A wire through which an electrical current
circulates must be made of a conducting material (its atoms must have outer
electrons which can easily be separated from the atom and which travel through the material). These materials
are metals (copper, aluminium and other metallic alloys).
The characteristic magnitudes of wires are: resistance,
which opposes the flow of electric current, and width, which
depends on the electrical network, as it must not become too hot and melt (or set something on fire). These
two magnitudes are related.
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In addition to the alternating current cables that reach
houses, the telephone is connected to a source of direct current with a low
voltage and intensity, which transmits signals by using the change in intensity
caused by the microphone.
Since the end of the twentieth century, there are signals transmitted by
electromagnetic waves (television signals, for example) which are detected by
antennae and are amplified and transmitted along coaxial cables, protected by
a metallic shield.
Nowadays, there are also singals transmitted through the Internet by means of
computers, which use small voltages and intensities with an order of magnitude
of a few milliamperes. |
Cables that carry information (and not just current) must be protected with
metallic shields so that the data transmitted is not perturbed by external signals.
The colour of the plastic jacket does not affect the conduction of electricity, it is just
used by the electrician to know which cable is connected to a high voltage, which is
connected to ground, etc. just by looking at the end of the cables. The ground cable is usually
yellow and green and it usually goes from the electrical sockets in a building to a metal spike
buried in the foundation. If part of the current goes through the case of an electrical appliance
because of a contact problem in the wires, most of the current will leave through the ground cable
instead of going through a person's body.
