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We associate all kinds of events and devices with electric current: electric light,
electric transport, electric sound, etc. They are too numerous to mention.
However, there are only three basic effects of an electric current and all the other
applications follow from them: a) magnetic effect ) b) chemical effect, c) heating
The magnetic effect of current is the basis for most electromechanical devices.
Near a current there is a magnetic field and this exerts a force on other currents or
magnetic materials.
The presence of magnetic materials such as iron can make the forces
thousands of times greater than the currents acting alone, and yet it is the current
which controls the magnet.
Loudspeakers and electric motors are other applications of electromagnetism.
The materials themselves may retain the magnetism and become permanent
magnets which exert their own influence. Permanent magnets are the basis for
some of the simpler devices. The compass needle responds to the magnetic field
of the Earth which is itself a permanent magnet.
When a lead acid battery is charged the acid becomes more concentrated and
hydrogen and oxygen are liberated. As the battery discharges the acid gets
weaker and lead oxide on the positive plate is charged to lead sulphate. These
processes are examples of the chemical effect of a current, i.e. electrolysis.
Electrolysis is used to purify metals such as copper and aluminium and to deposit
metals onto surface, e.g., silver plating.
An electric fire is the most obvious example of the heating effect of a current.
The element of the fire is just highly resistive wire which glows red hot as the
current passes through it.
If the wire is very thin it is heated white hot and a greater proportion of light to
heat is released as in the tungsten filament lamp. Hotter still and more dramatic
are the effects of arc welding and fork lightning when large amounts of electrical
energy are concentrated to give temperature sufficient to melt metals.