This video is about Lenzi's law.
Lenzi's law states that the direction of the induced current is such as to oppose the
change that created the current.
This law is basically an energy conservation law.
It implies that work cannot be done without some sort of opposition.
For example, when you do work by pushing a box across the floor, there is some opposition
due to the friction force.
We will see in a moment that when a conductor is moved in a magnetic field and a current
is induced, there is always some opposition to the movement of the conductor.
Let's explore how exactly this opposition happens.
We'll use the example of the moving rod in a magnetic field, just like we've done
in the previous video.
Let's begin by using Fleming's right hand rule for induction effects.
Our goal here is to find the direction of the induced current.
If you watched the previous video, you have already seen how to do this.
Point your thumb in the direction of the conductor's motion, so to the right, and
your index finger in the field direction, so out of the page or screen.
If you've done this correctly, your middle finger should be pointing downwards, showing
you the direction of the induced conventional current.
Let's label this current direction on the diagram.
Based on what we learned in subtopic 5.4, this induced current creates a magnetic field.
This magnetic field interacts with the original magnetic field that is pointing out of the
page.
As a result, there will be a force acting on the conductor, so in this example, the
moving rod.
To find the direction of this force, we will use Fleming's left hand rule for motor effects.
As a reminder, we are using this rule to find the direction of the force that acts on the
conductor due to the interaction of the magnetic field and the induced current.
So point your middle finger in the direction of the conventional current, which here is
downwards, and line up your index finger with the field direction which is out of the page.
If you have done this correctly, your thumb will show the force direction, which here
is pointing to the left.
Let's show this force on our diagram.
So we can see that the force arising due to the interaction of the induced current and
the magnetic field opposes the motion of the conductor.
More generally, the direction of the induced current is such as to oppose the change that
created the current.
Let's see an important final point.
As we have seen, as the conductor moves in the magnetic field, there is a force opposing
the conductor's motion that arises due to the induced current.
Therefore, in order to keep the conductor moving at constant speed, an external force
must be present to balance out the force that is opposing the motion.
A key point in electromagnetic induction is that the work done by this external force
appears as electrical energy in the conductor, so we say that there is an induced EMF in
the conductor.
This wraps up our discussion of Lenz's Law.
In the next video, we'll learn about magnetic flux and magnetic flux density.