Page 51 - Teach Yourself Electricity and Electronics

Energy and the watt hour 31

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 these
units 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.

Table 2-2. Common prefix
multipliers.

Prefix Fraction
pico- 0.000000000001
(one-trillionth)
nano- 0.000000001
(one-billionth)
micro- 0.000001
(one-millionth)
milli- 0.001
(one-thousandth)
kilo- 1000
mega- 1,000,000
giga- 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. Then
you 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 by
dividing through either by E (to get I) or by I (to get E).

Energy and the watt hour

There is an important difference between energy and power. You’ve probably heard the
two terms used interchangeably, as if they mean the same thing. But they don’t. Energy
is 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 of
power, dissipated for one second of time. In electricity, you’ll more often encounter the
watt hour or the kilowatt hour. As their names imply, a watt hour, abbreviated Wh, is
the equivalent of 1 W dissipated for an hour (1 h), and 1 kilowatt hour (kWh) is the
equivalent of 1 kW of power dissipated for 1 h.
An energy of 1 Wh can be dissipated in an infinite number of different ways. A
60-watt bulb will burn 60 Wh in an hour, or 1 Wh per minute. A 100-W bulb would burn
1
1 Wh in /100 hour, or 36 seconds. A 6-watt Christmas tree bulb would require 10 min-
1
utes ( /6 hour) to burn 1 Wh. And the rate of power dissipation need not be constant; it
could be constantly changing.