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 so as 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 a-c
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.
See
Electric
Motors and Generators.
