Page 97 - Teach Yourself Electricity and Electronics

Quiz 77

component fails, and then a third. It’s hard to predict the current and power distribu-
tion in an array when its resistor values are all different. So it’s hard to know whether
any of the components in such a matrix are going to burn out.
If you need a resistance with a certain power-handling capacity, you must be sure
the network can handle at least that much power. If a 50-W rating is required, and a cer-
tain combination will handle 75 W, that’s alright. But it isn’t good enough to build a cir-
cuit that will handle only 48 W. Some extra tolerance, say 10 percent over the minimum
rating needed, is good, but it’s silly to make a 500-W network using far more resistors
than necessary, unless that’s the only convenient combination given the parts available.
Nonsymmetrical series-parallel networks, made up from identical resistors, will in-
crease the power-handling capability. But in these cases, the total resistance will not be
the same as the value of the single resistors. The overall power-handling capacity will al-
ways be multiplied by the total number of resistors, whether the network is symmetri-
cal or not, provided all the resistors are the same. In engineering work, cases sometimes
do arise where nonsymmetrical networks fit the need just right.

Resistive loads in general

The circuits you’ve seen here are good for illustrating the principles of dc. But some of
the circuits shown here have essentially no practicality. You’ll never find a resistor con-
nected across a battery, along with a couple of meters, as shown in Fig. 4-7, for exam-
ple. The resistor will get warm, maybe even hot, and it will eventually drain the battery
in an unspectacular way. Aside from its educational value, the circuit does nothing of
any use.
In real life, the ammeter and voltmeter readings in an arrangement such as that
shown in Fig. 4-7 would decline with time. Ultimately, you’d be left with a dead, cold
battery, a couple of zeroed-out meters, a potentiometer, and some wire.
The resistances in the diagrams like Fig. 4-7 are always put to some use in electri-
cal and electronic circuits. Instead of resistors, you might have light bulbs, appliances
(60-Hz utility ac behaves much like dc in many cases), motors, computers, and radios.
Voltage division is one important way in which resistors are employed. This, along with
more details about current, voltage, and resistance in dc circuits, is discussed in the
next chapter.

Quiz

Refer to the text in this chapter if necessary. A good score is at least 18 correct answers.
The answers are in the back of the book.
1. Suppose you double the voltage in a simple dc circuit, and cut the resistance in
half. The current will become:
A. Four times as great.
B. Twice as great.
C. The same as it was before.
D. Half as great.