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Alternating voltages and currents
With alternating current (a.c.), the flow of electric charge periodically
reverses direction, whereas with direct current (d.c.), the flow of electric charge is
only in one direction. In a power station, electricity can be made most easily by
using a gas or steam turbine or water impeller to drive a generator consisting of a
spinning magnet inside a set of coils. The resultant voltage is always ‘alternating’ by
virtue of the magnet’s rotation.
Alternating voltage can be carried around the country via cables far more
effectively than direct current because a.c. can be passed through a transformer and
a high voltage can be reduced to a low voltage, suitable for use in homes. The
electricity arriving at your home is alternating voltage. Electric light bulbs and
toasters can operate perfectly from 230 volts a.c. Other equipment such as
televisions has an internal power supply which converts the 230 volts a.c. to a low
d.c. voltage for the electronic circuits. How is this done? There are several ways, but
the simplest is to use a transformer to reduce the voltage to, say, 12 volts a.c. This
lower voltage can be fed through a ‘rectifier’ which combines the negative and
positive alternating cycles so that only positive cycles emerge. A.c. is the form in
which electric power is delivered to businesses and residences. The usual waveform
of an a.c. power circuit is a sine wave. In certain applications, different waveforms
are used, such as triangular or square waves. Audio and radio signals carried on
electrical wires are also examples of alternating current. The frequency of the
electrical system varies by country; most electric power is generated at either 50 or
60 hertz. Some countries have a mixture of 50 Hz and 60 Hz supplies, notably
Japan. A low frequency eases the design of electric motors, particularly for hoisting,
crushing and rolling applications, and commutator-type traction motors for
applications such as railways.
However, low frequency also causes noticeable flicker in arc lamps and
incandescent light bulbs. The use of lower frequencies also provides the advantage
of lower impedance losses, which are proportional to frequency. 16.7 Hz frequency is still used in some European rail systems, such as in Austria, Germany, Norway,
Sweden and Switzerland. Off-shore, military, textile industry, marine, computer
mainframe, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits
of reduced weight of apparatus or higher motor speeds.

Exercise 2.
1. Define keywords of this article.
2. Compose alternative title for the text
3. Make a plan
4. What is the main thought of the text?
5. Write an abstract for this article.
Exercise 4.
Write pharases, using these words:
residence
railway
transformer
impeller
voltage
Exercise 5.
Make new sentences with the phrases and words below.
Magnitude, angular velocity, phase difference, transformer equation,
inductive reactance