Electromagnetic induction

Main article: Faraday's law of induction

Electromagnetic induction is the production of voltage across a conductor moving through a 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. Around 1830 to 1832 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:

${\mathcal {E}}=-{{d\Phi _{B}} \over dt},$

Thus:

${\mathcal {E}}$ is the electromotive force (emf) in volts

Φ_B 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

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

where

${\mathcal {E}}$ is the electromotive force (emf) in volts

N is the number of turns of wire

Φ_B 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

Applications

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

• Induction Sealing
• Induction motors
• Electrical generators
• Transformers
• Contactless charging of rechargeable batteries
• The 6.6 kW Magne Charge system for Battery electric vehicles
• Induction cookers
• Induction welding
• Inductors
• Electromagnetic forming
• Magnetic flow meters
• Transcranial magnetic stimulation
• Faraday Flashlight
• Graphics tablet
• Wireless energy transfer
• Electric Guitar Pickups
• Hall effect meters
• Current transformer meters
• Clamp meter
• Audio and video tapes
• Rowland ring

See also

• Maxwell's equations for further mathematical treatment.
• Faraday's law of induction
• Inductance
• Eddy current
• Lenz's law
• Moving magnet and conductor problem
• Gravitational induction

References

• 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

• BIGS animation The induction
• A free java simulation on motional EMF
• Two videos demonstrating Faraday's and Lenz's laws at EduMation