QCE Physics - Unit 3 - Electromagnetism

Electromagnetic Induction | QCE Physics

Learn electromagnetic induction for QCE Physics, including flux, Faraday's law, Lenz's law, motional EMF, transformers and electromagnetic waves.

Updated 2026-06-17 - 4 min read

Electromagnetic induction is the process of producing an electromotive force, or EMF, from a changing magnetic environment. In QCE Physics, this topic connects magnetic flux, Faraday's law, Lenz's law, transformers and electromagnetic waves.

Magnetic flux

Magnetic flux measures how much magnetic field passes through an area:

$ \Phi = BA\cos\theta $

where $B$ is magnetic flux density, $A$ is area and $\theta$ is the angle between the magnetic field and the normal to the area. Flux is largest when the field is perpendicular to the plane of the loop and zero when the field is parallel to the plane of the loop.

Flux can change if:

• magnetic field strength increases or decreases
• loop area changes
• the loop rotates, changing the angle between the field and the loop's normal
• the loop moves into or out of the field
• the magnet or field source moves into or out of the loop

That change is what matters for induction.

Faraday's law and Lenz's law

Faraday's law relates induced EMF to rate of change of flux:

$ emf = -N\frac{\Delta\Phi}{\Delta t} $

The number of turns $N$ matters because each turn contributes to the total induced EMF. Faster flux change produces larger EMF.

The negative sign is Lenz's law. It means the induced EMF drives current in a direction that opposes the change in magnetic flux that caused it. Lenz's law is an energy-conservation rule. If induction assisted the change instead of opposing it, energy would appear without an external input.

Moving conductors

An EMF can be induced across a conductor moving through a magnetic field because charges in the conductor experience magnetic force. A rod moving perpendicular to a field separates charges, producing a potential difference across its ends.

For QCE, the general Faraday form is often enough:

$ emf = -N\frac{\Delta(BA_\perp)}{\Delta t} $

The practical reasoning is: if motion changes the effective area exposed to the field, it changes flux and induces EMF.

Transformers

Transformers use electromagnetic induction to change AC potential difference. An alternating current in the primary coil produces a changing magnetic flux in the core. That changing flux induces an EMF in the secondary coil.

For an ideal transformer:

$ \frac{V_p}{V_s} = \frac{N_p}{N_s} $

and:

$ I_pV_p = I_sV_s $

If the secondary has more turns than the primary, it is a step-up transformer. If the secondary has fewer turns, it is a step-down transformer. Ideal transformer equations assume negligible energy losses, so real transformers may be slightly less efficient.

Electromagnetic waves

Electromagnetic waves are linked oscillating electric and magnetic fields that can travel through a vacuum. Accelerating or oscillating charges produce electromagnetic radiation. The changing electric field produces a changing magnetic field, and the changing magnetic field produces a changing electric field.

This concept connects Unit 3 electromagnetism to Unit 4 quantum theory, where light is treated both as an electromagnetic wave and as photons depending on the evidence being explained.

Worked example

Quick check

Sources

• QCAA Physics subject page
• QCAA Physics 2025 syllabus
• QCAA Physics 2025 formula and data book
• OpenStax University Physics Volume 2: Lenz's Law
• OpenStax University Physics Volume 2: Electromagnetic Induction