Question Video: Understanding the Function of a Commutator Physics

The diagram shows a direct-current motor. The coil of the motor is simultaneously shown at three different angles to the magnetic field of the motor. Which of the diagrams correctly shows the paths of currents through each of the coils, from the positive terminal of the motor to the negative one? [A] Diargram A [B] Diargram B [C] Diargram C [D] Diargram D

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Video Transcript

The diagram shows a direct-current motor. The coil of the motor is simultaneously shown at three different angles to the magnetic field of the motor. Which of the diagrams correctly shows the paths of currents through each of the coils, from the positive terminal of the motor to the negative one?

Our diagram shows the coil of the motor at three different positions. We will label the position where the coil is horizontal position one. Then when the coil is rotated clockwise by about 45 degrees, we will label this as position two, and then when it is rotated a further 90 degrees, position three. The diagram also shows us that the positive terminal of the motor is on the left and the negative terminal is on the right, meaning that this brush on the left is connected to the positive terminal, and the brush on the right is connected to the negative terminal.

These two semicircles make up the commutator, and this rotates with the coil as it moves. Let’s take a look at the commutator in each of these three positions to work out what the direction of the current in the coil will be. Let’s start with position one. As we can see the left-hand half of the commutator is connected to the positive terminal and the right-hand half of the commutator is connected to the negative terminal. This means that current will enter the coil from the left and exit it on the right.

Let’s draw this on our main diagram. As we can see, current will move clockwise around the coil, moving into the diagram on the left and out of the diagram on the right. We can check any of our four diagrams to see if they match this. And we can see that diagrams (A), (B), and (C) all show the correct direction of current in position one, whereas diagram (D) doesn’t. Therefore, we can rule out option (D). Now, let’s take a look at the commutator in position two. The commutator in position two looks very similar to position one, except we see this is rotated clockwise by 45 degrees.

Once again, the left-hand half of the commutator is connected to the positive terminal and the right-hand half is connected to the negative terminal. So the same as in position one, current enters the coil on the left and exits on the right. Just as in position one, the direction of current on the left-hand side of the coil is into the diagram and on the right-hand side of the coil is out of the diagram.

Let’s check this against our remaining options of (A), (B), and (C). Immediately, we can see that diagram (A) shows the incorrect direction of current, in position two, diagram (A) showing current going out of the diagram on the left and into the diagram on the right. This is incorrect. So we can rule out option (A). Whereas diagrams (B) and (C) both showed the correct direction of current.

So on to position three. Position three is rotated around 90 degrees clockwise from position two. And this means that the half of the commentator that was on the right in position two is now on the left in position three. Similarly, the half of the commutator that was on the left in position two is now on the right.

And what this actually means is that even though it’s a different half of the commutator contacting the positive and negative brushes, current still enters the coil from the left and exits the coil on the right. And we can draw this on our diagram. And we can see that once again the direction of current on the left-hand side of the coil is into the diagram and on the right-hand side of the coil is out of the diagram.

Looking at the remaining options of (B) and (C), we see that in diagram (B) the direction of current on the left-hand side of the coil in position three is into the diagram and is out of the diagram on the right-hand side of the coil, so this is correct, whereas diagram (C) shows the opposite. In position three, the direction of current on the left-hand side of the coil is out of the diagram and is into the diagram on the right-hand side. This is incorrect. So we can rule out option (C).

So we ruled out options (A), (C), and (D). And we’ve shown that diagram (B) shows the correct direction of current for each of the positions of the coil. Therefore, the diagram that correctly shows the paths of current through each of the coil, from the positive terminal of the motor to the negative one, is diagram (B).

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