Free Lenz's Law MCQs with Answers

1. Lenz's law states that the direction of an induced current is such that it

• 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 face a north pole, which resists the approach. This opposition is why work must be done to move the magnet, and that work is the source of the electrical energy produced. Lenz's law is therefore conservation of energy applied to induction.

2. The minus sign in the expression for Faraday's law expresses

• 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. Without it, an induced current could reinforce the change producing it and energy would be created from nothing. The magnitude of the emf is given by the rest of the expression.

3. A magnet dropped down a long vertical copper tube falls unusually slowly because

• 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 the fall. Copper is not itself magnetic, so the effect disappears entirely if a plastic tube is used, which is the standard control for this demonstration. The same principle gives eddy current braking in trains.

4. The back emf in an electric motor

• 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 therefore the current fall as the motor speeds up. At the instant of starting there is no back emf, which is why the starting current is very large and why motors need starters. Overloading a motor slows it, reduces the back emf and can burn out the windings.

5. A metal ring is placed on the core of a coil carrying alternating current. The ring

• 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 ring jumps. Cutting a slit in the ring stops the induced current from circulating and the effect disappears, which is the standard control. The ring also warms up because of the current flowing in its own resistance.

6. The energy delivered by an induced current ultimately comes from

• 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 converted into electrical energy. This is why a bicycle dynamo makes pedalling harder once the lamp is switched on. A magnetic field is not consumed in the process.

7. A loop moves toward a stationary magnet. The induced current will:

• A. Enhance the motion
• B. Try to stop the approach
• C. Become zero
• D. Become alternating

Explanation: By Lenz's law the induced current sets up a field that opposes the change producing it, so the loop is repelled and work must be done to keep pushing it towards the magnet. That work is the source of the electrical energy generated, which makes Lenz's law a statement of energy conservation. A current that enhanced the motion would create energy from nothing.