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Theme D — Fields

D.4.3 Faraday’s Law of Induction — IB Physics HL Revision Notes & Topical Questions

5 min read 8 parts Lesson 37 of 39 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

D.4.3 Faraday's Law of Induction — IB PHYSICS HL notes: State Faraday's law and use ε = N(ΔΦ)/Δt to find the e.m.f. induced by a changing flux linkage.Statement — the magnitude of an induced e.m.f.

  • Number of turns
  • Plane perpendicular to the field
  • Plane parallel to the field
  • Lenz's law
  • Alternating supply
  • Motional e.m.f

Part 1 of 6Faraday's law

Statement
  • Statement the magnitude of an induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage
Equation
  • ε = N(ΔΦ)/Δt
Symbols
  • ε in V, N(ΔΦ) the change in flux linkage in Wb turns and Δt in s
Graph
  • Graph the e.m.f. is the gradient of a flux linkage–time graph

Part 2 of 6What changes the flux linkage

Magnetic flux density B
Cross-sectional area A
Angle θ between the field and the normal
Number of turns
  • Number of turns it multiplies the e.m.f.

Part 3 of 6A rotating coil

A rotating coil
Plane perpendicular to the field
  • Plane perpendicular to the field flux linkage is a maximum but the e.m.f. is zero because there is no change of flux
Plane parallel to the field
  • Plane parallel to the field flux linkage is zero but the e.m.f. is a maximum because the rate of change is greatest
Cutting
  • Cutting an e.m.f. is induced only if the conductor cuts field lines

Part 4 of 6Calculating an average e.m.f.

1
Convert B to tesla and the area to m2.
2
Work out the initial and final angles between the normal and the field.
3
Find N, BA cos θ for the start and the end.
4
Subtract to find the change in flux linkage.
5
Divide by the time interval to find the average e.m.f.
  • Angles
    • if a coil in the field plane turns through φ, the angle to the normal is 90° − φ

Part 5 of 6Link to other ideas

Lenz's law
  • Lenz's law the minus sign in ε = −N(ΔΦ)/Δt gives the direction
Alternating supply
  • Alternating supply an a.c. supply produces an alternating e.m.f. with the same frequency
Motional e.m.f.
  • ε = BLv follows from the rate of flux cutting

Part 6 of 6More from the specification

More from the specification
Induced electromotive force and energy
  • Induced electromotive force and energy the magnitude of induced EMF equals the time rate of change of magnetic flux, conserving electromagnetic energy.
Closed circuit
  • Complete loop with a source in which current can flow
Constant speed
  • Speed that does not change.
Electromagnetic induction
  • Production of an e.m.f. by relative movement of a conductor and a magnetic field.
Current
  • Flow of electrons in a circuit.
Voltmeter
  • Instrument that measures p.d., connected in parallel.
Primary
  • Input coil.
Secondary
  • Output coil.
Angular speed
  • Magnitude of the angular velocity.
Mutual induction
  • Induction of an e.m.f. in one circuit by a change of current in a neighbouring circuit.
Potential difference
  • Work done by a unit charge passing through a component.
Alternating current
  • Current that continually reverses direction.
No change in flux
  • No change in flux a magnet held still inside or outside a coil induces no e.m.f., because the flux linkage is not changing.

Key terms

TermMeaning
Faraday's lawThe magnitude of the induced e.m.f. equals the rate of change of magnetic flux linkage.
Rate of change of flux linkageThe change in NΦ divided by the time taken.
Average e.m.f.The change in flux linkage divided by the time interval.
Weber turnsThe unit of flux linkage.

Avoid these mistakes

1
Saying an e.m.f. needs a large flux — it needs a changing flux linkage.
2
Using the change in flux without multiplying by N.
3
Using the total flux instead of the rate of change.
4
Mixing up the angle to the plane with the angle to the normal.
5
Forgetting to convert the time or the area into SI units.

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