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Magnetic Fields: Question 9

Syllabus 20.5

Multiple choice A2 1 mark

A bar magnet is held above a horizontal circular coil of wire, with its north pole facing downward toward the coil. The coil is connected to a sensitive ammeter. The magnet is released and falls, north pole first, straight down toward the coil.

While the magnet is still above the coil and approaching it (before it reaches the coil), what is the direction of the current induced in the coil, when viewed from above?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify the changing flux

As the magnet falls toward the coil, north pole first, the magnetic field it produces at the coil grows stronger, so the downward magnetic flux linking the coil is increasing. By Faraday’s law, this changing flux induces an e.m.f. and hence a current in the coil.

Step 2: Apply Lenz’s law

Lenz’s law states that the direction of an induced current is always such as to oppose the change in flux that produces it. Since the flux through the coil is increasing (downward, toward the coil, from the approaching north pole), the induced current must create its own magnetic field that opposes this increase. That is, a field pointing upward through the coil, back toward the approaching magnet.

Physically, this means the coil’s upper face must behave like a north pole, so that it repels the approaching north pole of the falling magnet, resisting the motion that is increasing the flux (consistent with energy conservation: work must be done against this repulsion to keep the magnet falling, and this work becomes the electrical energy of the induced current).

Step 3: Find the current direction from the required polarity

By the right-hand grip rule, a coil face behaves as a north pole (field emerging toward the viewer on that side) when the current flows anticlockwise as viewed from that same side. Since the required north pole faces upward, toward a viewer looking down from above, the current must flow anticlockwise when viewed from above.

Step 4: Why the other options are wrong

  • B (clockwise): this would make the coil’s upper face a south pole, which would attract the approaching north pole and reinforce the increasing flux, violating Lenz’s law.
  • C (alternating): the flux is increasing in the same sense throughout the approach, so the induced current direction stays constant (though its magnitude may change as the magnet gets closer and the rate of change of flux changes). It does not alternate.
  • D (no current until contact): an e.m.f. is induced whenever flux linkage is changing, which begins as soon as the magnet starts approaching, not only once it reaches the coil.

Final answer

  • The induced current flows anticlockwise\boxed{\text{anticlockwise}}, when viewed from above, option A.