Simple applications of electromagnetic induction
Key idea: H2 Physics lessons on magnetic flux, induction laws, applications and ideal transformers.
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The core idea
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Big question: How do generators and moving conductors turn motion into electrical energy?
A conductor cutting magnetic field lines develops an e.m.f.; for perpendicular motion ε = Blv. Rotating coils repeatedly change flux linkage and produce alternating e.m.f. Eddy currents can provide braking or heating, with Lenz's law accounting for the opposing mechanical effect and energy transfer.
Explain a rotating-coil generator
A coil rotating steadily in a uniform magnetic field has flux linkage NBA cos ωt. Faraday's law gives sinusoidal e.m.f. NBAω sin ωt. Slip rings connect the rotating coil to an external circuit while allowing the output polarity to alternate.
Maximum flux and maximum e.m.f. occur a quarter-cycle apart: when flux is greatest its gradient is zero; when flux is zero it changes fastest. Increasing N, B, A or ω increases peak e.m.f.
Check your understanding: At what coil orientation is induced e.m.f. greatest?
When the coil's normal is perpendicular to B, so flux is zero but changing most rapidly.
Account for magnetic braking and heating
Changing flux through bulk conductors drives eddy currents. Their magnetic effects oppose the motion or flux change, producing braking and transferring mechanical energy into internal energy.
Eddy currents are useful in induction heating and contactless braking but unwanted in transformer cores. Laminating a core interrupts current loops and raises their resistance, reducing heating loss without removing the desired magnetic flux.
Check your understanding: Why does a magnetic brake slow an object without contact?
Motion changes flux and induces currents whose magnetic forces oppose that motion; kinetic energy becomes internal energy.
Key ideas to keep
- Motion parallel to the field gives no motional e.m.f.
- An induced current requires a closed conducting path.
- Mechanical work supplies the electrical or thermal energy produced.
See the reasoning
Worked example
Explain eddy-current braking as an energy transfer
Question: Explain eddy-current braking in a conducting plate moving through a non-uniform magnetic region.
Step 1: Identify the changing flux
Why: Motion through a non-uniform field changes flux through loops in the plate.
Working: Different parts of the conductor enter and leave the magnetic region.
Step 2: Apply Faraday and Lenz
Why: Changing flux induces circulating currents whose effects oppose the cause.
Working: Eddy-current magnetic forces oppose the plate's motion.
Step 3: Complete the energy account
Why: The braking force does not destroy energy.
Working: Mechanical energy becomes internal energy through resistive heating in the plate.
Answer: Motion changes flux through loops within the plate, inducing circulating currents. Their fields oppose the flux change and hence the motion. The opposing force removes mechanical energy, which becomes internal energy through resistive heating.
Check: An external agent must do work to maintain steady motion through the braking region.
Use a hint if needed
Practise with support
Try this
A rod's speed triples at fixed B, l and perpendicular geometry. State the motional e.m.f. factor.
Hint: The simple expression requires mutually perpendicular rod, velocity and field.
Check your answer
E = Blv, so it triples.
Now work without the hint
Practise independently
Your turn
Explain one generator-effect application and one eddy-current application using flux change, Lenz's law and energy.
Check your answer
For a moving conductor or generator, cutting field lines changes linkage and creates motional e.m.f.; mechanical input becomes electrical output. In eddy-current braking, induced loops oppose motion and convert mechanical energy to internal energy. Both obey energy conservation through Lenz's law.
Avoid these traps
Common mistakes
Common mistake
Motion anywhere in a magnetic field always induces Blv.
What is wrong with this reasoning?
Show better thinking
The simple form requires the conductor to cut field lines with mutually perpendicular B, l and v components.
Common mistake
Eddy-current braking destroys mechanical energy.
What is wrong with this reasoning?
Show better thinking
Mechanical energy is transferred mainly to internal energy through resistive heating.
Write for the examiner
Exam guidance
Link every application to a named flux change and an energy source.
Exam-style practice [6 marks]
A 0.40 m conductor moves at 7.0 m s⁻¹ perpendicular to 0.25 T. Find e.m.f. and explain why an applied force is needed when current flows.
Plan before you answer
- Use ε = Blv.
- Use Lenz's law for the opposing force.
- Name the mechanical energy input.
Mark your answer and compare the model
Marking points
Tick each point only if your answer states it clearly.
Model answer
E = Blv = 0.70 V. Induced current in the field experiences a magnetic force opposing the motion by Lenz's law, so an external force must do work that supplies electrical and thermal energy.
Come back in three days
Check what stayed with you
Recall question
Why do laminations reduce unwanted transformer-core eddy currents?
Check the answer
Insulated laminations interrupt large conducting loops, raising path resistance and reducing loop area, current and resistive energy loss.
Syllabus and review details
This lesson covers the listed H2 Physics 9478 outcomes. Flux uses area perpendicular to B; flux linkage is NΦ for N linked turns. Faraday's law uses the rate of change of linkage and Lenz's law fixes polarity from the change being opposed. The simple Blv motional-e.m.f. form requires mutually perpendicular conductor length, velocity and uniform field. Ideal transformer ratios assume common linked flux, alternating operation and no winding or core losses. More advanced induction applications are not required here.
- GCE A-Level H2 PhysicsTopic 18(f) · 2027Checked against the syllabus · partial topic coverageOfficial 9478 syllabus
Course and syllabus information
- Course
- GCE A-Level H2 Physics
- Edition
- GCE A-Level H2 Physics 2027