Q: What does A-Level Physics: 18) Electromagnetic Induction Guide cover? A: Magnetic flux, Faraday's law, Lenz's minus sign and the transformer equation - this post unpacks Section V Topic 18 of the 2026 H2 Physics syllabus.
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 ϕ=BA, drill the chain rule for E=−dtd(Nϕ)
Reviewed by
Chee Wei Jie·Academic Advisor (Physics)
, and practise sketching transformer energy flow diagrams under timed conditions.
Concrete example: how to use this page
If a coil moves into a magnetic field, ask what changes: field strength, area, angle, or time. Then use Faraday's law for size and Lenz's law for direction.
Keep the induction toolkit tied to the rest of the electromagnetism arc via the H2 Physics notes hub; it bundles this chapter with electromagnetism, AC circuits, and Modern Physics extensions.
Lenz's law and sensitive-balance route
Searches for "sensitive balance" can mean two different H2 Physics tasks. SEAB 9478 Topic 17 names the current-balance method for measuring magnetic flux density; Topic 18 names Faraday's and Lenz's laws for induction. Use the route below before choosing a rule.
Search or question clue
First page to use
Why
Lenz's law sensitive balance
This page first, then the Topic 17 force page
Lenz's law decides the induced current direction; Topic 17 explains the magnetic force that changes a balance reading.
Supplied current, known wire length, and mass change
The calculation is usually F=Δmg followed by B=F/Il.
Moving magnet, coil, changing flux, or induced current
This page
The direction comes from opposing the flux change, not from treating the current as externally supplied.
Induction decision map
Use this map before writing equations. Induction questions usually go wrong when students calculate a magnitude first and only later realise they tracked the wrong changing quantity.
What the question shows
First move
Main route
Misconception check
A loop or coil in a magnetic field
Ask whether B, area, angle, or number of turns is changing.
Compute flux linkage first, then use Faraday's law for the induced e.m.f. magnitude.
Large flux alone is not enough; the flux linkage must change with time.
Direction of induced current
Name the change in flux before choosing a current direction.
Use Lenz's law: the induced current opposes the change that produced it.
Oppose the change, not necessarily the original magnetic field.
Straight conductor moving through a field
Check whether the length, velocity, and field are mutually perpendicular.
Use E=BLv only for the perpendicular cutting-field-lines case.
Motion parallel to field lines gives zero motional e.m.f.
Rotating coil or generator graph
Track the angle between the field and the area normal.
Differentiate flux linkage, or read e.m.f. from the slope of the flux-linkage graph.
Maximum flux does not mean maximum e.m.f.; maximum e.m.f. occurs when flux is changing fastest.
Transformer
Check that the flux is alternating and links both coils.
Use turns ratio for voltage and ideal power balance for current.
A transformer needs changing flux, so the simple transformer model is not a DC device.
Why students find this topic hard
Electromagnetic induction is hard because the question often hides the important step before the equation. You must identify the changing flux, decide the induced direction, and only then calculate the magnitude. The equations are short; the hard part is knowing which direction to point things.
Failure mode 1: Direction of induced current
Students know Faraday's Law but cannot consistently determine the direction of induced EMF or current in non-trivial geometries - rotating coils, partial loops, or a bar moving on rails. The root cause is applying Lenz's Law as a memorised rule rather than understanding the underlying energy argument: the induced current must oppose the change because any other direction would create energy from nothing.
Failure mode 2: Rotating coil scenarios
The sinusoidal EMF from a rotating coil requires understanding which component of the area vector is changing as the coil rotates. Students frequently confuse the angle between B and the area normal, particularly when a question uses a non-standard starting orientation or asks about the instant when EMF is maximum versus zero.
Failure mode 3: Lenz's Law energy argument
Students can state Lenz's Law but cannot deploy the energy conservation reasoning to verify their answer. The check is straightforward: if removing the opposing effect would allow the induced current to reinforce the flux change indefinitely, energy would be created from nothing - so any answer that does not oppose the change is wrong by construction.
Direction check: After determining the direction of induced current, ask yourself: does this current act to reduce the flux change that caused it? If yes, your direction is correct. If your answer would instead amplify the change, you have the direction reversed. A current that reinforces its own cause would produce perpetual motion - that is the tell.
Moving-conductor direction checkpoint
For a rod or loop moving through a magnetic field, decide the direction in two passes: first name the flux change, then check that the magnetic force opposes the motion that caused the induction.
Step
Question to ask
What to decide
Common trap
1
Is the flux into or out of the page increasing or decreasing?
The induced field must oppose that change.
Opposing the original field instead of opposing the change in flux.
2
What current direction would create that induced field?
