In experiment #1, a bar magnet is moved towards a conducting wire loop axially, with the magnet's north pole facing the loop. In experiment #2, the same process as in experiment #1 is repeated except that the south pole of the magnet faces the loop. Which one of the following statements is true in this context?
- A.The direction of current in the loop will be of opposite nature in both the experiments.
- B.The direction of current in the loop will be the same in both the experiments.
- C.No current will flow in either of the two experiments.
- D.More current will flow in the loop in experiment #1.
Correct answer
Explanation
The correct answer is A, the current flows in opposite directions in the two experiments. Reversing the pole that approaches the loop reverses the change in magnetic flux, so the induced current reverses too.
When a magnet moves towards a closed loop, the magnetic flux through the loop changes and an electric current is induced (Faraday's law of electromagnetic induction). By Lenz's law, the induced current always opposes the change that causes it. With the north pole approaching, the face of the loop towards the magnet becomes a north pole to push it back. With the south pole approaching, that face becomes a south pole instead, so the current must circulate the other way.
B is wrong: the same direction would mean the loop ignores which pole is approaching, which contradicts Lenz's law.
C is wrong: a moving magnet changes the flux through the loop, so current does flow in both cases.
D is wrong: with the same magnet moved in the same way, the size of the current is the same; only its direction changes.
Exam tip: Faraday's law gives the size of the induced EMF; Lenz's law gives its direction (it opposes the change).
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