Basic HTML Version
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AC and DC currents
I
n DC currents the charges flow in one direction only
and the polarity of applied voltage is fixed. Abattery
is a
steady
DC voltage source because it has fixed
polarity and a steady voltage output.
An AC current, an alternating voltage source,
periodically reverses or alternates in polarity so the
resulting alternating current periodically reverses its
direction too. Current usually flows from the negative
terminal of the voltage source, through the circuit,
and back to the positive terminal, but when polarity
reverses, the current reverses as well.
Batteries,
fuel cells
and solar cells all produce direct
current (DC). The positive and negative terminals of a
battery are always, respectively, positive and negative.
Current always flows in the same direction between
these two terminals.
The power that comes from a
power plant
, on the
other hand, is alternating current (AC). The direction
of the current reverses, or alternates, 60 times per
second (in the U.S.) or 50 times per second (in Europe,
for example). In the United States a wall socket
supplies 120-volt, 60-cycle AC power, available
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to the consumer; in Europe a wall socket supplies
220-volts, 50 cycle AC power.
The big advantage of alternating current for the
power grid
is the fact that it is relatively easy to
change the voltage of the power, using a device called
a transformer. Power companies save a great deal of
money this way, using very high voltages to transmit
power over long distances.
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to make available
: to supply for someone’s use
Two Kinds of Current
21
Circle the main points of this excerpt and list the differences between an AC and a DC current.
TEXT 5
Bicycle
dynamos
produce
alternating current (AC), so
are more correctly called
alternators.
fuel cell
cella a combustibile
power grid
linea di distribuzione dell’energia
power plant
centrale elettrica
steady
costante
TECHNICAL ENGLISH
22
Now tick (
T
) for True or (
F
) for False.
a
When the polarity of the applied voltage reverses, the direction of current
flow also reverses.
b
A battery is a DC voltage source because it cannot reverse the polarity
across its output terminals.
c
The power that comes from a plant is DC current.
d
In DC currents is quite easy to change voltage.
Multimeter display closeup with
voltage and amps for AC and DC
current.
T F
T F
T F
T F
Progetto FOR ELECTRONICS_unit_1.indd 15
10/01/