Use the right-hand grip rule around the loop.
Choosing clockwise or anticlockwise before naming the flux change.
3
For a sliding rod, what force acts on the current-carrying rod?
Use F=BIL direction to check that the force resists the motion.
Accepting a current direction that would pull the rod faster in the same direction.
4
If the circuit is open, where do charges pile up?
State the polarity or induced e.m.f., but no continuous current flows.
Drawing a loop current when there is no closed circuit.
Worked check: a vertical rod slides to the right on horizontal rails in a magnetic field into the page. The loop area increases, so into-page flux increases. The induced current must produce an out-of-page field, so the loop current is anticlockwise. In the moving rod, the current is upward; with B into the page, the magnetic force on the rod is leftward, opposing the rightward motion.
Misconception check: Lenz's law does not say the induced current always makes a field opposite to the original field. It makes a field that opposes the change in flux.
1 Magnetic flux (ϕ) and flux linkage
1.1 Definition
Magnetic flux is the product of magnetic flux density (B) and the area (A)perpendicular to that field:
ϕ=BA
1.2 Flux linkage
For a coil with N identical turns, the magnetic flux linkage is
Nϕ=NBA.
IP Exam Cue: The unit for flux is the weber (Wb=T⋅m2). Always show the superscript “2” - examiners dock marks for m2 written as m.
2 Faraday's and Lenz's laws
2.1 Faraday's law
A changing magnetic flux linkage induces an e.m.f.:
E=−dtd(Nϕ).
The magnitude is proportional to the rate of change of Nϕ.
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 Current-balance connection
When a sensitive-balance setup appears in an induction question, use two layers:
Induction layer: identify the change in magnetic flux and use Lenz's law to decide the induced current direction.
Force layer: once the induced current direction is known, use the Topic 17 motor-effect rule to decide the force on the conductor or magnet assembly.
That two-layer sequence prevents the common error of using Fleming's left-hand rule before deciding which way the induced current flows.
2.5 What affects E?
Factor
Why it matters
Speed of motion
Faster change increases dtdϕ.
Number of turns
More turns raise Nϕ.
Coil area
Larger A intercepts more field lines.
Field strength
Bigger B gives bigger ϕ.
Flux-change setup checkpoint
Before substituting into Faraday's law, separate the flux value from the reason it changes.
Quantity changing
What changes in Nϕ=NBAcosθ
What to write first
Common trap
Field strength B
The field gets stronger or weaker while the coil geometry stays fixed.
Express B as a function of time or use ΔB/Δt.
A large constant B gives no induced e.m.f. if nothing changes.
Area A
More or less of the loop lies inside the field.
Track the area inside the field, not the whole loop area.
Using full area while only part of the loop is in the field.
Angle θ
The area normal turns relative to the field.
Decide whether flux uses cosθ from the normal or convert if the angle is given from the plane.
Treating maximum flux and maximum e.m.f. as the same instant.
Turns N
More linked turns increase total flux linkage.
Multiply single-turn flux by N before taking the change rate.
Forgetting that Faraday's law uses flux linkage, not just flux.
Misconception check: induction depends on the rate of change of flux linkage. A high flux value by itself is not enough; the graph must have a slope.
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:
VpVs=NpNs.
Transformer ratio checkpoint
Before using transformer equations, decide whether you are comparing voltage, current, or power. The turns ratio sets the voltage ratio; ideal power balance then fixes the current ratio.
Transformer clue
What changes
What stays linked
Common trap
Ns>Np
Secondary voltage is higher than primary voltage.
Ideal output power equals ideal input power.
Saying the transformer creates extra energy.
Ns<Np
Secondary voltage is lower than primary voltage.
Lower voltage means higher current for the same ideal power.
Reducing both voltage and current in a step-down transformer without accounting for power.
Load current is given
Use Ps=VsIs first.
Then compare with Pp=VpIp
Efficiency is less than 100%
Output power is smaller than input power.
Losses are the input-output power difference.
Applying ideal power balance after the question gives non-ideal data.
Worked check: an ideal transformer steps 240V down to 12V. The voltage ratio is 20:1, so a 2.0A secondary current corresponds to Ps=12×2.0=24W. For an ideal transformer, Pp=24W, so Ip=24/240=0.10A. The current is smaller on the high-voltage side.
Misconception check: step-up and step-down describe voltage, not power. In the ideal model, raising voltage lowers current for the same power.
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.
Step-down transformer: renders mains 230V 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 ∼100kHz.
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 ϕ or Nϕ - be ready to estimate gradients.
Need structured practice on Electromagnetic Induction? Our A-Level Physics tuition Singapore programme covers this topic with weekly problem sets and Paper 4 practical drills.
