Electricity produced by friction between two dissimilar
objects is known as static electricity. For example, if a glass rod is rubbed
with silk, the rod requires a positive charge and the silk has an equal negative
charge.
Conductors
Substances that can conduct electrical
charges are called conductors. All metals conduct electricity because they have
a large number of free electrons.
Insulators
Insulators have no free electrons and do not conduct
electrical charges. Most non-metals are insulators. For example wood, glass,
rubber, pure water.
Superconductors
The resistance of metal to the flow of current reduces with
decreasing temperature. At temperatures near absolute zero which means -2730
C metals have almost zero resistance and become superconductors.
Certain ceramics can be made to behave as superconductors at
relatively high temperatures of above 100k.
Semiconductors
They are good insulators in their pure crystalline form, but
their conductivity increases if small amounts of impurities are added to them.
After the addition of impurities, they become n-type and p-type semiconductors.
Lightning conductor
Lightning conductors are used to protect tall buildings from
lightning damage. Here a thick copper strip is fixed to an outside wall of the
building. The upper end of the strip is in the form of several sharp spikes
reaching above the highest part of the building. The lower end is connected to
a copper plate buried in the earth.
When clouds charged positively or negatively come near the
spitted conductor, the clouds induce an opposite charge on them that is
streamed off by the points.
In the process, the clouds get discharged and an intense spark
discharge, which might otherwise pass through the building causing heavy
damage, is averted.
Magnetic effect of
current
A current-carrying wire has a magnetic field around it.
Electromagnets are produced in this way. Strong electromagnets find a lot of
applications in industry for lifting and transporting steel plates. They are
used in electric bells, and telephone receivers.
The heating effect of the current
When a charge moves in a conductor, it does work, which
results in heating the conductor.
If the resistance of a wire is R ohms and the current flowing
through it is I ampere, the heat produced per second in the wire is I2
R Joules.
The heating effect of current is used in geysers, electric
irons, toasters, ovens, room heaters and so on.
Some facts about electricity
Ic chips for computers are usually made of silicon.
Semiconductor is made of silicon and germanium.
Soft iron would be most suitable for making an electromagnet.
Nichrome wire is used as a heating element in many appliances.
Because it has high resistivity and resists oxidation in air when red hot.
Energy conversions in a loudspeaker are electrical to
mechanical to sound.
Energy conversion takes place in a microphone from sound to
mechanical to electrical.
A transformer is a device for stepping up AC voltage.
The advantage of AC over DC is that it can be transmitted
over long distances with minimum power loss.
The material used for a fuse wire should have a low melting
point.
One should not connect several electrical appliances to
the same point socket. Because of this can damage the domestic wiring due to
overheating.
Air is completely removed from an electric bulb to prevent
oxidation of tungsten filament.
When an electric bulb breaks, there is a mild bang due to the
air rushing in to fill the evacuated space.
A common fluorescent tube contains mercury vapour at low
pressure.
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