Video Transcript
The diagram shows a bar magnet
moving away from a solenoid. This induces an electric current in
the solenoid, which creates its own magnetic field in turn. Which end of the solenoid is the
north pole of the induced magnetic field?
This question is about the magnetic
field created by the solenoid because of the moving bar magnet. Specifically, it’s asking us
whether the solenoid’s magnetic field is oriented so that its north pole is on the
left end of the solenoid, at point A, or the right end of the solenoid, at point
B.
To answer this question, let’s
recall some information about electromagnetic induction and Lenz’s law. First, we know that when a solenoid
experiences a changing magnetic field, which is occurring due to this bar magnet
moving to the left, a current is induced in the solenoid. This is electromagnetic
induction. We also know that when a current is
present in a solenoid, it produces its own magnetic field. This brings us to Lenz’s law, which
states that when current is created through electromagnetic induction, the direction
of the current is such that it generates a magnetic field opposing the change in the
original magnetic field. This means that the solenoid’s own
magnetic field must oppose the change in the original magnetic field. And remember, here, the original
magnetic field is due to this bar magnet.
So, to answer this question, we
need to identify which direction this solenoid’s magnetic field must be oriented in
order to oppose the change in the original bar magnet’s field. We know that every magnetic field
has a north and south pole, and the field produced by a solenoid is no
exception. So, in order to help visualize the
poles of the solenoid’s magnetic field, we can choose to model the solenoid as a bar
magnet with its north and south poles. So the question comes down to,
should its north pole be oriented on the left or right end?
Let’s imagine both scenarios. First, let’s suppose that the
solenoid’s north pole is on the left end. Notice then that two like magnetic
poles are facing toward each other. We know that like magnetic poles
repel. So this repulsive force would push
the original bar magnet further to the left, away from the solenoid, which is the
direction that it was heading to begin with. So here, instead of opposing the
original change in magnetic field, that change in the original magnetic field would
be aided by the repulsive force due to the like magnetic poles. This does not obey Lenz’s law.
Now, let’s imagine the solenoid’s
magnetic field being oriented the other way, with its north pole on the right
end. Notice here that we have opposite
magnetic poles facing toward each other. We know that opposite magnetic
poles attract. So this attractive force would act
to pull the original bar magnet to the right, which is opposite to the direction
that it was originally moving. So here the solenoid’s magnetic
field does oppose the change in the original magnetic field. This obeys Lenz’s law.
Thus, using our understanding of
electromagnetic induction and Lenz’s law, we were able to see that the solenoid’s
induced magnetic field must be oriented with its north pole on the right end of the
solenoid. Going back to the original diagram
given to us, we see that B represents the right end of the solenoid. So this is our final answer. The north pole of the induced
magnetic field is at point B.
