Moving a coil across a magnetic field will produce a current (see image below) .This is called electromagnetic induction. The direction of the current depends on how the coil is moved. The current produced in called the induced current.
Electromagentic current is key to how a transformers, generator elecgtric motors and solenoids works. Michael Faraday is generally credited with the discovery of induction in 1831. He formulated the elecromotive force (EMF) produced around a closed path is proportional to the rate of the change in magnetic flux by any surface bounded by that path -- (simplify this). This means that an electric current will be induced in any circuit when the magnetic flux through a surface by the conductor changes. This applies whether the field itself changes in trength or the conductors is moved through it.
An important application from electromagnetic induction is the generator. A generator is a device that converts mechanical energy into electrical energy (see types of energy). Generators in power plants are responsible for producing 99% (check this) of energy used in the united states.A simple generator is composed of a loop of wire located between the poles of a magnet. The loop is attached to a rod or axle that can rotate. When the loop of wire is rotated or cuts through the magnetic field lines a current of electricity is induced. As the loop of wire continues to rotate there will be a point where the wire is parrallel to the lines of force of the magnetic field. At this point no current is produced. Further rotation moves the loop to a position where the lines of force are cut in the opposite direction. This will cause the direction of current to flow in the opposite direction.Since the direction of current changes with each rotation of the loop the current in called Alternating Current (AC) --
Need a few images on this page.
why 120 times/second change in AC?
What is a transformer?
A transformer is a device that increases or decreases voltage of alternating current. The current in one coil induces a current in another coil.
A transformer consists of two coils (one coil is the primary coil the other is the secondary coil) wrapped around a metal core. (see images ---) When an alternating current passes through the primary coil and magnetic field in induced -- electromagnetic induction causes a current to flow in the secondary coil. If the number of loops of wire is the same in both coils the induced voltage will be the same in the secondary coil. If the number of loops in the secondary coil are greater than the primary coil the voltage will be greater in the secondary coil. This is an example of a step up transformer. See how do transformers work