Free Electromagnetic Induction MCQs with Answers

• A. the magnetic flux through it
• B. the rate of change of magnetic flux linkage
• C. the current in it
• D. the resistance of the circuit

Explanation: It is the rate of change that matters, not the flux itself, so a coil sitting in a strong but steady field has no emf induced in it at…

• A. maximum
• B. zero
• C. half the maximum
• D. reversed in direction

Explanation: With the magnet at rest the flux linkage is constant, so its rate of change is zero and no emf is induced however strong the magnet.

• A. assists the change producing it
• B. opposes the change producing it
• C. is always clockwise
• D. is independent of the change

Explanation: The induced current sets up a field that opposes the flux change, so pushing a north pole towards a coil induces a current making the near…

• A. that the emf is always negative
• B. Lenz's law, that the induced effect opposes the change causing it
• C. that flux always decreases
• D. an error of convention with no physical meaning

Explanation: The sign encodes the direction of the induced emf rather than its magnitude, stating that it acts to oppose the change in flux.

• A. copper is magnetic and attracts it
• B. air resistance inside the tube is very large
• C. induced eddy currents in the tube produce a field opposing the magnet's motion
• D. the magnet loses its magnetism as it falls

Explanation: The changing flux induces circulating eddy currents in the tube wall, and by Lenz's law their magnetic field opposes the motion, retarding…

• A. self induction only
• B. mutual induction between two coils linked by a common changing flux
• C. electrostatic attraction
• D. the motor effect

Explanation: Alternating current in the primary produces a continuously changing flux, which the laminated iron core carries to the secondary, where it…

• A. more turns on the secondary than the primary
• B. fewer turns on the secondary than the primary, giving a lower output voltage
• C. the same number of turns on both
• D. no core

Explanation: Voltage divides in the ratio of the turns, so fewer secondary turns give a lower voltage, and for an ideal transformer the current rises…

• A. 2200 V
• B. 22 V
• C. 220 V
• D. 2.2 V

Explanation: The turns ratio is 10 to 1 down, so the voltage falls by the same factor, from 220 V to 22 V.

• A. high voltage travels faster
• B. the current is then small, so the I squared R heating loss in the cables is small
• C. high voltage cables are cheaper
• D. consumers need high voltage

Explanation: For a given power, raising the voltage lowers the current in proportion, and since the loss goes as the square of the current, a tenfold…

• A. make it lighter
• B. increase the flux through it
• C. reduce energy lost as heat through eddy currents in the core
• D. insulate the two coils from each other

Explanation: Thin sheets separated by insulating varnish break up the circulating paths available to eddy currents, so much less energy is dissipated…

• A. induces an emf in a neighbouring coil
• B. opposes any change in the current flowing through itself
• C. stores charge like a capacitor
• D. converts alternating current to direct current

Explanation: A changing current changes the coil's own flux, which by Lenz's law induces a back emf opposing that change, so an inductor resists sudden…

• A. henry
• B. weber
• C. tesla
• D. farad

Explanation: One henry is the inductance of a coil in which a current changing at one ampere per second induces a back emf of one volt.

• A. perpendicular to the magnetic field
• B. parallel to the magnetic field
• C. at 45 degrees to the field
• D. irrelevant, since the emf is constant

Explanation: The emf depends on the rate of change of flux, not on the flux itself, and that rate is greatest when the coil is edge on to the field…

• A. Moving the magnet faster
• B. Using a stronger magnet
• C. Increasing the number of turns on the coil
• D. Increasing the resistance of the coil

Explanation: The induced emf depends on flux, turns and speed of change, none of which is altered by the coil's resistance; resistance affects the…

• A. adds to the supply voltage
• B. opposes the supply voltage and limits the current drawn
• C. has no effect on the current
• D. appears only when the motor is stationary

Explanation: As the armature spins it acts as a generator, inducing an emf that by Lenz's law opposes the supply, so the effective driving voltage and…

• A. is attracted into the coil
• B. is thrown upwards, because the induced current opposes the changing flux
• C. remains unaffected
• D. melts instantly

Explanation: The changing flux induces a current in the closed ring, and by Lenz's law the ring's field opposes the coil's, so the two repel and the…

• A. the output power equals the input power
• B. the output voltage equals the input voltage
• C. the output current equals the input current
• D. no flux passes through the core

Explanation: Ideal means lossless, so all the power delivered to the primary appears at the secondary, and voltage and current change in inverse…

• A. direct current is too dangerous
• B. steady direct current gives no changing flux, so no emf is induced in the secondary
• C. the core would melt
• D. direct current cannot flow in a coil

Explanation: Induction requires a changing flux, and a steady current produces a steady flux, so the secondary voltage is zero except at the instants…

• A. the magnetic field, which is used up
• B. the mechanical work done against the opposing force predicted by Lenz's law
• C. nothing, since it is created by induction
• D. the resistance of the circuit

Explanation: Because the induced effect opposes the motion, whoever moves the magnet or turns the generator must do work, and that mechanical work is…

• A. 0.4 V
• B. 0.1 V
• C. 4 V
• D. 2.5 V

Explanation: The motional emf is BvL, that is 0.2 multiplied by 4 multiplied by 0.5, giving 0.4 V.