Faraday's law of induction (more generally, the law of electromagnetic
induction) states that the induced emf (electromotive force) in a closed
loop equals the negative of the time rate of change of magnetic flux through the loop.
This simply means that the induced emf is proportional to the rate of change of the magnetic flux through a coil.
In layman's terms, moving a conductor (such as a metal wire) through a magnetic field produces a voltage. The resulting
voltage is directly proportional to the speed of movement: moving the conductor twice as fast produces twice the voltage. (The
magnetic field, the direction of movement, and the voltage are all at right angles to each other. Whenever movement creates
voltage, Fleming's right hand rule describes the direction of the
voltage.)
The relation between the rate of change of the magnetic flux through the surface S enclosed by a contour C and
the electric field along the contour:
where
- E is the electric field,
- dl is an infinitesimal element of the contour C,
- B is the magnetic field.
The directions of the contour C and of 
are assumed to be related by the
right-hand rule.
Equivalently, the differential form of Faraday's law is
which is one of the Maxwell equations.
In the case of an inductor coil where the electric wire makes N turns, the formula
becomes:
where emf is the induced electromotive force and dΦ/dt is the time-rate of change of magnetic flux Φ. The direction of
the electromotive force (the negative sign in the above formula) was first given by Lenz's
law.
This principle is used for measuring the flow of electrically conductive liquids and slurries. Such instruments are called
Magnetic Flow Meters. The induced voltage U generated in the magnetic field B due to a conductive liquid moving at velocity v is
thus given by:
,
where L is the distance between electrodes in the magnetic flow meter.
Faraday's law, along with the other laws of electromagnetism, was later
incorporated into Maxwell's equations, unifying all of electromagnetism.
Faraday's law of induction is based on Michael Faraday's experiments in
1831. The effect was also discovered by Joseph Henry at about
the same time, but Faraday published first.[1][2]
Lenz's law gives the direction of the induced electromotive force (emf) and current
resulting from electromagnetic induction. German physicist Heinrich Lenz formulated it in 1834
Practical Demonstration
A brief but informative video demonstrating Faraday's Law may be watched at EduMation.
See also
References
- ^ Ulaby, Fawwaz (2001-01-31).
Fundamentals of Applied Electromagnetics, 2nd edition, Prentice Hall, p. 232. ISBN 0-13-032931-2.
- ^ Joseph Henry. Distinguished Members Gallery, National Academy of Sciences. Retrieved on
2006-11-30.
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