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Electromagnetic Induction
I INTRODUCTION Electromagnetic Induction, in electricity, the creation of an electromotive force (voltage) in a conductor moving across a magnetic field. The effect was discovered by the British physicist Michael Faraday and led directly to the development of the rotary electric generator, which converts mechanical motion into electric energy.
II
ELECTRIC GENERATOR
When a conductor, such as a wire, moves through
the gap between the poles of a magnet, the negatively charged electrons in the
wire will experience a force along the length of the wire and will accumulate at
one end of it, leaving positively charged atomic nuclei, partially stripped of
electrons, at the other end. This creates a potential difference, or voltage,
between the ends of the wire. If the ends of the wire are connected by a
conductor, a current will flow around the circuit. This is the principle behind
the rotary electric power generator, in which a loop of wire is spun through a
magnetic field to produce a voltage and generate a current in a closed circuit
III
ELECTRIC TRANSFORMER
Induction occurs only if the wire moves at right
angles to the direction of the magnetic field. This motion is necessary for
induction to occur, but it is a relative motion between the wire and the
magnetic field. Thus, an expanding or collapsing magnetic field can induce a
current in a stationary wire. Such a moving magnetic field can be created by a
surge of current through a wire or electromagnet. As the current in the
electromagnet rises and falls, its magnetic field grows and collapses (the lines
of force move outward, then inward). The moving field can induce a current in a
nearby stationary wire. Such induction without mechanical motion is the basis of
the electric transformer.
A transformer usually consists of two adjacent
coils of wire wound around a single core of magnetic material. It is used to
couple two or more AC circuits by employing the induction between the coils.
IV
SELF-INDUCTION
When the current in a conductor varies, the
resulting changing magnetic field cuts across the conductor itself and induces a
voltage in it. This self-induced voltage is opposite to the applied voltage and
tends to limit or reverse the original current. Electric self-induction is thus
analogous to mechanical inertia. An inductance coil, or choke, tends to smooth
out a varying current, as a flywheel smooths out the rotation of an engine. The
amount of self-induction of a coil, its inductance, is measured by the
electrical unit called the henry, named after the American physicist Joseph
Henry, who discovered the effect. The inductance is independent of current or
voltage; it is determined only by the geometry of the coil and the magnetic
properties of its core.