Unit: electricity and magnetism
Year: 1989 Question#: 39
Question: A bar magnet is dropped through a wire loop as shown in the diagram. As the south pole approaches the loop, the electron flow induced in the loop
(1 ) is clockwise
(2 ) is counterclockwise
(3 ) attracts the south pole
(4 ) speeds up the magnet
What is this question really asking?
Explanation by: Allison B.
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Answer 1
is clockwise
INCORRECT
A clockwise current would induce a magnetic field like the one shown in the picture. This magnetic field would attract the magnetic field of the moving magnet, which violates Lens' Law (which says that a moving magnetic field creates a magnetic field which opposes the magnetic field that created it).
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Answer 2
is counterclockwise
CORRECT
The moving magnetic field of the magnet will create a magnetic field in the wire by inducing a current. The magnetic field created will oppose the magnetic field of the magnet (Lens' Law). So, using the first hand rule, the current must be counterclockwise in order to create a magnetic field in opposition to the magnetic field of the permanent magnet.
return to top Answer 3
attracts the south pole
INCORRECT
A south pole sould be attracted by a north pole. The magnetic field produced by a current in a wire has direction but no poles. So the created magnetic field cannot attract the south pole. return to top
Answer 4
speeds up the magnet
INCORRECT
Lens' Law states that a created magnetic field opposes the field that created it, so it wouldn't help it out by speeding it up. return to top
What's this question really asking?
This question is asking if you understand how Lens' Law works. return to top
