Question Video: Finding the Direction of Current in the Coil of a DC Motor | Nagwa Question Video: Finding the Direction of Current in the Coil of a DC Motor | Nagwa

Question Video: Finding the Direction of Current in the Coil of a DC Motor Physics • Third Year of Secondary School

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The diagram shows a direct current motor. The blue arrows shown represent forces that act on the coil. Which of the terminals π‘Ž or 𝑏 is the positive terminal of the motor?

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Video Transcript

The diagram shows a direct current motor. The blue arrows shown represent forces that act on the coil. Which of the terminals π‘Ž or 𝑏 is the positive terminal of the motor?

In the diagram, we are shown a motor with a magnetic field that goes from north to south, so from left to right. And the blue arrows represent electromagnetic forces that are acting on the coil. The question asks us, which of the terminals π‘Ž or 𝑏 is the positive terminal of the motor? In other words, we must find the direction of current in the motor.

To do this, we will consider the left-hand side of the coil. We can use a right hand rule to work out the direction of current through this side of the coil. The right hand rule tells us that if we point our fingers in the direction of current in that side of the coil and then bend our fingers by 90 degrees to point in the direction of the magnetic field in the motor, which in this example is out of the diagram, then if we outstretch our thumb, this will point in the direction of force acting on that side of the coil.

In the question, we have a very similar setup to this. When we look at the left-hand side of the coil, the force on the coil is also acting upwards. However, the magnetic field, rather than being out of the diagram, is from left to right. So if we look at our right hand rule example from the top down, if we want to match this to our diagram on the left, all we have to do is rotate this 90 degrees counterclockwise, where we can see that force is upwards. The magnetic field is from left to right. And because we’re viewing the diagram from the front, the direction of current is out of the diagram.

So adding this to our diagram on the left, the direction of current in the left-hand side of the coil is out of the diagram. And this means that the direction of current on the right-hand side of the coil is into the diagram. And we can carry on and annotate the direction of current for the rest of the coil, where we can see that charge flows into the coil from the right and out of the coil from the left, meaning the direction of current is from 𝑏 to π‘Ž. And we know that charge flows from positive to negative. So 𝑏 must be the positive terminal, and π‘Ž must be the negative terminal. 𝑏 is the positive terminal of the motor.

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