Page 43 - Teach Yourself Electricity and Electronics

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CHAPTER

Electrical units

THIS CHAPTER EXPLAINS SOME MORE ABOUT UNITS THAT QUANTIFY THE
behavior of direct-current circuits. Many of these rules apply to utility alternating-cur-
rent circuits also. Utility current is, in many respects, just like direct current because
the frequency of alternation is low (60 complete cycles per second).

The volt

In chapter 1, you learned a little about the volt, the standard unit of electromotive force
(EMF) or potential difference.
An accumulation of static electric charge, such as an excess or shortage of elec-
trons, is always, associated with a voltage. There are other situations in which voltages
exist. Voltage is generated at a power plant, and produced in an electrochemical reac-
tion, and caused by light falling on a special semiconductor chip. It can be produced
when an object is moved in a magnetic field, or is placed in a fluctuating magnetic field.
A potential difference between two points produces an electric field, represented
by electric lines of flux (Fig. 2-1). There is always a pole that is relatively positive, with
fewer electrons, and one that is relatively negative, with more electrons. The positive
pole does not necessarily have a deficiency of electrons compared with neutral objects,
and the negative pole might not actually have a surplus of electrons with respect to neu-
tral things. But there’s always a difference in charge between the two poles. The nega-
tive pole always has more electrons than the positive pole.
The abbreviation for volt is V. Sometimes, smaller units are used. The millivolt
(mV) is equal to a thousandth (0.001) of a volt. The microvolt (µV) is equal to a mil-
lionth (0.000001) of a volt. And it is sometimes necessary to use units much larger than
one volt. One kilovolt (kV) is equal to one thousand volts (1,000). One megavolt (MV)
is equal to one million volts (1,000,000) or one thousand kilovolts.
In a dry cell, the EMF is usually between 1.2 and 1.7 V; in a car battery, it is most

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