Put a phone down on a charging pad, and it starts filling up with no cable, no metal contacts touching anything. It looks like magic. It isn't — it's a GCSE and A-Level physics topic doing exactly what it says on the syllabus: electromagnetic induction, the same idea that runs power-station transformers, scaled down to fit under your phone.
GCSE coreThe one rule induction runs on
A changing magnetic field, passing through a coil of wire, induces a voltage in that coil — and if the coil is part of a complete circuit, that voltage drives a current. Note the word "changing": a steady, unmoving magnetic field induces nothing at all. This is exactly why a charging pad uses alternating current — current that reverses direction many thousands of times a second — rather than a steady current: the constant reversing is what keeps the magnetic field changing, which is the only thing induction actually responds to.
The pad's coil carries alternating current, creating a magnetic field that keeps changing direction. That changing field induces a current in the phone's coil, with no physical contact needed.
Exam-style question
A student places their phone at the edge of a wireless charging pad, rather than centred on it, and notices it charges much more slowly. Explain why. [3 marks]
Show model answer and mark breakdown
- Moving the phone off-centre reduces how closely the phone's coil is aligned with the pad's coil (1 mark)
- This means less of the pad's changing magnetic field actually passes through the phone's coil (1 mark)
- A smaller changing magnetic field induces a smaller EMF (and current) in the phone's coil, so less energy is transferred per second (1 mark)
Examiner focus: link the misalignment to the amount of magnetic field actually passing through the coil, not just 'they're further apart' — distance and alignment both reduce the overlap, but the mechanism is about field linkage.
Common mistake: saying the pad simply 'doesn't work as well at the edges' without explaining why in terms of the magnetic field and induced current.
Two coils, one invisible link
Inside the charging pad is a coil of wire carrying that alternating current, generating a magnetic field that expands and collapses in sync with it. Inside the phone, positioned close to the pad, is a second coil. That constantly-changing magnetic field from the pad passes through the phone's coil and induces its own alternating current there — which the phone's internal electronics then convert into the steady direct current the battery actually needs to charge.
Why this is the same physics as the National Grid
A wireless charger and a step-down transformer at your local substation are doing the identical trick: one coil carrying alternating current, a changing magnetic field, and a second coil picking up an induced current from it. A mains transformer wraps both coils around a shared iron core to make the magnetic linking far more efficient; a phone charger has to do it across an open air gap, which is why it's slower and generates more waste heat than plugging in directly. Same principle, very different engineering trade-off. If electromagnetic induction, transformers or any other GCSE/A-Level physics topic needs explaining with the actual mechanism rather than just the formula to memorise, that's exactly what our GCSE physics tutoring is for — see the full learning pathway here.
Frequently asked questions
Why does the phone need to be aligned so precisely on the pad?
Induction only works well when the two coils are closely aligned, because the strength of the induced current depends on how much of the changing magnetic field from the pad's coil actually passes through the phone's coil. Move it off-centre and less of that field overlaps, so less current gets induced — which is why misaligned phones charge slowly or not at all.
Is wireless charging less efficient than a cable?
Yes, noticeably — a typical wireless charger loses more energy as heat than a cable connection does, because induction across an air gap is inherently less efficient than direct electrical contact. That's a real, physics-based trade-off for the convenience, not just a manufacturing shortcut.
Is this the same physics as the National Grid's transformers?
Yes, exactly the same principle — electromagnetic induction. A wireless charger is essentially a tiny transformer with its two coils separated by air instead of both wound around the same iron core, which is also why it's far less efficient than a proper transformer.
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About the author
Sudershan Soni
Founder & Lead Tutor at Mostak Services — an MSc-qualified Mathematics, Science, Computer Science & STEM tutor with 20+ years of professional experience, teaching students from 11+ and GCSE to A-Level and beyond, online worldwide.
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