Energy and the watt hour

Chapter Energy and the watt hour

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

  • You will often hear about milliwatts (mW), microwatts (µW), kilowatts (kW)and megawatts (MW). You should, by now, be able to tell from the prefixes what theseunits represent. But in case you haven’t gotten the idea yet, you can refer to Table 2- 2.This table gives the most commonly used prefix multipliers in electricity and electron-ics, and the fractions that they represent. Thus, 1 mW 0.001 W; 1 µW 0.001 mW 0.000001 W; 1 kW 1,000 W; and 1 MW 1,000 kW 1,000, 000 W.Energy and the watt hour31Table 2-2. Common prefixmultipliers.PrefixFractionpico-0.000000000001(one-trillionth)nano-0.000000001(one-billionth)micro-0.000001(one-millionth)milli-0.001(one-thousandth)kilo-1000mega-1,000,000giga-1,000,000,000(one billion)tera-1,000,000,000,000(one trillion)Sometimes you need to use the power equation to find currents or voltages. Thenyou should use I P/E to find current, or E P/I to find power. It’s easiest to remem-ber that P EI (watts equal volt-amperes), and derive the other equations from this bydividing through either by E (to get I) or by I (to get E).Energy and the watt hourThere is an important difference between energy and power. You’ve probably heard thetwo terms used interchangeably, as if they mean the same thing. But they don’t. Energyis power dissipated over a length of time. Power is the rate at which energy is expended.Physicists measure energy in joules. One joule is the equivalent of one watt ofpower, dissipated for one second of time. In electricity, you’ll more often encounter thewatt hour or the kilowatt hour. As their names imply, a watt hour, abbreviated Wh, isthe equivalent of 1 W dissipated for an hour (1 h), and 1 kilowatt hour (kWh) is theequivalent of 1 kW of power dissipated for 1 h.An energy of 1 Wh can be dissipated in an infinite number of different ways. A60-watt bulb will burn 60 Wh in an hour, or 1 Wh per minute. A 100-W bulb would burn1 Wh in 1/100 hour, or 36 seconds. A 6-watt Christmas tree bulb would require 10 min-utes (1/6 hour) to burn 1 Wh. And the rate of power dissipation need not be constant; itcould be constantly changing.