Lenz's law

The Lenz's law (also: Lenz'sches law or rule of Lenz) is a statement about the direction of the electric current in electromagnetic induction, named after Emil Lenz. This first published his ideas in 1833, referring to the previous work, Michael Faraday, and André -Marie Ampère.

From today's perspective formulated Lenz's law more generally, as it originally did Lenz, this one emphasizes primarily changes in magnetic flux (see below) as the starting point of the induction:

Seen in Lenz's law an inference of general Faraday's law of induction:

Lenz's law in the doctrine

Apart from its importance in the history of physics has Lenz's law, especially in high school physics a high priority. In university education and in research, the rule is represented as an aspect of the induction law and the Maxwell equations.

Explanation

The electromagnetic induction is one of the basic phenomena of electron physics. The induction law establishes a relationship between magnetic fields and electrical stresses and is especially useful for understanding electric machines necessary.

Lenz's law states that the induced current tries to prevent a change of magnetic flux. The change of magnetic flux is the law of induction ( a part of the Maxwell equations ) corresponding to the cause of the development of the induction current.

The Lenz's law is directly related to the conservation of energy: the energy for the development of the electric fields comes from the magnetic field. Your physical statement corresponds to the minus sign in the law of induction, which reads in integral form as follows:

On the left side is the induced voltage ( integration of electric field strength over a closed path ) on the right, the temporal change of the magnetic flux ( integration of the scalar product of the magnetic flux density and the surface normal vector of the area enclosed by the path area A ).

Abstract: The induced voltage always acts of their cause ( change in magnetic flux ) counter.

Application Examples

• All electric motors and speakers and pull magnets work on the principle
• In that variant of a maglev that after EDS principle ( electrodynamic levitation ) ( found, for example, in Japanese design ) to work, induce magnets on the vehicle by the vehicle movement in a so-called reaction rail on the route of eddy currents. These eddy currents in turn generate a magnetic field which opposes the field of vehicle magnets. So these two fields repel each other, causing the vehicle hovers at sufficiently high speed on the road. The competing "EMS " Design of a magnetic levitation train as it was, for example, implemented the Transrapid, does not use this principle against it.
• The Lenz's law acts, inter alia, when shielding against the magnetic portion of electromagnetic fields. An external field produces a surface current in the screen. This induced in the shield current generated by the Lenz's law, an opposing field, the incident external magnetic field is superimposed destructive. The effect of this shielding can be detected via the measurement size of screening.
• As a demonstration experiment ( Thomson shear ring test, according to Elihu Thomson) is placed vertically, so that the iron core out looking upwards, for example, a magnetic coil with 600 turns and a 20 cm long, straight iron core. This core should - be composed of mutually insulated metal plates so that eddy currents do not convert the energy into heat - as is usual with transformers. These rod-shaped iron core a matching ring is pushed out of aluminum, which surrounds those as closely as possible, but not applied. In principle, there is a transformer with a shorted secondary coil. If you put on the coil for a few seconds, either a pulse or alternating current ( 50 Hz), a strong magnetic field builds up in the iron core, which induces a very strong current in the ring. Its magnetic field is (according to Lenz's law ) directed opposite to the coil. Therefore, the single aluminum " turn " pushes off from the reel and flies upward ( Gaußkanone ). Experimentally it has been found that excess heights of 50 m can be obtained when a 10 - uF capacitor is used as a power source, which has been previously charged to 2500 volts. With 230 V AC mains voltage, the ring flies about 2 m high.
• The latter effect is the special case of an accident -like event which may occur in practical operation, namely, when a large magnetic field suddenly collapses. This can happen in research magnet, if a superconducting magnet " quenching " (i.e., the temperature of the superconductor exceeds ). Result is that the magnetic coil has an ohmic resistance. Thus, the electric current in the coil is suddenly reduced, thus correspondingly fast and the magnetic field. When such a magnet metallic conductor loops are then provided in the vicinity, such as those reacting aluminum ring. Since they are located outside the magnet, pull up the rest of this field and the induced field ( north and south poles ), and everything is drawn with great force in the magnets, which can have devastating effects. To hedge against it for such magnets must be provided in the immediate vicinity that no conductor loops occur. If structures ( for example, frame structures for racks) would represent a conductor loop of the circuit and thus the formation of induced magnetic field induction is avoided, for example by inserting a voltage-resistant enough, insulating piece