Comprehensive revision pack
9478 Section V, Topic 18 Syllabus outcomes
Candidates should be able to:
(a) define magnetic flux as the product of magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density.
(b) show an understanding of and use the concept of magnetic flux linkage.
(c) recall and use Φ=BA and NΦ=NBA to solve problems, where N is the number of turns.
(d) infer from appropriate experiments on electromagnetic induction: (i) that a changing magnetic flux can induce an e.m.f., (ii) that the direction of the induced e.m.f. opposes the change producing it, and (iii) the factors affecting the magnitude of the induced e.m.f.
(e) recall and solve problems using Faraday's law of electromagnetic induction and Lenz's law.
(f) explain simple applications of electromagnetic induction.
(g) show an understanding of the principle of operation of a simple iron-core transformer and recall and solve problems using NpNs=VpVs=IsIp
Concept map (in words)
Flux linkage changes drive induced e.m.f. Use ϕ=BA to compute flux, then multiply by turns N. Faraday's law links the rate of change to induced voltage via E=−dtd(Nϕ); Lenz's law fixes the opposition direction. In moving conductors, the shortcut E=BLv applies. Transformers rely on alternating flux sharing a core between windings.
Key relations
Relation
Comment
ϕ=BAcosθ
Area component must be perpendicular to B to count toward flux.
Nϕ=NBAcosθ
Multiply single-coil flux by turn count for total linkage.
E=−dtd(Nϕ)
Minus sign encodes Lenz's law; magnitude follows the change rate.
E=BLv
For a length L moving perpendicular to B at speed v.
E(t)=NBAωsin(ωt)
Faraday's law applied to a rotating coil of angular speed ω.
VpVs=NpNs
VpIp=VsIs
η=PinPout×100%
Derivations & reasoning to master
Bar cutting flux: derive E=BLv from changing linkage or Lorentz-force arguments.
Rotating coil waveform: compute ϕ=BAcosθ, differentiate to recover the sinusoidal e.m.f.
Lenz's minus sign: articulate energy conservation argument to justify opposition.
Transformer losses: relate eddy currents, hysteresis, copper losses to design features (laminations, soft iron, thick windings).
Worked example 1 - induced e.m.f. in rotating coil
A 200-turn coil of area 0.015m2 rotates at 25Hz in a 0.35T uniform field. Find the maximum induced e.m.f. and write its time dependence.
Solution: E0=NBAω with ω=2πf. Therefore E(t)=E0sin(2πft).
A transformer steps 240V down to 12V for a 2.0A load. The primary current is 0.12A. Determine the efficiency and estimate the power loss (assume input minus output is mainly copper loss).
Method: Calculate Pout=VsIs and Pin=VpIp; the difference approximates copper loss through I2R.
Calculation: compute Pout and Pin, then η and the loss from the power difference.
Pout=12×2.0=24W,Pin=240×0.12=28.8W.
η=28.824×100%=83%(3 s.f.),Ploss≈28.8−24=4.8W.
Practical & data tasks
Use datalogger to record induced e.m.f. vs time when dropping a magnet through a coil; integrate area to estimate flux change.
Build a simple generator with hand crank and LED; relate speed to brightness (frequency).
Perform transformer experiment measuring VpVs vs NpNs; evaluate deviations and discuss causes.
Common misconceptions & exam traps
Forgetting the cosine factor in ϕ=BAcosθ when the area is tilted.
Dropping the negative sign in E=−dtd(Nϕ) (lose credit for direction).
Confusing flux ϕ (webers) with flux density B (tesla).
Assuming transformers work with DC (they require changing flux).
Quick self-check quiz
Define magnetic flux. - Product of magnetic flux density and perpendicular area ϕ=BA.
What does Lenz's law state? - Induced e.m.f. acts to oppose the change causing it.
How can you increase induced e.m.f. in a generator? - Increase N, A, B or rotation speed so ∣E∣ grows.
Why are transformer cores laminated? - To reduce eddy current losses.
If a conductor moves parallel to magnetic field lines, what is induced e.m.f.? - Zero: E=0 (no flux change).
Revision workflow
Re-derive E=BLv and the rotating-coil waveform E(t)=NBAωsin(ωt) weekly to keep intuition sharp.
Practise interpreting flux linkage graphs and calculating gradients quickly.
Summarise transformer loss mechanisms (I2R, eddy currents, hysteresis) and their mitigation strategies on a cheat sheet.
Attempt past practical questions involving induced e.m.f. measurements and graph analysis.
Practice Quiz
Test yourself on the key concepts from this guide.
