Question Video: Finding the Direction of Current in a DC Motor Physics

The diagram shows a direct-current motor. The direction of current through the coil of the motor is shown. Which of the following diagrams correctly shows the direction of current through the coil when it has rotated counterclockwise by 90°? [A] Diagram A [B] Diagram B

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Video Transcript

The diagram shows a direct current motor. The direction of the current through the coil of the motor is shown. Which of the following diagrams correctly shows the direction of current through the coil when this is rotated counterclockwise by 90 degrees?

On our diagram, we can see three things. First, we can see that the coil of the motor is rotating counterclockwise. Secondly, we can see that the magnetic field is from left to right because it’s going from the north pole, which is on the left, to the south pole, which is on the right. Third, the direction of current in the coil of the motor is into the coil on the left and out of the coil on the right. And this is because we have the positive terminal of the motor on the left and the negative terminal on the right.

We’re then shown two possible diagrams, (A) and (B). The motion of the coil in both of these diagrams is still counterclockwise, and the direction of the magnetic field is still from left to right because the north and south pole in each diagram is in the same place. The positive–negative terminals of motor are also the same in that the positive terminal is on the left and the negative terminal is on the right. What has changed is the orientation of the coil, which, we are told in the question, has rotated counterclockwise by 90 degrees. So the coil started in this orientation on the left and has now rotated counterclockwise by 90 degrees to what we see in the diagrams on the right.

And the question asks us to find out which of these diagrams correctly shows the direction of current through the coil of the motor. And diagram (A) shows current going into the coil from the right and out of the coil from the left, whereas diagram (B) shows current going into the coil from the left and out of the coil from the right. And we can immediately see that in diagram (A) that if current is going into the coil of the motor from the right and out of the motor to the left that the direction of current in this diagram is from negative to positive. And we know that can’t be true, so we can be fairly sure that diagram (A) is incorrect.

But to understand why, let’s look more closely at the motor. This shows an end-on view of the commutator and the brushes of the motor. Now these semicircles are the commutator, and they rotate with the coil. And these rectangles are brushes, and they remain stationary. As we can see, whichever half of the commutator is on the left will be in contact with the positive brush, and whichever half was on the right will be in contact with the negative brush. So if we imagine rotating our coil by something like 45 degrees, the direction of current is still into the coil from the left and out of the coil to the right.

If we keep rotating the coil, even though which half of the commutator is connected to the positive and negative brushes has changed, the direction of current is still into the coil from the left and out of the coil to the right. So even though it’s rotated, the direction of current is still the same. So when we look at our question, even though it’s rotated counterclockwise by 90 degrees, the direction of current will still be in from the left and out to the right, from positive to negative. And this corresponds to diagram (B).

The diagram that correctly shows the direction of current through the coil when it is rotated counterclockwise by 90 degrees is diagram (B).

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