ac Waves and the hertz

Chapter ac Waves and the hertz

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • 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 0.00000000000000000016joule (there are 18 zeroes after the decimal point and before the 1). The physicists write1.6× 10–19 to represent this. It is the energy gained by a single electron in an electricfield of 1 V. Atom smashers are rated by millions of electron volts (MeV) or billions ofelectron 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 electronvolts (TeV).Another energy unit, employed to denote work, is the foot pound (ft-lb). This isthe work needed to raise a weight of one pound by a distance of one foot, not includingany friction. It’s equal to 1.356 joules.All of these units, and conversion factors, are given in Table 2-3. Kilowatt hours andwatt 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, youneed 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’llneed scientific notation.34 Electrical unitsTable 2-3. Energy units.To convert to joulesConversely,Unitmultiply bymultiply byBtu10550.000948or 9.4810–4eV1.6 10–196.21018erg0.000000110,000,000or 10–7or 107ft-lb1.3560.738Wh36000.000278or 2.78 10–4kWh3,600,0000.000000278or 3.6 x 106or 2.78 10–7ac Waves and the hertzThis chapter, and this whole first section, is concerned with direct current (dc), thatis, 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. Itreverses direction periodically, exactly once every1/120 second. It goes through a com-pletecycle every1/60 second. Every repetition is identical to every other. This is alter-nating current (ac). In some countries, the direction reverses every 1/100 second, andthe cycle is completed every 1/50 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 thisdifference. The effective voltage for an ac wave is never the same as the instantaneous