A-Level · 26 September 2026 · 6 min read
Magnetic fields and induction: force, flux and emf questions
The word “magnetic” does not identify one equation. A wire may carry current and experience a force, or a circuit may have an emf induced as its flux linkage changes. Some devices involve both effects, but the first job is to identify which quantity the question asks you to explain or calculate.
These original examples practise Edexcel IAL A2 fields and induction. Each starts with a physical description and ends with a condition check. The accompanying notes provide the concept reference; use the worked questions to test whether you can select and defend the relevant model.
Separate the force angle from the flux angle
In F = BIL sin θ, θ is the angle between the current direction and magnetic field. In Φ = BA cos α, α is the angle between the field and the normal to the loop’s plane. These angles refer to different geometrical objects. Copying an angle from the diagram without identifying its reference direction can reverse the expected result.
A wire parallel to the field has zero magnetic force in this model. A loop whose normal is parallel to the field has maximum flux. A loop’s plane parallel to the field has zero flux. Sketch a short normal arrow on every coil diagram so you can explain these statements rather than treating sine and cosine as interchangeable choices.
Worked example: force on a current-carrying wire
A straight 0.12 m length of wire lies perpendicular to a uniform 0.40 T field and carries 3.0 A. The force magnitude is F = BIL = 0.40 × 3.0 × 0.12 = 0.144 N. The length is the part within the field, not necessarily the full length of the circuit. Use conventional current direction when applying your direction rule.
If the wire makes 30° to the field instead, F = 0.144 sin 30° = 0.072 N. If current reverses, force direction reverses. If both current and field reverse, the force direction returns to its original direction. Predicting those reversals is a useful check that you understand the vector interaction.
Flux is not flux linkage
Magnetic flux through one loop describes the field passing through its area, accounting for orientation. Flux linkage NΦ includes the number of turns linking that flux. A coil with twice as many identical turns in the same field has twice the linkage, although the flux through each turn has not doubled.
Changes can come from field strength, loop area within the field, orientation, or some combination. A coil need not visibly move for induction to occur: changing the magnetic field can change flux linkage through a stationary coil. Conversely, motion by itself does not guarantee a changing flux linkage.
Worked example: average induced emf
A 250-turn coil has area 3.0 × 10⁻³ m² per turn. Its normal remains parallel to a uniform field that falls from 0.20 T to 0.05 T in 0.15 s. The flux change per turn has magnitude AΔB = 4.5 × 10⁻⁴ Wb. The linkage change is 0.1125 Wb turns, so the average emf magnitude is 0.1125/0.15 = 0.75 V.
For a closed, purely resistive 5.0 Ω circuit with other effects neglected, this average emf corresponds to an average current magnitude of 0.15 A over the interval. The current would be constant only if the emf were constant, for example with a constant field-change rate in this model. An open circuit can still have an induced emf even though there is no complete path for sustained current. Do not describe emf and current as if they were the same measured quantity.
Explain Lenz’s law through a named change
Imagine a conducting loop entering a region where the field points into the page. The inward flux through the loop increases. The induced current produces a field out of the page, opposing that increase; viewed from the page, this is anticlockwise current. The induced field opposes the change, not automatically the original field in every situation.
If the same loop leaves the region, inward flux decreases and the induced current produces an inward field, now clockwise. Reusing the first direction would miss the changed condition. A good explanation names the initial flux, whether it increases or decreases, the required induced field and then the current direction.
Use graph slopes and energy to check your answer
An emf–time pattern depends on the slope of the flux-linkage–time graph. Constant linkage gives zero induced emf, a constant nonzero slope gives constant emf, and a steeper slope gives a larger magnitude. A maximum linkage is not automatically a maximum emf: at a smooth turning point its instantaneous slope is zero.
For mechanical induction, explain where the electrical energy comes from. An external agent may need to do work against a magnetic effect to maintain motion. Then practise one force calculation, one linkage calculation and one direction explanation. Underline the angle definition, number of turns and time interval before checking the arithmetic.
Questions, explained
Choose a question for a direct answer, then explore the explanation and supporting resources. Each answer has its own link to save or share.
What is the difference between magnetic flux and flux linkage?
Flux describes the magnetic field through one loop, including its area and orientation. Flux linkage is N times that flux when N turns link the same flux. Faraday’s law uses the rate of change of flux linkage, so forgetting the number of turns can give an emf that is too small.
Can there be induced emf without induced current?
Yes. A changing flux linkage can produce emf even when the circuit is open. A sustained current also requires a complete conducting path and depends on circuit resistance and other circuit properties. Do not infer current merely from the presence of an induced emf.
Does Lenz’s law mean the induced field always opposes the original field?
No. It opposes the change in flux causing the induction. If the original flux increases, the induced effect acts against the increase. If that flux decreases, the induced effect acts to maintain it. State the direction of the change before deciding the induced current direction.
Can a moving conductor have zero induced emf?
Yes, depending on its motion and geometry. Motion that does not produce the relevant separation of charge or change in circuit flux linkage need not create the expected emf. Analyse the specified arrangement instead of assuming that any motion in any magnetic field is sufficient.
Is emf greatest when magnetic flux is greatest?
Not necessarily. Emf depends on the rate of change of flux linkage, not its value. At a smooth maximum of linkage the slope is zero, so the instantaneous induced emf is zero. Read the gradient of a linkage–time graph rather than its height.