Question Video: Identifying the North Pole of an Induced Magnetic Field | Nagwa Question Video: Identifying the North Pole of an Induced Magnetic Field | Nagwa

# Question Video: Identifying the North Pole of an Induced Magnetic Field Physics • Third Year of Secondary School

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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?

03:43

### 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.

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