Question Video: Identifying the Direction of the Magnetic Field in a Direct Current Motor | Nagwa Question Video: Identifying the Direction of the Magnetic Field in a Direct Current Motor | Nagwa

Question Video: Identifying the Direction of the Magnetic Field in a Direct Current Motor Physics • Third Year of Secondary School

Which of the following diagrams, showing a direct current motor, correctly labels the poles of the magnet in the motor? [A] Diagram A [B] Diagram B

03:39

Video Transcript

Which of the following diagrams showing a direct current motor correctly labels the poles of the magnet in the motor?

So both of these diagrams show the same motor. The motor is rotating clockwise. So if we look at the point where the coil is parallel to the magnetic field, the left-hand side of the coil will move upwards and the right-hand side of the coil will move downwards. Next, let’s look at the direction of current in the motor. We see that the positive terminal is on the left and the negative terminal on the right. Again, we can look at the point in time when the coil is horizontal.

If we follow the path that the current takes around the coil, we can see that the direction of current on the left side of the coil is into the diagram and on the right-hand side of the coil is out of the diagram. Finally, we can look at the magnetic field across the motor, which we know is horizontal, but the left-hand diagram shows it going from left to right because the north pole is on the left and the south pole is on the right. And the right-hand diagram shows it going from right to left because the north pole is on the right and the south pole is on the left.

And the question asked us, “Which of these diagrams correctly shows the poles of the magnet in the motor?” And to do that, we need to find the correct direction of the magnetic field. To work this out, we will use a right hand rule. The right hand rule tells us that if we point our fingers in the direction of current in a wire and then bend our fingers so that they are pointing in the direction of the magnetic field across the wire and then we outstretch our thumb, then the thumb will be pointing in the direction that the wire will move.

We will apply this to the left-hand side of the coil. For the left-hand side of the coil, we know that the direction of current is into the diagram. And we know that the coil moves upwards. And we would like to work out if the direction of the magnetic field is to the left or to the right. In the example of the right-hand rule that we just looked at, current was from right to left, field was out of the diagram, and the motion of the wire was upwards.

Now this actually isn’t too far from what we have in these question diagrams. In both cases, the motion of the wire is upwards. In fact, all we need to do to make our example look like our question is to rotate our current and our field directions by 90 degrees, which gives us the direction of current pointing into the diagram, just as we do in our question. And the direction of the magnetic field is from right to left. So the correct direction of the magnetic field in the question diagram is from right to left as well.

Another way to think about this is, how can we rotate our hand so that it matches the question diagram? As we’ve seen, the thumb is pointing in the correct direction for the motion of the wire. But instead of the current pointing from right to left, we need to rotate our hand so that the part of our hand that used to point from right to left now points into the diagram. And then we see that our fingers that used to point out of the diagram now point from right to left, again telling us that the magnetic field must be from right to left in this motor.

But which diagram does this correspond to? Well, remember that in diagram (A), we saw that magnetic field was from left to right. So diagram (A) can’t show the correct direction of magnetic field. However, in diagram (B), the magnetic field was from right to left, which is exactly what we’re looking for. And if we look at diagram (B) closer, we see that the north pole is on the right and the south pole is on the left. So we’ve ruled out diagram (A). And we’ve shown that diagram (B) shows the correct direction of magnetic field in the motor. So the diagram that correctly labels the poles of the magnet in the motor is diagram (B).

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