Electromagnetic induction: flux, laws and transformers
Key idea: Define the linked quantity before taking its rate of change, use Lenz's law as an energy-consistent direction rule, and keep simple applications and ideal-transformer ratios within their stated assumptions.
Before you start: Electromagnetic Forces objective chainCurrent Electricity objective chain
By the end, you can
- Define and calculate magnetic flux and flux linkage with correct orientation.
- Infer induction behaviour from experiments and apply Faraday's and Lenz's laws.
- Explain simple motional-e.m.f. and eddy-current applications using energy conservation.
- Explain simple iron-core transformer operation and apply ideal voltage/current ratios.
Starting-point self-check
1. Check your starting point
Attempt all four groups without notes and mark the first area, linkage, rate, polarity, application or ideal-model decision you cannot justify. Use the recorded topic diagnostic above when you want scoring and a personalised repair plan.
Simple applications of electromagnetic induction 18(f)
Question 1
A 0.25 m rod moves at 4.0 m s⁻¹ perpendicular to a 0.30 T field and its length. Find the motional e.m.f. and explain the energy source.
Check the model response
E = Blv = (0.30)(0.25)(4.0) = 0.30 V. Mechanical work maintaining the motion supplies the electrical energy; current, if the circuit is closed, produces an opposing magnetic force.
repair
2. Repair the common breaks
Use only the correction matching an error, then retry the corresponding diagnostic.
Simple applications of electromagnetic induction 18(f)
Check this idea
Misconception: Motion anywhere in a magnetic field always induces Blv.
Repair: The simple form requires the conductor to cut field lines with mutually perpendicular B, l and v components.
Check this idea
Misconception: Eddy-current braking destroys mechanical energy.
Repair: Mechanical energy is transferred mainly to internal energy through resistive heating.
worked example
3. Follow four worked models
Follow how each solution fixes area orientation, linkage sign, energy pathway or ideal-transformer assumptions before calculating.
Simple applications of electromagnetic induction 18(f)
Model 1
Explain eddy-current braking in a conducting plate moving through a non-uniform magnetic region.
Check the model response
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.
guided practice
4. Guided practice
Use each hint only to select the correct perpendicular area, linkage rate, motional geometry or turns ratio.
Simple applications of electromagnetic induction 18(f)
Question 1
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 the model response
E = Blv, so it triples.
independent practice
5. Independent practice
Solve without repair notes and state sign conventions, field geometry, circuit closure and ideal-transformer assumptions.
Simple applications of electromagnetic induction 18(f)
Question 1
Explain one generator-effect application and one eddy-current application using flux change, Lenz's law and energy.
Check the model response
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.
Practice exit check
6. Practice assessment
Use this as extra closed-book practice, then complete the separate recorded assessment in your plan.
Simple applications of electromagnetic induction 18(f)
Question 1
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.
Check the model response
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.
Re-test practice
7. Delayed re-test practice
Return after at least three days and solve these fresh contexts without reopening earlier responses. The recorded plan enforces the delay and uses a separate re-test family for selected-response skill-group evidence.
Simple applications of electromagnetic induction 18(f)
Question 1
Why do laminations reduce unwanted transformer-core eddy currents?
Check the model response
Insulated laminations interrupt large conducting loops, raising path resistance and reducing loop area, current and resistive energy loss.