QCE Physics - Unit 3 - Electromagnetism

Transformers and Electromagnetic Waves | QCE Physics

Learn QCE Physics transformers, changing magnetic flux, Faraday's law, Lenz's law and electromagnetic wave production.

Updated 2026-06-17 - 4 min read

Transformers and electromagnetic waves both come from the same deep idea: changing electric and magnetic conditions create new fields. In QCE Physics, this connects Faraday's law, Lenz's law, changing magnetic flux and oscillating electric charges.

From changing flux to induced EMF

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

$ \Phi = BA\cos\theta $

If the flux through a loop changes, an EMF is induced. Faraday's law is:

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

The number of turns $N$ matters because each turn experiences the changing flux. More turns means a larger induced EMF for the same rate of flux change.

The negative sign represents Lenz's law: the induced effect opposes the change that produced it. This is not an extra formula trick. It is a conservation-of-energy statement. If the induced current helped the change, energy could appear without an external source.

Transformer structure

A transformer has a primary coil, a secondary coil and usually a soft iron core. The primary coil is connected to an alternating voltage source. The alternating current produces a changing magnetic field in the core. That changing field creates changing flux through the secondary coil, inducing an EMF across it.

The ideal transformer relationship is:

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

If the secondary coil has more turns than the primary coil, the transformer steps voltage up. If the secondary coil has fewer turns, it steps voltage down.

In an ideal transformer, power is conserved:

$ I_pV_p=I_sV_s $

So stepping voltage up reduces current, and stepping voltage down increases current.

Why transformers use alternating current

Direct current that has settled to a constant value produces a steady magnetic field. A steady flux does not induce a continuous EMF in the secondary coil. Alternating current keeps changing direction and magnitude, so the magnetic flux keeps changing.

This is why transformer questions often mention AC, changing flux or alternating current. If the current is not changing, there may be a brief induced EMF when the circuit is switched on or off, but not a sustained transformer output.

Electromagnetic waves

An electromagnetic wave is a travelling disturbance made of oscillating electric and magnetic fields. The fields are perpendicular to each other and to the direction of travel. They can travel through a vacuum, which is why sunlight reaches Earth.

Oscillating electric charges produce electromagnetic waves. The changing electric field is linked to a changing magnetic field, and the changing magnetic field is linked back to a changing electric field. This self-sustaining field pattern moves through space at the speed of light in a vacuum.

Use the wave relationship:

$ v=f\lambda $

For electromagnetic waves in a vacuum, $v=c$, so:

$ c=f\lambda $

The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. They are all electromagnetic waves; they differ in frequency, wavelength and photon energy.

Worked example

Explaining Lenz's law in words

For explanation questions, avoid saying only "it opposes motion". Lenz's law is more precise: the induced current or EMF is in the direction that opposes the change in magnetic flux. If a magnet is pushed into a coil, the induced current creates a magnetic field that resists the increase in flux. If the magnet is pulled out, the induced current resists the decrease in flux.

Quick check

Sources

• QCAA Physics subject page
• QCAA Physics 2025 syllabus
• QCAA Physics 2025 formula and data book
• OpenStax University Physics Volume 2: Faraday's Law
• OpenStax College Physics 2e: Electromagnetic Waves