Question Video: Identifying the Direction of Rotation of a Coil in a DC Motor Physics

Which of the diagrams showing a direct current motor correctly represents the direction of the rotation of the motor? The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor. [A] Diagram A [B] Diagram B

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Video Transcript

Which of the diagrams showing a direct current motor correctly represents the direction of the rotation of the motor? The coil of the motor is simultaneously shown at four different angles to the magnetic field of the motor.

Let’s start with the left-hand diagram. As we can see, there is a magnetic field across the motor from north to south, which on this diagram means it is from left to right. We are also shown the direction of current through the motor. And we can follow the path that the current takes from positive to negative when the coil is parallel to the magnetic field. We can see that the current is perpendicular to the magnetic field along two edges of the coil, this one on the left, where it is traveling into the diagram and this one on the right, where it is traveling out of the diagram. Finally, we are shown the direction of rotation of the motor, which shows us that the left-hand side of the coil will move downwards and the right-hand side of the coil will move upwards.

The question asks us to work out whether this is showing the correct direction of rotation of the motor. And to do this, we can use a right-hand rule where we will consider this side of the coil here if we take our right hand and point our fingers along the direction of current in the wire, so, in this case, into the diagram. Next, we rotate our hand so that when we bend our fingers, they will point in the direction of the magnetic field. In this case, we need to rotate our hand 90 degrees counterclockwise so that when we bend our fingers, they point in the direction of the magnetic field, which is from left to right. Then, we out-stretch our thumb, and this will point in the direction that the wire will move, which in this case is downwards.

So, our right-hand rule suggests that if our direction of current is into the diagram and our magnetic field is pointing from left to right, then the motion of the wire will be downwards. And because the left-hand side of the coil is moving downwards, the direction of rotation of the motor must be counterclockwise, which is what is shown on the diagram. So, this means diagram (A) is a good candidate for the diagram that shows the correct direction of rotation of the motor.

Now, let’s take a look at diagram (B). Diagram (B) shows the same magnetic field, the same direction of current, except this time it shows the motor rotating clockwise. Now, if we take a look at the left-hand wire of the motor coil when the coil is parallel to the magnetic field, the direction of current is into the page, and the magnetic field is from left to right. This setup is identical to the one shown in diagram (A). So, we already know that this side of the coil will move downwards and the right-hand side of the coil will move upwards, which means the direction of rotation of the motor in diagram (B) is actually counterclockwise. So, the diagram is showing the incorrect direction of rotation. So, we can rule out diagram (B).

We found that diagram (A) is a good candidate for the one showing the correct direction of rotation of the motor, and we ruled out diagram (B). So, the diagram that correctly represents the direction of rotation of the motor is diagram (A).

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