A-Level Physics — 18) Electromagnetic Induction (IP-Friendly Guide)

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14 Jul 2025, 00:00 Z

TL;DR
Induction is the bridge between electricity and magnetism that powers bike dynamos, phone chargers and the entire national grid.
For Paper 2 and Paper 4, memorise \(\phi = BA\), drill the chain rule for \(\mathscr{E} = -\dfrac{\mathrm{d}(N\phi)}{\mathrm{d}t}\), and practise sketching transformer energy flow diagrams under timed conditions.

1 Magnetic flux \((\phi)\) and flux linkage

1.1 Definition

Magnetic flux is the product of magnetic flux density \((B)\) and the area \((A)\) perpendicular to that field:

\[ \phi = BA \]

1.2 Flux linkage

For a coil with \(N\) identical turns, the magnetic flux linkage is
\[ N\phi = NBA. \]

IP Exam Cue: The unit for flux is the weber \((\pu{Wb} = \pu{T.m2})\). Always show the superscript “2” — examiners dock marks for \(\pu{m2}\) written as \(\pu{m}\).

2 Faraday's and Lenz's laws

2.1 Faraday's law

A changing magnetic flux linkage induces an e.m.f.:
\[ \mathscr{E} = -\frac{\mathrm{d}(N\phi)}{\mathrm{d}t}. \]
The magnitude is proportional to the rate of change of \(N\phi\).

2.2 Lenz's law

The negative sign shows that the induced e.m.f. acts to oppose the change producing it — a direct consequence of energy conservation.

2.3 Required practical

Push a bar magnet into a coil connected to a galvanometer: the needle deflects; pull it out and the needle swings the opposite way. The faster you move, or the stronger the magnet, the larger the deflection.

2.4 What affects \(\mathscr{E}\)?

Factor	Why it matters
Speed of motion	Faster change increases \(\dfrac{\mathrm{d}\phi}{\mathrm{d}t}\).
Number of turns	More turns raise \(N\phi\).
Coil area	Larger \(A\) intercepts more field lines.
Field strength	Bigger \(B\) gives bigger \(\phi\).

Timing hack: In WA calculations, write the chain rule in one line to earn a method mark even if algebra slips.

3 Power transformers

3.1 Principle of operation

Two coils share a laminated soft-iron core, enabling almost all magnetic flux from the primary to link the secondary via mutual induction.

3.2 Ideal transformer equation

Assuming perfect coupling and zero losses:
\[ \frac{V_s}{V_p} = \frac{N_s}{N_p}. \]

3.3 Efficiency and real-world tweaks

Thin insulated laminations in the core slash eddy-current losses and cut heating.
High-grade silicon steel further reduces hysteresis loss.

3.4 IP-level application checks

Step-up transformer: boosts voltage, lowers current, cuts I\(^2\)R transmission loss.
Step-down transformer: renders mains \(\pu{230 V}\) safe for the USB-C charger on your desk.

4 Everyday induction heroes

Bicycle dynamo: converts wheel rotation into light.
Credit-card stripe reader: reads data via changing magnetic flux.
Wireless phone charging: coils in the pad and phone form a tightly-coupled transformer at \(\sim \pu{100 kHz}\).

Parents' insight: These real objects make abstract equations tangible, boosting engagement during tuition sessions.

5 Three tuition take-aways

Master the minus sign. Most lost marks come from forgetting Lenz's direction.
Sketch before solving. Draw flux linkage vs time graphs to see slopes.
Practise with data-logger traces. Paper 4 often gives non-linear graphs of \(\phi\) or \(N\phi\) — be ready to estimate gradients.

6 Further reading

7 Call-to-action

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Last updated 14 Jul 2025. Next review when SEAB issues the 2027 draft syllabus.