Lenz law

Lenz's law, in electromagnetism, statement that an induced electric current flows in a direction such that the current opposes the change that induced it. This law was deduced in 1834 by the Russian physicist Heinrich Friedrich Emil Lenz (1804-1865).
Permanent bar magnet induces an electric current in the coil.
Fig. 1: Permanent bar magnet induces an electric current in the coil.
Thrusting a pole of a permanent bar magnet through a coil of wire, for example, induces an electric current in the coil; the current in turn sets up a magnetic field around the coil, making it a magnet. Lenz's law indicates the direction of the induced current. (The direction of the induced current from Lenz's law contributes the minus sign in Faraday's law of induction.) Because like magnetic poles repel each other, Lenz's law states that when the north pole of the bar magnet is approaching the coil, the induced current flows in such a way as to make the side of the coil nearest the pole of the bar magnet itself a north pole to oppose the approaching bar magnet.
Upon withdrawing the bar magnet from the coil, the induced current reverses itself, and the near side of the coil becomes a south pole to produce an attracting force on the receding bar magnet.
Demonstrating the effect of Lenz's law
Fig. 2: Demonstrating the effect of Lenz's law
If the current increases in magnitude, the flux linking the coil also increases. For this coil, therefore, an induced voltage is developed across the coil due to the change in current through the coil. The polarity of this induced voltage tends to establish a current in the coil that produces a flux that will oppose any change in the original flux. In other words, the induced effect ($e_{ind}$) is a result of the increasing current through the coil as shown in [Fig. 2].
However, the resulting induced voltage will tend to establish a current that will oppose the increasing change in current through the coil. Keep in mind that this is all occurring simultaneously. The instant the current begins to increase in magnitude, there will be an opposing effect trying to limit the change. It is "choking" the change in current through the coil. Hence, the term choke is often applied to the inductor or coil.
In fact, we will find shortly that the current through a coil cannot change instantaneously. A period of time determined by the coil and the resistance of the circuit is required before the inductor discontinues its opposition to a momentary change in current. Recall a similar situation for the voltage across a capacitor in Chapter 8. The reaction above is true for increasing or decreasing levels of current through the coil. This effect is an example of a general principle known as Lenz's law, which states that
an induced effect is always such as to oppose the cause that produced it.

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