Retrieval Practice

Electromagnetic Induction — AQA A-Level Physics

Q1. State Faraday's law of electromagnetic induction.
The magnitude of the induced EMF is equal to the rate of change of flux linkage: ε = NΔΦ/Δt.
Q2. State Lenz's law.
The induced EMF always acts in a direction to oppose the change in flux linkage that is producing it.
Q3. Why is there a minus sign in ε = −NΔΦ/Δt?
  • The minus sign represents Lenz's law.
  • It shows that the induced EMF opposes the change in flux linkage.
  • If the induced EMF aided the change, energy would be created from nothing, violating conservation of energy.
Q4. State four methods of inducing an EMF.
  • Moving a wire through a magnetic field, moving a magnet into or out of a coil, changing the current in a nearby coil (mutual induction), rotating a coil in a magnetic field.
  • All work by changing the flux linkage.
Q5. Derive the expression ε = BLv for a straight conductor moving through a field.
  • In time Δt, the wire sweeps out area ΔA = LvΔt.
  • Change in flux: ΔΦ = BΔA = BLvΔt.
  • By Faraday's law: ε = ΔΦ/Δt = BLvΔt/Δt = BLv.
Q6. Write expressions for the flux linkage and the EMF for a coil rotating in a uniform field.
  • Flux linkage: NΦ = BAN cos(ωt).
  • Induced EMF: ε = BANω sin(ωt).
  • The EMF is the rate of change of the flux linkage.
Q7. State the expression for peak EMF and identify when it occurs during rotation.
  • Peak EMF: ε₀ = BANω.
  • It occurs when sin(ωt) = 1, which is when the coil is edge-on to the field (flux linkage = 0, rate of change is maximum).
Q8. On a graph of flux linkage against time, at what points is the EMF (a) zero and (b) maximum?
(a) EMF is zero at the peaks and troughs of the flux linkage graph (gradient = 0). (b) EMF is maximum where the flux linkage graph crosses zero (gradient is steepest).
Q9. What are eddy currents and why do they cause energy loss?
  • Eddy currents are circulating currents induced in the bulk of a conductor by a changing magnetic flux.
  • They dissipate energy as heat due to the electrical resistance of the metal.
Q10. Explain how laminations in a transformer core reduce eddy current losses.
  • The core is made of thin iron sheets insulated from each other.
  • The insulating layers break up the eddy current loops, increasing the resistance of eddy current paths.
  • Smaller loops carry smaller currents, reducing I²R heating losses.
Q11. Describe how a search coil can be used to measure magnetic flux density.
  • Place a small coil of known area A and N turns in the field, perpendicular to B.
  • Either pull it out quickly or rotate it 90° and measure the induced EMF.
  • Use Faraday's law to calculate B: for a quick pull, B = ε_avg × Δt / (AN).
Q12. A magnet is dropped through a vertical coil. Describe the EMF induced as it falls through.
  • As the magnet approaches, flux linkage increases and EMF builds in one direction.
  • As it passes through the centre, EMF drops to zero momentarily.
  • As it exits, flux linkage decreases and EMF is induced in the opposite direction.
  • The exit pulse is sharper and taller because the magnet is moving faster (due to gravity).
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