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Right hand generator rule

Let's learn how to use our right hand to remember the direction of the induced current when a wire is moved in a magnetic field. Created by Mahesh Shenoy.

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Video transcript

- [Instructor] In a previous video, we've seen that a changing magnetic field can induce an electric current. And so one way of doing this would be to move a bar like that towards and away from a coil. Or maybe you can hold the magnet stationary and move the coil closer and farther away from the magnet. Turns out that moving the coil is a little bit more convenient. So in this video we will see how to remember the direction of the electric current induced when we move a coil or when we move a wire in a magnetic field. So let's consider a magnetic field due to two pole pieces of a magnet. You can imagine these are two separate magnets, or these are two poles of a horseshoe magnet. The reason we're choosing this is because if we had a single magnet, then the field lines would be pretty curved as we saw before and will be very difficult to understand in what direction the current will be produced. But over here, if you use arrangement like this, then at least near the center, the field will be pretty straight. But if we go farther away, of course, the field will now start curving like this. But at least in the center the field will be straight. It'll be easier to analyze what direction the current goes. To induce a current, we need a coil, but instead of a coil, we can just move a wire. So let's introduce a wire over here. And we can move this wire up and down like this. So as we move this wire, notice it starts cutting the magnetic field. And whenever it does that, an electric current will be induced in this wire. Of course, we need a closed circuit for that, so we can imagine the wire from here gets connected to some galvanometer somewhere, which I've not shown over here. So let's say we move this wire up like this. We move the wire up. So we push it up. Then, it turns out if you do the experiment, the current generated in this wire, the current induced in this wire is going to be out of the screen, all right. Somewhat like this. So the current will flow out of the screen this way. And if you were to push it down, the current will reverse. The current direction will also depend upon the direction of the magnetic field. If you reverse the direction of the magnetic field, the whole current again will reverse. So now the big question is how do we remember this? We will not worry about why the current is outwards. We'll just say that the experiment shows that, it shows us it's that way. But how do you remember this? That's the big question we want to answer. So this can be remembered by using something called the right hand generator rule. So what we do is we take our right hand, and we stretch three fingers, the thumb, the forefinger, and the middle finger. This way says that they are perpendicular to each other, all of them. So this is perpendicular to this. This is perpendicular to this if you see carefully. And even these two are perpendicular to each other. Stress them that they're all perpendicular to each other. Then, the thumb represents the direction in which you are pushing the wire. So, F for force, in what direction the wire is being pushed. The forefinger will tell us in what direction the magnetic field is. And the symbol for, the letter for magnetic field is B. It's not M. I don't know why. Then, the middle finger gives us the direction of the current. And so if we are to use this right hand rule over here, we can see the force is up, the magnetic field, the forefinger is this way. And the middle finger is pointing out of the screen just as our current. And if you were to move this wire down, then this current direction would now reverse. Now, can you use your right hand generator rule one more time to convince yourself of this? Make sure that the field forefinger is to the left, but this time make sure the thumb is pointing downwards and see what direction the middle finger points. Go ahead, try this. All right, if you have done it, it might look somewhat like this. The force is down. Magnetic field is to the left. Now notice the current, that is your middle finger, is pointing inwards into the screen just like what we got here. So just for practice, let's take another example. Here we have the magnetic field coming out of the screen this way. And the conductor is going to be moved, let's say, upwards. So we'll move the conductor up like this, cutting the magnetic field. Can you figure out in what direction the current will run in this conductor? Again, pause the video, and see if you can try this yourself. All right, we have to bring in our right hand, and if you align it according to the magnetic field and the push or the motion of the conductor, it would look somewhat like this. The forefinger points in the direction of the magnetic field, and the thumb must point in the direction of the motion of the conductor, in the direction in which we are pushing the conductor. Then notice the middle finger points to the right. That means the current in this conductor will flow to the right. So this is how we use our right hand generator rule to figure out the direction of the current when any conductor or any wire is moved in a magnetic field.