Most electrical energy is generated by
electromagnetic induction. However, electricity can be produced by
other means. Batteries use electrochemistry to produce low voltages.
An electrolyte is a solution of
chemicals in water such that the chemical separates into positively
and negatively charged ions when dissolved. The charged ions react
with the conducting electrodes and release energy, as well as give up
their charge
A fixed electrode potential is
associated with the reaction at each electrode; the difference
between the two electrode potentials drives a current around an
external circuit. The electrolyte must be sealed into a safe
container to make a suitable battery.
‘Dry’ cells use an electrolyte in
the form of a gel or thick paste. A primary cell releases electricity
as the chemicals react, and the cell is discarded once all the active
chemicals have been used up, or the electrodes have become
contaminated. A secondary cell uses a reversible chemical reaction,
so that it can be recharged to regenerate the active chemicals.
The fuel cell is a primary cell which
is constructed so that the active chemicals (fuel) pass through the
cell, and the cell can be used for long periods by replenishing the
chemicals. Large batteries consist of cells connected in series or
parallel to increase the output voltage or current.
Electricity can be generated directly
from heat. When two different materials are used in an electrical
circuit, a small electrochemical voltage (contact potential) is
generated at the junction. In most circuits these contact potentials
cancel out around the circuit and no current flows.
However, the junction potential varies
with temperature, so that if one junction is at a different
temperature from the others, the contact potentials will not cancel
out and the net circuit voltage causes current to flow (Seebeck
effect). The available voltage is very small, but can be made more
useful by connecting many pairs of hot and cold junctions in series.
The thermocouple is used mostly for
measurement of temperature by this effect, rather than for the
generation of electrical power. The efficiency of energy conversion
is greater with semiconductor junctions, but metal junctions have a
more consistent coefficient and are preferred for accurate
measurements.
The effect can be reversed with
suitable materials, so that passing an electric current around the
circuit makes one junction hotter and the other colder (Peltier
effect). Such miniature heat pumps are used for cooling small
components.
Certain crystalline chemicals are made
from charged ions of different sizes. When a voltage is applied
across the crystal, the charged ions move slightly towards the side
of opposite polarity, causing a small distortion of the crystal.
Conversely, applying a force so as to distort the crystal moves the
charged ions and generates a voltage.
This piezoelectric effect is used to
generate high voltages from a small mechanical force, but very little
current is available. Ferromagnetic materials also distort slightly
in a magnetic field. The magnetostrictive effect produces low
frequency vibration (hum) in ac machines and transformers.
Electricity can be produced directly
from light. The photovoltaic effect occurs when light falls on
suitable materials, releasing electrons from the material and
generating electricity. The magnitude of the effect is greater with
short wavelength light (blue) than long wavelength light (red), and
stops altogether beyond a wavelength threshold.
Light falling on small photovoltaic
cells is used for light measurement, communications and for proximity
sensors. On a larger scale, semiconductor solar cells are being made
with usable efficiency for power generation.
Light is produced from electricity in
incandescent filament bulbs, by heating a wire to a sufficiently high
temperature that it glows. Fluorescent lights produce an electrical
discharge through a low pressure gas. The discharge emits ultraviolet
radiation, which causes a fluorescent coating on the inside of the
tube to glow.
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