Electromagnetic Induction

Electromagnetic induction is the process of generating an electromotive force (EMF) — and therefore an electric current — in a conductor by changing the magnetic flux through it. It was discovered by Michael Faraday in 1831, and independently by Joseph Henry around the same time.

The key insight: you do not need a battery to drive current through a wire. A moving magnet near a coil of wire, or a coil moving through a magnetic field, will induce a current. The relative motion between the conductor and the magnetic field is what matters.

Faraday's Law: The induced EMF in a circuit is proportional to the rate of change of magnetic flux through the circuit. More field lines cut per second = greater induced EMF and current.

How to Increase Induced Current

Use a stronger magnet (more magnetic flux).
Move the magnet faster (greater rate of change of flux).
Increase the number of turns in the coil (each turn contributes its own induced EMF — they add up).
Insert a soft iron core inside the coil (concentrates and strengthens the magnetic field).

Lenz's Law

Lenz's Law states that the induced current always flows in a direction that opposes the change causing it. This is a consequence of the conservation of energy — if the induced current helped the change instead of opposing it, you would be getting energy for free.

Lenz's Law: Induced current opposes the change in magnetic flux that caused it.

AC vs DC Current

Type	Description	Source
Direct Current (DC)	Flows in one direction only. Constant magnitude (ideally).	Batteries, solar cells, DC generators
Alternating Current (AC)	Reverses direction repeatedly at a fixed frequency. Forms a sinusoidal waveform.	AC generators (alternators), mains electricity supply

South Africa mains supply: 230 V AC at 50 Hz (50 complete cycles per second)

The AC Generator (Alternator)

An AC generator converts kinetic (mechanical) energy into electrical energy using electromagnetic induction. Its key components are:

Coil of wire (armature): rotates within the magnetic field.
Permanent magnets (or electromagnets): provide the magnetic field.
Slip rings: rotating metal rings that maintain electrical contact with the coil as it spins.
Brushes: stationary carbon contacts that press against the slip rings to carry current to the external circuit.
External circuit: where the electrical energy is used.

As the coil rotates, the angle between the coil and the magnetic field changes continuously. This causes the magnetic flux through the coil to change sinusoidally, inducing an alternating EMF that drives an alternating current. The result is a sinusoidal voltage waveform.

Faster rotation → higher frequency AND higher peak voltage (more field lines cut per second). In SA power stations, turbines spin at 3 000 rpm to produce 50 Hz.

The DC Generator

A DC generator works on the same principle as an AC generator, but uses a split-ring commutator instead of slip rings. The commutator reverses the connections to the external circuit every half-turn, so the output is always in the same direction — a pulsating DC. A smooth DC output requires multiple coils offset at different angles.

Transformers

A transformer is a device that changes (steps up or steps down) the voltage of an AC supply using mutual induction — the changing magnetic field produced by the primary coil induces an EMF in the secondary coil. Transformers only work with AC, not DC, because DC does not produce a continuously changing magnetic flux.

A transformer consists of:

Primary coil (Np turns): connected to the input voltage (Vp).
Secondary coil (Ns turns): delivers the output voltage (Vs).
Soft iron core: channels the magnetic field from the primary to the secondary coil efficiently.

Vp / Vs = Np / Ns · Vp × Ip = Vs × Is (100% efficiency)

Type	Turns ratio	Voltage effect	Current effect	Use
Step-up transformer	Ns > Np	Vs > Vp (voltage increases)	Is < Ip (current decreases)	Power transmission lines — high voltage, low current reduces energy loss
Step-down transformer	Ns < Np	Vs < Vp (voltage decreases)	Is > Ip (current increases)	Household supply — reduces high transmission voltage to safe 230 V

Why Transmit at High Voltage?

Power lines carrying electricity from power stations to homes lose energy as heat in the wires (P_loss = I² × R). By using a step-up transformer to increase voltage and decrease current, the same amount of power is transmitted with much less heating loss. A step-down transformer at the other end reduces the voltage to a safe level for homes and businesses.

Why AC mains? AC voltage can be easily increased or decreased using transformers; DC cannot. This makes long-distance electrical transmission vastly more efficient with AC.

Applications

Power stations: Steam or water turbines drive AC generators (alternators).
National grid transmission lines: Step-up transformers raise voltage to ~400 kV for efficient long-distance transmission.
Substations: Step-down transformers reduce voltage for industrial and domestic use.
Electric motors: Reverse of a generator — electrical energy converted to kinetic energy.
Microphones: Sound waves vibrate a coil in a magnetic field, inducing a small varying current that represents the sound.
Induction cooktops: High-frequency AC induces eddy currents directly in a metal pot, heating it without an open flame.

Answer all questions in your exercise book. Show all calculations with units. Draw and label diagrams where asked.

Question 1 · (5 marks)

A transformer has a primary coil with 200 turns and a secondary coil with 50 turns. The input voltage is 240 V. (a) Is this a step-up or step-down transformer? (b) Calculate the output voltage (Vs). (c) If the primary current is 0.5 A, calculate the secondary current, assuming 100% efficiency. Show all working.

Question 2 · (4 marks)

Draw and label a diagram of an AC generator. Your diagram must include and label: the coil, the two magnets (with poles), slip rings, brushes, and the external circuit. Explain in two sentences how it produces AC.

Question 3 · (4 marks)

Compare AC and DC current. For each: (a) define it, (b) describe its voltage-time graph, (c) give an example of a source. Then state ONE reason why the national grid uses AC rather than DC.

Question 4 · (3 marks)

Explain why electricity is transmitted at very high voltages over long distances. In your answer, refer to: the formula for power loss (P = I²R), the role of a step-up transformer at the power station, and the role of a step-down transformer at the substation.

Question 5 · (3 marks)

A student pushes a bar magnet into a coil of wire connected to a galvanometer. The needle deflects. State THREE changes the student could make to get a larger deflection, and explain the physics behind each change using Faraday's Law.

Question 6 · (6 marks)

A power station must deliver 100 000 W to a town through a transmission cable with a resistance of 5 Ω. Two transmission options are compared:

Option	Voltage	Current (P = VI)	Cable resistance
A (low voltage)	1 000 V	100 A	5 Ω
B (high voltage)	10 000 V	10 A	5 Ω

(a) Calculate the power lost as heat (P_loss = I²R) for Option A and for Option B. Show all working. (3 marks)
(b) Calculate what percentage of the 100 000 W transmitted is lost as heat in EACH option. (2 marks)
(c) Using your answers, explain why national grids step voltage up so high before transmitting electricity over long distances. (1 mark)

Electromagnetic Induction