change, so no current is induced.

Magnetic Flux: The Key Concept

To understand induction, it helps to understand magnetic flux. Magnetic flux describes how much magnetic field passes through a given area, such as the area inside a loop of wire.

Think of magnetic field lines as invisible lines passing through a loop. Magnetic flux is related to how many of those lines pass through the loop. It depends on:

• The strength of the magnetic field.
• The area of the loop.
• The angle between the magnetic field and the loop.

You can change magnetic flux in several ways:

• Move a magnet closer to or farther from a coil.
• Change the strength of the magnetic field.
• Change the size or shape of the loop.
• Rotate the loop in a magnetic field.

When magnetic flux changes, an electromotive force, or emf, is induced in the loop. If the circuit is complete, this emf drives a current.

Faraday's Law of Induction

Faraday's law describes how much voltage is induced when magnetic flux changes. For a coil with multiple turns, the relationship is:

This equation tells us that the induced voltage is larger when:

• The change in magnetic flux is larger.
• The change happens more quickly.
• The coil has more turns.

Lenz's Law: Why the Current Opposes Change

Faraday's law includes a minus sign. That minus sign represents Lenz's law, which explains the direction of the induced current.

Lenz's law states that an induced current flows in a direction that opposes the change in magnetic flux that caused it.

Pushing the Magnet Toward the Coil

1. The magnetic flux through the coil increases.
2. The induced current creates its own magnetic field.
3. That induced field opposes the approaching magnet.
4. The side of the coil facing the magnet behaves like a north pole, pushing against the incoming north pole.

Pulling the Magnet Away

1. The magnetic flux through the coil decreases.
2. The induced current reverses direction.
3. The induced magnetic field now tries to maintain the original flux.
4. The side of the coil facing the magnet behaves like a south pole, attracting the receding north pole.

Faraday's Original Experiment

Faraday demonstrated induction using two coils wrapped around an iron ring. One coil was connected to a battery, and the other was connected to a galvanometer, an instrument that detects electric current.

When Faraday closed the switch in the first coil, current flowed and created a magnetic field. The changing magnetic field induced a brief current in the second coil, causing the galvanometer needle to move. When he opened the switch, the magnetic field collapsed, and the galvanometer moved in the opposite direction.

How Generators Use Induction

An electric generator is one of the most important applications of electromagnetic induction. It converts mechanical energy into electrical energy by rotating a coil or magnet within a magnetic field.

As the coil rotates, the magnetic flux through it changes continuously. This changing flux induces a voltage, which drives current through an external circuit.

Faraday built an early generator, now called the Faraday disc, in 1831. It consisted of a copper disc rotating between the poles of a magnet. As the disc spun, it cut through magnetic field lines, inducing a voltage between the centre and the edge of the disc.

Modern power stations use the same basic principle on a much larger scale. Whether the energy source is steam, water, wind, or gas, the generator relies on rotating machinery and changing magnetic flux to produce electricity.

How Transformers Use Induction

A transformer uses electromagnetic induction to change the voltage of alternating current. It consists of two coils of wire wrapped around a shared iron core.

How It Works

1. Alternating current flows through the primary coil.
2. This current creates a changing magnetic field in the iron core.
3. The changing magnetic field passes through the secondary coil.
4. The changing flux induces a voltage in the secondary coil.

By changing the number of turns in each coil, a transformer can increase or decrease voltage. This is why transformers are essential in electrical grids, allowing electricity to be transmitted efficiently at high voltage and then reduced to safer levels for homes and businesses.

Induction Cooktops and Wireless Charging

Electromagnetic induction is not limited to power stations. It appears in many everyday technologies.

Wireless charging is the same basic principle as a transformer, but with an air gap instead of a shared iron core. The efficiency depends on how well the coils are aligned and how close they are to each other.

Eddy Currents: Induction in Solid Conductors

When a solid conductor experiences a changing magnetic field, circulating currents can form within the material. These are called eddy currents. They can produce heat, create magnetic forces, or cause energy losses in some machines.

In transformers and electric motors, engineers often use laminated cores—thin layers of conductive material separated by insulation—to reduce unwanted eddy currents and improve efficiency.

In other applications, eddy currents are useful. Induction braking systems, for example, use eddy currents to slow moving metal parts without physical contact.