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Electricity · Magnetism
Free space · Lorentz force law · emf · Electromagnetic induction · Faraday’s law · Lenz's law · Displacement current · Maxwell's equations · EM field · Electromagnetic radiation · Liénard-Wiechert Potential · Maxwell tensor · Eddy current

Electromagnetic induction is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field.

Michael Faraday is generally credited with the discovery of the induction phenomenon in 1831 though it may have been anticipated by the work of Francesco Zantedeschi in 1829[citation needed]. Around 1830 [1] to 1832 [2] Joseph Henry made a similar discovery, but did not publish his findings until later.


Technical details

Faraday found that the electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path.

In practice, this means that an electrical current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor changes. This applies whether the field itself changes in strength or the conductor is moved through it.

Electromagnetic induction underlies the operation of generators, all electric motors, transformers, induction motors, synchronous motors, solenoids, and most other electrical machines.

Faraday's law of electromagnetic induction states that:

 \mathcal{E} = -{{d\Phi_B} \over dt},


\mathcal{E} is the electromotive force (emf) in volts
ΦB is the magnetic flux in webers

For the common but special case of a coil of wire, composed of N loops with the same area, Faraday's law of electromagnetic induction states that

 \mathcal{E} = - N{{d\Phi_B} \over dt}


\mathcal{E} is the electromotive force (emf) in volts
N is the number of turns of wire
ΦB is the magnetic flux in webers through a single loop.

A corollary of Faraday's Law, together with Ampere's and Ohm's laws is Lenz's law:

The emf induced in an electric circuit always acts in such a direction that the current it drives around the circuit opposes the change in magnetic flux which produces the emf.

The direction mentioned in Lenz's law can be thought of as the result of the minus sign in the above equation


The principles of electromagnetic induction are applied in many devices and systems, including:

See also


  1. ^ "ThinkQuest : Site Unavailable". Retrieved 2009-11-06. 
  2. ^ "Joseph Henry". Retrieved 2009-11-06. 
  • David J. Griffiths (1998). Introduction to Electrodynamics (3rd ed.). Prentice Hall. ISBN 0-13-805326-X. 
  • Paul Tipler (2004). Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics (5th ed.). W. H. Freeman. ISBN 0-7167-0810-8. 
  • J.S. Kovacs and P. Signell, Magnetic induction (2001), Project PHYSNET document MISN-0-145.

External links


Study guide

Up to date as of January 14, 2010

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The laws of Electromagnetic induction

The phenomena of electromagnetic induction are described in two laws.

The first of these Faraday's law, states: "when the magnetic flux through a coil is made to vary, a voltage is set up. The magnitude of the induced voltage is proportional to the rate of change of flux".

The second law , known as Lenz's law concerns the direction of the induced voltage. " A change of flux threading a closed circuit induces a voltage and sets up a current ; the direction of this current is such that its magnetic field tends to oppose the change in flux"

The opposition to the change of flux through a circuit depends on the presence of current in the circuit; if the coil is open-circuited, the induced voltage cannot set up a current and there is no opposition.

Simple English

Electromagnetic induction is where a current is produced in a conductor through a changing magnetic flux.

Magnetic flux

When a coil is introduced near a magnet (usually a bar magnet), then the magnetic lines of force passing through the coil is called magnetic flux. Magnetic flux is represented by the symbol {\Phi}, therefore we can say that {\Phi} = BAcos(a) and the resulting unit will be Tm^2, where T is the unit for magnetic field and m^2 is the unit for area.

The changing magnetic flux generates an electromotive force (EMF). This force then pushes free electrons in a certain way, which in turn creates a current.

Faraday's Law

Michael Faraday found that an electromotive force is generated when there is a change in magnetic flux in a conductor.

His laws state that:

\mathcal{E} = {-{d\Phi} \over dt}


\mathcal{E} is the electromotive force, measured in volts;

{d\Phi} is the change in magnetic flux, measured in webers;

dt is the change in time, measured in seconds.

In the case of a solenoid:

\mathcal{E} = {-N{d\Phi} \over dt}


N is the number of loops in the solenoid.

Lenz's Law

The negative sign in both equation above is a result of Lenz's law, named after Heinrich Lenz. His law states that the electromotive force (EMF) produces a current that opposes the motion of the changing magnetic flux.


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