Page 54 - Teach Yourself Electricity and Electronics

34 Electrical units

Physicists also use, in addition to the joule, a unit of energy called the electron volt
(eV). This is an extremely tiny unit of energy, equal to just 0.00000000000000000016
joule (there are 18 zeroes after the decimal point and before the 1). The physicists write
1.6 × 10 –19 to represent this. It is the energy gained by a single electron in an electric
field of 1 V. Atom smashers are rated by millions of electron volts (MeV) or billions of
electron volts (GeV) of energy capacity. In the future you might even hear of a huge lin-
ear accelerator, built on some vast prairie, and capable of delivering trillions of electron
volts (TeV).
Another energy unit, employed to denote work, is the foot pound (ft-lb). This is
the work needed to raise a weight of one pound by a distance of one foot, not including
any friction. It’s equal to 1.356 joules.
All of these units, and conversion factors, are given in Table 2-3. Kilowatt hours and
watt hours are also included in this table. You don’t really need to worry about the ex-
ponential notation, called scientific notation, here. In electricity and electronics, you
need to be concerned only with the watt hour and the kilowatt hour for most purposes,
and the conversions hardly ever involve numbers so huge or so miniscule that you’ll
need scientific notation.

Table 2-3. Energy units.

To convert to joules Conversely,
Unit multiply by multiply by
Btu 1055 0.000948
or 9.48 10 –4
eV 1.6 10 –19 6.2 10 18
erg 0.0000001 10,000,000
or 10 –7 or 10 7
ft-lb 1.356 0.738
Wh 3600 0.000278
or 2.78 10 –4
kWh 3,600,000 0.000000278
or 3.6 x 10 6 or 2.78 10 –7

ac Waves and the hertz

This chapter, and this whole first section, is concerned with direct current (dc), that
is, current that always flows in the same direction, and that does not change in intensity
(at least not too rapidly) with time. But household utility current is not of this kind. It
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reverses direction periodically, exactly once every /120 second. It goes through a com-
1
plete cycle every /60 second. Every repetition is identical to every other. This is alter-
1
nating current (ac). In some countries, the direction reverses every /100 second, and
1
the cycle is completed every /50 second.
Figure 2-8 shows the characteristic wave of alternating current, as a graph of volt-
age versus time. Notice that the maximum positive and negative voltages are not 117 V,
as you’ve heard about household electricity, but close to 165 V. There is a reason for this
difference. The effective voltage for an ac wave is never the same as the instantaneous