Electromagnetism | OCR A-Level Physics B (Advancing Physics) (H557)
Electromagnetism
- 136 questions
- 9 subtopics
- The physics content, examined on all three papers
- Component 01, Component 02 and Component 03
Electromagnetism is examined in all three written papers — the specification states that Components 01, 02 and 03 each assess content from across all the teaching modules, so nothing is confined to one paper.
It covers magnetic flux, B-fields and flux linkage, electromagnetic induction: Faraday's and Lenz's laws, graphs of current, flux and induced e.m.f., the dynamo, the transformer, eddy currents and real transformers, forces on current-carrying conductors, magnetic circuits, permeance and machine design and investigating induction, flux density and transformers.
Sample questions from Electromagnetism
Answer each one closed book first, then open the answer.
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Magnetic flux, B-fields and flux linkage
A coil of 500 turns encloses a flux of 1.0 × 10⁻³ Wb. Calculate the flux linkage.
Show the answer
Flux linkage = NΦ = 500 × 1.0 × 10⁻³ = 0.50 Wb-turns. -
Electromagnetic induction: Faraday's and Lenz's laws
The flux linkage through a coil falls steadily from 0.50 Wb-turns to zero in 0.020 s. Calculate the magnitude of the induced e.m.f.
Show the answer
The magnitude of the e.m.f. is 0.50 ÷ 0.020 = 25 V. -
Graphs of current, flux and induced e.m.f.
At which points on a sinusoidal flux linkage graph is the induced e.m.f. zero?
Show the answer
The e.m.f. is zero at the peaks and troughs of the flux linkage, where the gradient is momentarily zero. -
The dynamo
In a rotating-coil generator, in which position is the induced e.m.f. zero?
Show the answer
The e.m.f. is zero when the plane of the coil is at right angles to the field, because the flux linkage is then at a maximum and momentarily not changing. -
The transformer
Write the equation relating the currents and turns of an ideal transformer.
Show the answer
For an ideal transformer I₂/I₁ = N₁/N₂, so the coil with fewer turns carries the larger current. -
Eddy currents and real transformers
In which direction do eddy currents flow?
Show the answer
They flow in whichever sense produces a magnetic field that opposes the change of flux causing them, as Lenz's law requires. -
Forces on current-carrying conductors
Explain the force between a magnet and a current-carrying coil in terms of induced poles.
Show the answer
The current in the coil makes it behave as a small magnet with its own north and south faces, and the attraction or repulsion between those poles and the poles of the magnet provides the force. -
Magnetic circuits, permeance and machine design
State the corresponding expression for the conductance of a conductor, and say why the comparison is useful.
Show the answer
Conductance is proportional to cross-sectional area divided by length and depends on the conductivity of the material, and permeance depends on the dimensions of the magnetic path in exactly the same way.
The 9 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Magnetic flux, B-fields and flux linkage | Recall questions on the B-field and the tesla, magnetic flux Φ = BA and the weber, flux linkage NΦ, a coil turned in a field, the continuity of flux lines and why they never cross, line spacing, and the fields of a bar magnet, a solenoid, a long straight wire and a uniform field. | 15 |
| Electromagnetic induction: Faraday's and Lenz's laws | Recall questions on Faraday's and Lenz's laws, ε = −d(NΦ)/dt and its minus sign, Lenz's law and conservation of energy, induced e.m.f. calculations and the effect of turns and time, the pole induced by an approaching magnet, the work needed to push it in, ε = BLv for a moving rod, and why a current needs a complete circuit. | 16 |
| Graphs of current, flux and induced e.m.f. | Recall questions on what a flux-linkage gradient gives, sinusoidal traces and where the e.m.f. peaks or vanishes, a magnet falling through a coil, areas under the graph, and transformer outputs. | 13 |
| The dynamo | Recall questions on how a dynamo induces an e.m.f. through relative motion, the alternating-current generator, coil positions of greatest and zero e.m.f., output frequency, ε = BLv for a sliding rod, faster rotation, soft-iron armatures, why a loaded generator is harder to turn, bicycle dynamos, commutators in direct-current dynamos, and the e.m.f. across an aircraft wing. | 14 |
| The transformer | Recall questions on how a transformer works and why it needs alternating current, the soft-iron core, V₁/V₂ = N₁/N₂ and I₂/I₁ = N₁/N₂ with calculations, ideal transformers and power balance, step-up and step-down turns, high-voltage transmission and cable losses, a direct supply, changing the primary turns, and the primary current with no load. | 16 |
| Eddy currents and real transformers | Recall questions on eddy currents in transformer cores and the energy they waste, laminated cores, their direction, electromagnetic braking, induction hobs and metal detectors, copper loss, flux leakage and hysteresis loss, transformer efficiency and cooling, and the jumping aluminium ring. | 14 |
| Forces on current-carrying conductors | Recall questions on F = ILB and when it applies, force, flux density and force per unit length calculations, Fleming's left-hand rule, currents parallel to a field, the force explained by flux lines and induced poles, parallel wires attracting and repelling, electric motors and moving-coil loudspeakers, the turning effect on a coil, reversed currents, wires at an angle, and power lines. | 16 |
| Magnetic circuits, permeance and machine design | Recall questions on magnetic circuits and their analogy with electric circuits, flux = permeance × current-turns, what permeance depends on, the conductance of a conductor, iron cores and air gaps, low-resistance windings, ampere-turns, why larger machines perform better and scaling by two, constant flux around a series circuit, soft magnetic cores, and why there is no magnetic insulator. | 15 |
| Investigating induction, flux density and transformers | Recall questions on the e.m.f. from a magnet falling through a coil, data loggers, increasing and controlling the e.m.f., reading its peak, equal pulse areas, the slowed magnet, measuring flux density with a digital balance and B = Δmg/(IL), zeroing, reversing the current, alignment and length, graphing the readings, and testing transformer turns ratios. | 17 |
How the guide is worked
Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.
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Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
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Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
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Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.
Read the full method, the return schedule and the research behind it.
Nearby topics
OCR A-Level Physics B (Advancing Physics) Active Recall Guide
Every topic, not just this one. 2,455 questions with their answers.