Faraday's Law of Electromagnetic Induction

In 1831, Michael Faraday discovered that a changing magnetic field induces a voltage (electromotive force, or EMF) in a conductor. This is called electromagnetic induction. No motion, no induction — the magnetic field must change to induce a current.

Faraday's Law

The induced EMF is proportional to the rate of change of magnetic flux through the circuit.

Greater rate of change → larger induced voltage → larger induced current

Ways to increase induced EMF:

1

Move the magnet or conductor faster (increase rate of flux change).

2

Use a stronger magnet (increase the magnetic field strength).

3

Use a coil with more turns of wire (each turn contributes to the EMF).

4

Use a coil with a larger cross-sectional area.

Lenz's Law

Lenz's law states that the direction of the induced current is always such that it opposes the change that caused it. This is consistent with conservation of energy — you must do work to overcome the opposing force to generate electricity.

Example: If a north pole of a magnet is moved toward a coil, the induced current flows in a direction that creates a north pole facing the magnet — opposing the approach. If the magnet is pulled away, the coil creates a south pole to attract (oppose the withdrawal). In both cases, the induced current resists the change.

Generators

A generator converts mechanical energy into electrical energy using electromagnetic induction. A coil of wire is rotated inside a magnetic field (or a magnet is rotated inside a coil). As the coil rotates, the magnetic flux through it changes continuously, inducing an alternating EMF.

AC Generator (Alternator)

Uses slip rings — produces alternating current (AC). The current reverses direction every half rotation. Used in power stations. Australia uses 50 Hz AC (50 cycles per second).

DC Generator (Dynamo)

Uses a split-ring commutator — produces direct current (DC). The commutator reverses the connection every half rotation so the external current always flows in one direction. Used in car alternators and some small motors.

Power station sequence: Fuel (coal, gas, nuclear, hydro) → heat → steam → turbine spins → generator coil rotates in magnetic field → AC electricity produced → step-up transformer → transmission lines → step-down transformer → homes.

Transformers

A transformer uses electromagnetic induction to change the voltage of an AC supply. It consists of two coils (primary and secondary) wound around an iron core. An alternating current in the primary coil creates a changing magnetic field in the core, which induces an EMF in the secondary coil.

V_p / V_s = N_p / N_s

V_p = primary voltage V_s = secondary voltage N_p = primary turns N_s = secondary turns

Step-up transformer

N_s > N_p → V_s > V_p. Increases voltage. Used at power stations to increase voltage to ~500 kV for long-distance transmission (reduces current, reducing energy lost as heat).

Step-down transformer

N_s < N_p → V_s < V_p. Decreases voltage. Used near homes to reduce voltage from transmission lines to safe 240 V for household use in Australia.

Ideal transformer — power conservation: V_pI_p = V_sI_s. Increasing voltage decreases current by the same factor. This minimises resistive power loss (P = I²R) during long-distance transmission.

Key Vocabulary

Term	Definition
Electromagnetic induction	The production of an EMF (voltage) in a conductor due to a changing magnetic flux.
Magnetic flux	The total magnetic field passing through a given area; depends on field strength and area.
Alternating current (AC)	An electric current that periodically reverses direction. Australia uses 50 Hz, 230 V AC.
Transformer	A device that uses electromagnetic induction to change the voltage of an alternating current supply.

Worked Examples

1

A transformer has 200 turns on the primary coil and 1,000 turns on the secondary. The primary voltage is 240 V. Find the secondary voltage.

Formula: V_p / V_s = N_p / N_s

Substitute: 240 / V_s = 200 / 1000

Rearrange: V_s = 240 × 1000 / 200 = 1,200 V

Answer: This is a step-up transformer. The secondary voltage is 1,200 V.

2

A step-down transformer converts 11,000 V to 240 V. The primary has 2,000 turns. How many turns are on the secondary coil?

Formula: N_s = N_p × V_s / V_p

Substitute: N_s = 2000 × 240 / 11000 = 480000 / 11000 ≈ 43.6 → 44 turns

Answer: Approximately 44 turns on the secondary coil.

3

Explain why electricity is transmitted at high voltage over long distances.

Power loss formula: P_loss = I²R, where R is the resistance of the transmission cables.

Key point: Transmission power P = IV. For a fixed power output, increasing V means I decreases (P = IV, so I = P/V).

Effect: If current is 10 times smaller, power lost as heat is 100 times smaller (since P ∝ I²).

Example: At 500,000 V instead of 240 V, current is about 2,000 times smaller, reducing heat losses dramatically and making long-distance transmission economically viable.

Knowledge Check

Select the correct answer for each question.

Question 1

What must change to induce an EMF in a conductor?

The temperature of the conductor The colour of the magnet The magnetic flux through the circuit The electrical resistance of the wire

Question 2

A transformer has a turns ratio of N_p:N_s = 1:10. If V_p = 50 V, what is V_s?

5 V 50 V 5,000 V 500 V

Question 3

Why do transformers only work with alternating current (AC) and not direct current (DC)?

DC creates a constant magnetic flux, so there is no changing flux to induce an EMF DC has too high a frequency DC is too dangerous for transformers DC cannot flow through iron cores

Question 4

According to Lenz's law, the induced current always:

Flows in the same direction as the changing flux Opposes the change that caused it Increases the magnetic flux Flows at 90° to the magnetic field

Question 5

Australia's power grid transmits electricity at very high voltages (e.g., 500,000 V). What is the primary reason for this?

High voltage is safer for workers on power lines High voltage increases the speed of electrons in the wire High voltage reduces current, which reduces power lost as heat in the cables High voltage reduces the need for transformers

Electromagnetic Induction