Magnetism & Electromagnetism

Electromagnetic Induction

Specific Learning Outcomes:

6.15 Know that a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it, and describe the factors that affect the size of the induced voltage.
6.16 Describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field, and describe the factors that affect the size of the induced voltage.

Electromagnetic induction is the process where a voltage (and potentially a current, if there's a complete circuit) is generated in an electrical conductor. This occurs when the conductor is placed in a changing magnetic field, or when the conductor moves through a magnetic field.

Key Principles

1. Changing Magnetic Field: A voltage is induced in a coil if the magnetic field passing through the coil changes. This can be achieved by:

Moving a magnet towards or away from the coil.
Moving the coil towards or away from a stationary magnet.
Changing the strength of the magnetic field (e.g., by altering the current in an electromagnet).

2. Relative Motion: A voltage is induced in a conductor when it moves relative to a magnetic field so that it "cuts" through the magnetic field lines.

Faraday's Law: EMF = −N × ΔΦ / Δt | The induced voltage equals the rate of change of magnetic flux through the coil times the number of turns.

Interactive Simulation: Faraday's Experiment

Drag the magnet left & right to move it through the coil. Watch the galvanometer and the EMF graph respond in real time.

Number of Turns (N) 5

Field Strength (B) 5

Notice: The faster you move the magnet, the larger the induced voltage. When the magnet is stationary, the EMF drops to zero because there is no change in magnetic flux.

Factors Affecting the Induced Voltage

The magnitude of the induced voltage depends on several factors (KLO 6.15 & 6.16):

Speed of Relative Motion: Faster motion → greater induced voltage.
Strength of the Magnetic Field (B): A stronger field induces a larger voltage.
Number of Turns (N): More turns in the coil significantly increase the induced voltage.
Area of the Coil (A): A larger coil area captures more flux, increasing the voltage.
Angle of Cutting Field Lines: Maximum voltage when the conductor moves perpendicular to field lines; zero when parallel.

Interactive Simulation: EMF Factor Explorer

Adjust each factor to see how it affects the peak EMF of a generator. The formula is Peak EMF = N × B × A × ω

Turns (N) 100

Field B (T) 0.50

Area A (m²) 0.100

ω (rad/s) 100

Generation of Electricity (KLO 6.16)

Electromagnetic induction is the core principle behind all electric generators. Electricity is generated by:

Rotating a coil of wire within a stationary magnetic field: As the coil spins, the amount of magnetic flux through it changes continuously, inducing a voltage.
Rotating a magnet within a stationary coil of wire: This achieves the same effect — the magnetic field through the coil changes, inducing a voltage.

This process generates an alternating voltage (AC), because the direction of flux change reverses periodically as the coil (or magnet) rotates.

Generator EMF: EMF(t) = N B A ω sin(ωt) | Peak EMF = N B A ω

Interactive Simulation: AC Generator

Watch the coil rotate between magnetic poles. The graph below shows the alternating voltage output in real time.

Speed (ω) 2.0

Field Strength (B) 2.0

Turns (N) 5

Observe how the voltage follows a sine wave. When the coil is edge-on to the field (cutting the most field lines per second), the voltage is at its peak. When the coil is face-on (momentarily not cutting any new lines), the voltage passes through zero.

Real-World Applications

Applications of Electromagnetic Induction:

Electric Generators: Convert mechanical energy (from turbines powered by steam, water, or wind) into electrical energy.
Transformers: Change AC voltages up or down for efficient power transmission. A changing magnetic field in one coil induces a voltage in a neighbouring coil.
Induction Cooktops: Generate changing magnetic fields that induce electric currents directly in the base of metallic cookware, heating it.
Dynamic Microphones: Sound waves vibrate a diaphragm attached to a coil near a magnet. The coil's movement induces a voltage representing the sound.
Contactless Payment / RFID: A reader emits a magnetic field that induces a current in the card's antenna, powering it for data exchange.
Alternators in Cars: Generate electricity to charge the battery and power electrical systems while the engine runs.