Video: Understanding Lenz’s Law

The direction of the current induced in the conductor by a changing _ is such that the magnetic field created by the induced current _ the initial changing magnetic field.

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Video Transcript

The direction of the current induced in the conductor by a changing blank is such that the magnetic field created by the induced current blank the initial changing magnetic field.

Now this question is talking about induced currents in a conductor by a changing something. It also tells us that the magnetic field created by the induced current is somehow related to the initial change in magnetic field.

Therefore, this statement looks very much like the definition of Lenz’s law. This is related to electromagnetic induction, which is when a change in magnetic flux induces a current in a conductor.

So the direction of the current induced in a conductor by a changing blank. Well, we just mentioned that, in electromagnetic induction, we have a changing magnetic field that induces a current. And so that’s what goes into our first blank. So the direction of the current induced in the conductor by a changing magnetic field is such that the magnetic field created by the induced current blank the initial changing magnetic field.

Now this latter part of the sentence is basically the whole point of Lenz’s law. Lenz’s law tells us that the magnetic field created by the induced current opposes the initial change in magnetic field. And so the word “opposes” goes into our second blank. But this may not be quite so clear. So let’s draw a diagram to help us visualise things.

Let’s start with a loop of conductor. This can be basically a wire. Then let’s introduce a magnet to our setup. Let’s say that is the north pole, which means that the magnetic field lines from this magnet look something like this.

From this diagram, we can clearly see that some of the magnetic field lines are passing through the coil. This is basically a direct indicator of the amount of magnetic flux passing through the coil. However, just because there’s magnetic flux passing through the coil doesn’t mean that there should be an electric current in the coil. That’s because it’s the change in magnetic flux through the coil that results in the induction of a current. And one way of doing this is moving the magnet relative to the coil.

Now we can see that we’ve moved the magnet closer to the coil. This means that more of the magnetic field lines pass through the coil. How quickly we move the magnet determines the current that’s produced in the coil. And that induced current will also generate its own magnetic field.

Now because we move the magnet in this direction, in the downward direction towards the coil, the change in magnetic flux is therefore also in the same direction. And so Lenz’s law tells us that the current in the coil generates its own magnetic field. And the current must be such that an opposing magnetic field is produced to this change in the initial magnetic field.

Now we’ve basically got a long coil. So the magnetic field lines produced by this coil will look something like this. They will go around the coil, but in which direction? Well, this magnetic field has to oppose the orange magnetic field through the center of the coil.

In other words, each one of these pink magnetic field lines has to be pointing in the upward direction inside of the coil. And so these magnetic field lines look something like this. We can then use our right-hand rule to determine the direction of the current flowing through the coil. And this right-hand rule is actually quite simple to use.

Basically, the thumb points in the direction of the current. And when we curl our fingers, then the fingers point in the direction of the magnetic field. And it’s basically as simple as that. So we can use this right-hand rule on any one of these pink current loops to see which direction the current is flowing in.

Let’s take a look at this particular field line here on the left. If we wrap our fingers in the direction of that field line, then our fingers will be going this way. Since we’re using our right hand, then the thumb would be coming out of the screen. Therefore, at this magnetic field line, the current in the coil is flowing in this direction.

We can do the same for all of the magnetic field lines we’ve drawn. But we don’t need to because we know the direction of the current. It’s flowing anticlockwise. The whole point of Lenz’s law then is that we change a flux through a coil. That changing flux results in a current through the coil. And the current is such that the magnetic field due to that current opposes the change in flux. And this is an important point.

The magnetic field due to the current doesn’t just oppose the magnetic field direction of the magnet. Instead, it opposes the change in flux. In other words, if we move the magnet this way, then the change in flux is in the same direction. And so the magnetic field due to the current is in this direction.

However, if we move that same magnet in the opposite direction, if we move it upwards, then even though the orange magnetic field lines still point downwards, that doesn’t matter. The current in the coil will flip around. And the magnetic field due to that current will be in this direction.

So it’s not the direction of the field lines from the magnet that matter, but it’s the direction of the change in flux. And that’s what Lenz’s law talks about.

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