Video: Eg17S1-Physics-Q20A

What is the rule used to determine the direction of the induced current in a straight wire moving in a magnetic field?

02:27

Video Transcript

What is the rule used to determine the direction of the induced current in a straight wire moving in a magnetic field?

To get a sense for the situation, let’s draw out a straight wire moving in such a field. Say that we have our straight wire here. And it’s in a uniform magnetic field. We’ve drawn this field pointing into the screen. But we could have picked any direction. This is just the one we chose. And then on top of all this, our straight wire is in motion in this field. In the situation the way we’ve drawn it, there can’t be any induced current in our straight wire. That’s because there is no closed loop for the current to travel through. But imagine that we create a track for our wire to run along. And we’ll say that this track is made of conducting material. This way there is a closed loop for current to travel through when current is induced by the magnetic field.

Our question asks us about just how it is we determine the direction of that induced current. That is what rule can we point to which helps us figure it out. A quick rule of thumb to help us figure out this current direction is to take our right hand, point our thumb in the direction of the motion of the straight wire, point our index finger in the direction of the magnetic field. And then, if the rest of our fingers are at right angles to our thumb and our forefinger, in this case pointed out of the screen at us, then it’s those fingers that point in the direction of the induced current in the straight wire. The name for this is Fleming’s right hand — since we’re using our right hand — rule. Let’s see how this rule works for figuring out the direction of current flow in our sketch.

We take our right hand and we start by putting our thumb in the direction of the motion of our straight wire. As we’ve oriented it, that’s left to right. Then, we want to orient our hand so that our index finger can point in the direction of the magnetic field. And as we’ve drawn it, that’s into the screen. It takes a little bit of twisting and it looks like our fingers have disappeared, but actually they’re pointing into the screen. And so, our forefingers pointing that way with the field. And then, we point the rest of our fingers at 90 degrees to that. And the direction that they point is the direction of the induced current in our straight wire.

Over on our sketch then, that means that the current induced here is moving in an anticlockwise direction. And that’s because on the straight wire itself, the direction of the induced current is from bottom to top; it’s upward.

So in answer to our question, what rule we would use to determine current direction. We’ve seen that in the situation of a straight wire moving in a magnetic field, it’s Fleming’s right hand rule that lets us do that.

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