Lenz Law is a way to find the direction of induced current in a conducting loop. As per Faraday’s Law of electromagnetic induction, a current is induced in a conducting loop by the changing magnetic flux. This induced current itself produces a magnetic field and hence a magnetic flux. This magnetic flux may have the same sign as the original magnetic flux or may have the opposite sign. It strengthens the original magnetic flux if it has the same sign or weakens it if opposite sign.
Lenz Law states that “The direction of the induced current is such that it opposes the change that has induced it.” In other words, Effect Opposes the Cause. Here effect is induced current and cause is changing magnetic flux.
Thus, if the current is induced by increasing magnetic flux, it will weaken the original flux. If the current is induced by the decreasing flux, it will strengthen the original flux.
Illustration of Lenz Law
In figure (1) below, a magnet is brought toward a circular loop. The North Pole of magnet is facing the loop. As the magnet get closer to the loop, the magnetic field through the loop increases. Due to which, the flux of the magnetic field through the closed loop increases. Mind here that, the cause is increasing magnetic flux and effect is induced current. Therefore as per Lenz Law, the effect i.e. induced current shall oppose the cause. Therefore, induced current will produce a magnetic flux which will weaken the original magnetic flux. The direction of original magnetic field is going into the plane of conducting loop. Therefore, the induced current shall produce magnetic field toward the magnet or away from the plane of conducting loop to oppose the cause. Hence, the direction of induced current is anticlockwise by Screw Rule.
Consider the figure (2). As shown in the figure, the wire AB is sliding toward left. Therefore, the area ABCD decreases with the slide of wire AB. Notice here that magnitude and direction of magnetic field is constant. But as the area ABCD is decreasing, the flux linkage Ø = BA will decrease. Therefore, from Faraday’s Law, a current will be induced in the circuit ABCD.
Now we will use Lenz Law to find the direction of induced current. Here, the cause is decreasing magnetic flux and effect is induced current. Effect should oppose the cause. Therefore, the direction of induced current shall be such that it strengthens the magnetic flux. The direction of original magnetic flied is going into the plane of your device screen. Induced current should also produce magnetic filed in the same direction which means the direction of induced current should be clockwise.