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Unit VII. Electrical power
I. Text A:
Electric Circuits
The concepts of electric charge and potential are very important in the study of
electric currents. When an extended conductor has different potentials at its ends, the free
electrons of the conductor itself are caused to drift from one end to the other. The potential
difference must be maintained by some electric source such as electrostatic generator or a
battery or a direct current generator. The wire and the electric source together form an
electric circuit, the electrons are drifting around it as long as the conducting path is
maintained.
There are various kinds of electric circuits such as: open circuits, closed circuits,
series circuits, parallel circuits and short circuits. To understand the difference between the
following circuit connections is not difficult at all. If the circuit is broken or «opened»
anywhere, the current is known to stop everywhere. The circuit is broken when an electric
device is switched off. The path along which the electrons travel must be complete
otherwise no electric power can be supplied from the source to the load. Thus the circuit is
“closed” when an electric device is switched on. When electrical devices are connected so
that the current flows from one device to another, they are said «to be connected in series».
Under such condi-tions the current flow is the same in all parts of the circuit as there is
only a single path along which it may flow. The electrical bell circuit is considered to be a
typical example of a series circuit. The “parallel” circuit provides two or more paths for
the passage of current. The circuit is divided in such a way that part of the current flows
through one path and part through another. The lamps in the houses are generally
connected in parallel.
The “short” circuit is produced when the current can return to the source of supply
without control. The short circuits often result from cable fault or wire fault. Under certain
conditions the short circuit may cause fire because the current flows where it was not
supposed to flow. If the current flow is too great a fuse is used as a safety device to stop
the current flow.