Electromagnetism

What Electromagnetism covers

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.

The 9 subtopics

Each one has its own page.

• Magnetic flux, B-fields and flux linkageRecall 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.Open →
• Electromagnetic induction: Faraday's and Lenz's lawsRecall 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.Open →
• 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.Open →
• The dynamoRecall 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.Open →
• The transformerRecall 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.Open →
• Eddy currents and real transformersRecall 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.Open →
• Forces on current-carrying conductorsRecall 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.Open →
• Magnetic circuits, permeance and machine designRecall 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.Open →
• Investigating induction, flux density and transformersRecall 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.Open →

What you need to know

Answer each one before you reveal it.

Take Electromagnetism further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Magnetic flux Φ vs Flux linkage NΦ

  Magnetic flux Φ: the amount threading one turn of a coil, the field's density times the area it passes through at right angles, measured in webers. Flux linkage NΦ: the amount threading a coil multiplied by how many turns that coil has, measured in weber-turns, and it is the rate of change of this that gives the induced e.m.f.

• Faraday's law vs Lenz's law

  Faraday's law: the induced e.m.f. equals the rate of change of flux linkage, which is what fixes its size. Lenz's law: the induced effect always opposes the change producing it, which is what fixes its direction and shows as the minus sign.

• Eddy current loss vs Copper loss

  Eddy current loss: energy wasted as heat by circulating flows induced in the body of the core, cut down by building that core from thin insulated laminations. Copper loss: energy wasted as heat in the resistance of the primary and secondary windings themselves.

Questions from this topic

Each one with the model answer underneath.

1. 1

   What is meant by the B-field at a point?

   Model answer

   The B-field, or magnetic flux density, is a vector quantity whose magnitude is the force per unit current per unit length on a straight conductor placed at right angles to the field at that point.

2. 2

   State the unit of magnetic flux density and express it in SI base units.

   Model answer

   The unit is the tesla (T), and 1 T is equal to 1 kg s⁻² A⁻¹.

3. 3

   Define magnetic flux and give the equation used for a uniform field.

   Model answer

   Magnetic flux is the product of magnetic flux density and the area it passes through at right angles, Φ = BA.

4. 4

   State the unit of magnetic flux and its equivalent in tesla and metres.

   Model answer

   The unit is the weber (Wb), and 1 Wb is equal to 1 T m².

5. 5

   A uniform field of flux density 0.25 T passes at right angles through a coil of cross-sectional area 4.0 × 10⁻³ m². Calculate the flux through the coil.

   Model answer

   Φ = BA = 0.25 × 4.0 × 10⁻³ = 1.0 × 10⁻³ Wb.

6. 6

   A coil of 500 turns encloses a flux of 1.0 × 10⁻³ Wb. Calculate the flux linkage.

   Model answer

   Flux linkage = NΦ = 500 × 1.0 × 10⁻³ = 0.50 Wb-turns.

What is inside Levo

Six ways to work the same material.

Electromagnetism revision questions

What does Electromagnetism cover in OCR A-Level Physics B?

9 subtopics: 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. Each one has its own page with questions and model answers.

How is Electromagnetism organised?

By the OCR H557 specification, subtopic by subtopic, so it matches the order you are taught it in.