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Physics · Unit 11

Electromagnetic Induction: MDCAT Notes and MCQs

Faraday's law

The emf induced in a circuit is directly proportional to the rate of change of magnetic flux through it: ε = −N ΔΦ/Δt. Flux can be changed by moving a magnet, changing current in a nearby coil, rotating a coil, or changing area. Motional emf of a rod: ε = BLv.

Lenz's law

  • The direction of the induced current is such that it opposes the change in flux that produces it (the minus sign in Faraday's law).
  • Consequence of conservation of energy: work must be done against the opposing force; that work appears as electrical energy. If the induced current aided the change, energy would be created from nothing.
  • Example: N-pole approaching a coil → near face becomes N (repels); receding → near face becomes S (attracts).

Transformer

  • Construction: two coils (primary and secondary) wound on a laminated soft-iron core. Works on mutual induction — needs AC (changing flux); does not work on steady DC.
  • Vs/Vp = Ns/Np = Ip/Is (ideal: VpIp = VsIs). Frequency is unchanged.
  • Step-up: Ns > Np → voltage ↑, current ↓. Step-down: Ns < Np → voltage ↓, current ↑.
  • Losses: eddy currents (reduced by laminating the core), hysteresis (soft iron), copper (I²R) losses (thick wires), flux leakage. Efficiency of large transformers ≈ 95–99%.

Power transmission

Power is stepped up to very high voltage (e.g. 132–500 kV) at the power station so that current is small; line loss P = I²R is greatly reduced. Step-down transformers near homes reduce voltage to 220 V for safe use.

PMDC learning outcomes for this unit
  • Induction & transformers: 11.1 State Faraday's law of electromagnetic induction.
  • Induction & transformers: 11.2 Account for Lenz's law to predict the direction of an induced current and relate it to conservation of energy.
  • Induction & transformers: 11.3 Describe the construction of a transformer and explain how it works.
  • Induction & transformers: 11.4 Describe how step-up and step-down transformers can be used to ensure efficient transfer of electricity along cables.

Electromagnetic Induction MCQs with answers

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Q1
According to Faraday's law, induced emf is proportional to:
AMagnetic field only
BResistance of the coil
CRate of change of magnetic flux
DMagnetic flux
Show answer
Answer: C (Rate of change of magnetic flux). ε = −N dΦ/dt.
Q2
Lenz's law is a consequence of the law of conservation of:
AMomentum
BEnergy
CCharge
DMass
Show answer
Answer: B (Energy). Opposition to change ensures work is done to produce electrical energy.
Q3
A rod of length 0.5 m moves at 4 m/s perpendicular to a 0.2 T field. The motional emf is:
A0.4 V
B4 V
C0.1 V
D1.6 V
Show answer
Answer: A (0.4 V). ε = BLv = 0.2 × 0.5 × 4 = 0.4 V.
Q4
A transformer works on the principle of:
AElectrostatic induction
BMutual induction
CSelf induction
DThermionic emission
Show answer
Answer: B (Mutual induction). Changing current in the primary induces emf in the secondary.
Q5
A transformer cannot be used with steady DC because:
ADC heats the core
BSteady DC produces no changing flux
CDC is too large
DDC has no voltage
Show answer
Answer: B (Steady DC produces no changing flux). Without changing flux there is no induced emf.
Q6
A step-up transformer has 100 turns in primary and 1000 in secondary. If input is 22 V, the output voltage is:
A2200 V
B220 V
C2.2 V
D22 V
Show answer
Answer: B (220 V). Vs = Vp × Ns/Np = 22 × 10 = 220 V.
Q7
The core of a transformer is laminated to reduce:
ACopper losses
BEddy current losses
CHysteresis losses
DFlux leakage
Show answer
Answer: B (Eddy current losses). Thin insulated laminations block eddy-current paths.
Q8
Electric power is transmitted at high voltage to:
AReduce I²R losses
BIncrease frequency
CIncrease current
DIncrease resistance
Show answer
Answer: A (Reduce I²R losses). Higher voltage means lower current for the same power, reducing heating losses.
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