Electromagnetic induction is one of the last topics in the O-Level Physics syllabus, which means it is often the one students revise last and understand least. It is also a favourite for Paper 2 structured questions, because it combines a chunk of explanation marks with a short calculation. Get the reasoning chain right and these become some of the fastest marks on the paper.
Here is how we teach it at Dojo, based on what actually appears in the mark schemes.
The one idea everything is built on
An e.m.f. is induced whenever there is a change in the magnetic flux linking a conductor. No change, no e.m.f. That is the whole topic in one line.
Examiners want you to say this properly. Two phrasings earn marks:
- "As the magnet moves, the magnetic flux linking the coil changes, so an e.m.f. is induced."
- "The conductor cuts magnetic field lines, so an e.m.f. is induced across its ends."
The size of the induced e.m.f. increases when the flux changes faster: move the magnet more quickly, increase the number of turns, or use a stronger magnet.
Lenz's law without the confusion
Lenz's law says the induced current flows in a direction that opposes the change producing it. In practice, questions ask you to justify a direction or explain why you must do work.
A reliable template:
- State what is changing (north pole approaching the coil).
- State what the coil does to oppose it (the near face becomes a north pole to repel the magnet).
- Deduce the current direction (anticlockwise when viewed from the magnet).
- If asked why work must be done, add: "Work done against the repulsive force is converted to electrical energy, consistent with conservation of energy."
That fourth point is the one most students miss, and it is frequently worth a mark.
Transformers: the four-step explanation
When asked "Explain how a transformer works", write the chain in order. Skipping a step loses marks even if your final sentence is correct.
- An alternating voltage across the primary coil produces an alternating current.
- This produces a changing magnetic flux in the soft iron core.
- The core links this changing flux to the secondary coil.
- The changing flux linking the secondary induces an alternating e.m.f. across it.
Because the flux must change, a transformer does not work on d.c. A steady direct current produces a constant flux, so no e.m.f. is induced in the secondary. That is a classic one-mark question.
The equations
For an ideal transformer:
$$\frac{V_s}{V_p} = \frac{N_s}{N_p}$$
and, since 100% efficiency means input power equals output power:
$$V_p I_p = V_s I_s$$
Worked example. A transformer steps 240 V down to 12 V to run a 24 W lamp. The primary has 1200 turns.
- Turns: $N_s = 1200 \times \frac{12}{240} = 60$ turns.
- Secondary current: $I_s = \frac{24}{12} = 2$ A.
- Primary current: $I_p = \frac{24}{240} = 0.1$ A.
Notice the pattern. Step the voltage down, and the current steps up. Students who memorise "more turns means more of everything" get this backwards every time.
Why the National Grid uses high voltage
This is the most predictable application question in the topic, and the answer must use $P_{loss} = I^2 R$.
Worked example. A station transmits 20 kW through cables of total resistance 2 Ω.
- At 250 V: $I = \frac{20000}{250} = 80$ A, so power lost $= 80^2 \times 2 = 12800$ W, which is 12.8 kW wasted.
- At 5000 V: $I = \frac{20000}{5000} = 4$ A, so power lost $= 4^2 \times 2 = 32$ W.
The mark-scheme sentence: "Transmitting at a higher voltage means a smaller current for the same power, and since power loss in the cables is proportional to $I^2$, less energy is wasted as heat."
Do not write "high voltage reduces resistance". The resistance of the cable is unchanged.
Efficiency losses and their fixes
Real transformers are not 100% efficient. Learn the loss with its remedy, since questions usually ask for both.
- Heating in the coils from their resistance. Use thick copper wire of low resistance.
- Eddy currents induced in the core. Use a laminated core, with layers insulated from each other.
- Flux leakage, where not all the flux links the secondary. Use a soft iron core and wind the coils on the same limb.
Common mistakes to avoid
- Writing "magnetic field" when the mark scheme wants "change in magnetic flux".
- Saying a transformer "increases power". It never does.
- Forgetting that the induced e.m.f. in an a.c. generator is maximum when the coil is parallel to the field, since that is when it cuts field lines fastest.
- Mixing up primary and secondary when substituting into the turns ratio. Label $V_p, N_p$ on the diagram before calculating.
Drill five past-year questions on this topic using the exact phrasing above, and you will find the marks come quickly and consistently.
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