Physics - Electromagnetic induction
Just as an electric current is able to produce a magnetic field, magnetic
fields can be used to create an electric current. When a current is induced
using a magnetic field, we say electromagnetic induction  has occurred.
Faraday discovered that whenever the amount of a magnetic field that is
passing through a loop of wire is altering, a electric current is induced.
There a number of ways in which a loop can be made to experience a changing
* a loop could be dragged into or out of a magnetic field
* a loop could be spun in a magnetic field
* a loop could be kept stationary and the strength of the field increased
* a loop could be altered in size while in a magnetic
Since there were a number of arrangements that could result in an induced
electric current, Faraday came up with an equation that would explain the
size of the induced current in any situation. To do this, the term magnetic
flux was invented. Flux is defined as the product of the magnetic field and
the area of the loop that is perpendicular to that field.
Once flux is defined as shown, it can simply be stated that
electromagnetic induction of a current occurs whenever there is a change in
flux. This statement covers all possible ways of creating electromagnetic
induction. The size of the induced EMF (electromotive force or voltage)
* the number of loops of wire
* the rate at which the flux value is changing.
The negative sign in the formula is a reminder of Lenz's law which states
that the direction of the induced voltage is such that it opposes the
change of flux that is occurring. A mechanism for determining the direction
of the induced voltage is discussed later.
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