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This video is about Lenzi's law.

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Lenzi's law states that the direction of the induced current is such as to oppose the

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change that created the current.

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This law is basically an energy conservation law.

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It implies that work cannot be done without some sort of opposition.

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For example, when you do work by pushing a box across the floor, there is some opposition

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due to the friction force.

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We will see in a moment that when a conductor is moved in a magnetic field and a current

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is induced, there is always some opposition to the movement of the conductor.

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Let's explore how exactly this opposition happens.

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We'll use the example of the moving rod in a magnetic field, just like we've done

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in the previous video.

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Let's begin by using Fleming's right hand rule for induction effects.

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Our goal here is to find the direction of the induced current.

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If you watched the previous video, you have already seen how to do this.

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Point your thumb in the direction of the conductor's motion, so to the right, and

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your index finger in the field direction, so out of the page or screen.

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If you've done this correctly, your middle finger should be pointing downwards, showing

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you the direction of the induced conventional current.

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Let's label this current direction on the diagram.

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Based on what we learned in subtopic 5.4, this induced current creates a magnetic field.

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This magnetic field interacts with the original magnetic field that is pointing out of the

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page.

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As a result, there will be a force acting on the conductor, so in this example, the

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moving rod.

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To find the direction of this force, we will use Fleming's left hand rule for motor effects.

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As a reminder, we are using this rule to find the direction of the force that acts on the

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conductor due to the interaction of the magnetic field and the induced current.

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So point your middle finger in the direction of the conventional current, which here is

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downwards, and line up your index finger with the field direction which is out of the page.

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If you have done this correctly, your thumb will show the force direction, which here

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is pointing to the left.

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Let's show this force on our diagram.

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So we can see that the force arising due to the interaction of the induced current and

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the magnetic field opposes the motion of the conductor.

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More generally, the direction of the induced current is such as to oppose the change that

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created the current.

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Let's see an important final point.

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As we have seen, as the conductor moves in the magnetic field, there is a force opposing

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the conductor's motion that arises due to the induced current.

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Therefore, in order to keep the conductor moving at constant speed, an external force

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must be present to balance out the force that is opposing the motion.

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A key point in electromagnetic induction is that the work done by this external force

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appears as electrical energy in the conductor, so we say that there is an induced EMF in

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the conductor.

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This wraps up our discussion of Lenz's Law.

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In the next video, we'll learn about magnetic flux and magnetic flux density.