Lenz's Law

Lenz's law is a basic law of electromagnetism that was formulated by the Russian physicist Heinrich Lenz

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Lenz's law is one of the basic laws of electromagnetism. Wherever electromagnetism exists, its effects will be felt when any changes occur.

Lenz's law was deduced in 1834 by the Russian physicist, Heinrich Friedrich Emil Lenz who lived between 1804 and 1865.

Lenz's Law definition

Lenz's law can easily be defined in a single sentence as below.

  • Lenz’s Law:   Lenz's law states that the current induced in a circuit due to a change or a motion in a magnetic field is so directed as to oppose the change that caused it.

Lenz’s Law applies to self inductance because an induced current has a direction such that its magnetic field opposes the change in magnetic field that induced the current.

Lenz's law also applies when there is physical motion, like that of a magnet

Lenz's law effects

Lenz's law can be felt in many ways and in some it can be seen that the conservation of energy laws are obeyed as a result. Physical examples of its presence can be seen in a variety of ways.

  • Permanent bar magnet and coil:   One of the easiest methods of demonstrating Lenz's law is to move a permanent bar magnet into a coil. As the field from the magnet moves across the coil, an electric current is induced. In turn this current sets up a magnetic field around the coil making the coil an electromagnet. Because like poles repel each other, Lenz's law enables us to predict that when the north pole of the bar magnet is approaching the coil, the current induced is in such a direction, that the side of the coil nearest the north pole of the bar magnet also becomes a north pole and in this way it opposes the approaching bar magnet. When the bar magnet is withdrawn from the coil, the induced current reverses itself, so that the side of the coil close to the bar magnet north pole becomes a south pole to produce an attracting force on the bar magnet being removed.
  • Dynamo:   Taking the bar magnet analogy further, it can be seen that turning a dynamo over will induce a current such that the movement of the dynamo is opposed. As a result, work has to be done to turn over the dynamo and generate current. In this way the mechanical energy put into turning the dynamo over, is converted into electrical energy and int his way the laws of the conservation of energy are preserved.
  • Slow build up in current in coil after switch on:   Another way in which Lenz's law makes its presence felt is when current in a circuit including an is tuned on. The fastest rate of change occurs just at switch on when a large jump in voltage is applied. A back EMF is built up, and as a result, the current in the circuit slowly builds up.
    The rise in current in an inductor after DC us switched on
    The rise in current when a steady voltage is applied to an inductor
  • Back EMF generated in coil at turn off:   Similarly when the current is turned off, Lenz's law comes onto play. The inductor opposes the change that has occurred when the circuit is broken and the current is suddenly stopped. As a result a large back EMF is induced which tries to ensure the current continues to flow. This back EMF can be very large and can cause arcing at the switch contacts.

    Where circuits are controlled by a transistor or FET, the back EMF can give rise to very large voltages that can destroy the device. To overcome this a suppressor diode is included across the inductor coil - in this case that of a relay. This is reverse biassed under normal operation, but when the back EMF is generated, which is in the opposite sense to that of the voltage under normal operating conditions, the diode starts to conduct, safely dissipating the back EMF.
    Diode included in relay drive circuit to suppress relay coil back EMF caused by Lenz's law at turn-off
    Diode included in relay drive circuit to suppress the relay coil back EMF caused by Lenz's law at turn-off
    This circuit is very simple, but very effective in suppressing the back EMF transients that could destroy the drive device.
  • Impeding alternating current flow:   Inductors impede the flow of alternating current flow as a result of Lenz's law. This is actually a very useful feature of inductors. By definition, an alternating current is constantly changing, and therefore as a result of Lenz's law a back EMF will be built up to counteract the current flow as a result of Lenz's law. This voltage is proportional to the inductance and the rate of change. In this way the induced voltage will work against the voltage causing the current to flow and in this way it will impede the current flow.

Lenz's law is one of the basic laws associated with inductors and inductance. It is at the core of understanding why inductors behave as they do: resisting any change, resisting the flow of alternating current, slowing the increase of current in a circuit, creating back EMFs and also causing work to be done when a dynamo generates electricity. This means the presence of Lenz's law is felt in many scenarios.

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