Page 92 - Teach Yourself Electricity and Electronics

72 Basic dc circuits

Problem 4-7
If the voltmeter reads 24 V and the ammeter shows 3.0 A, what is the value of the po-
tentiometer?
Use the formula R E/I and plug in the values directly, because they are expressed
in volts and amperes: R 24/3.0 8. 0 .
Note that you can specify this value to two significant figures, the eight and the
zero, rather than saying simply 8 . This is because you are given both the voltage and
the current to two significant figures. If the ammeter reading had been given as 3 A
1
1
(meaning some value between 2 /2 A and 3 /2 A), you would only be entitled to express
1
1
the answer as 8 (somewhere between 7 /2 and 8 /2 ). A zero can be a significant fig-
ure, just as well as the digits 1 through 9.
Problem 4-8
What is the value of the resistance if the current is 18 mA and the voltage is 229 mV?
First, convert these values to amperes and volts. This gives I 0.018 A and E
0.229 V. Then plug into the equation R E/I 0.229/0.018 13 . You’re justified in
giving your answer to two significant figures, because the current is only given to that
many digits.

Problem 4-9
Suppose the ammeter reads 52 uA and the voltmeter indicates 2.33 kV. What is the re-
sistance?
Convert to amperes and volts, getting I 0.000052 A and E 2330 V. Then plug
into the formula: R 2330/0.000052 45,000,000 45 M .

Power calculations

You can calculate the power, in watts, in a dc circuit such as that shown in Fig. 4-7, by
the formula P EI or the product of the voltage in volts and the current in amperes.
You might not be given the voltage directly, but can calculate it if you know the current
and the resistance.
Remember the Ohm’s Law formula for obtaining voltage: E IR. If you know I and
2
R, but don’t know E, you can get the power P by means of the formula P (IR)I I R.
That is, you take the current in amperes, multiply this figure by itself, and then multiply
the result by the resistance in ohms.
You can also get the power if you aren’t given the current directly. Suppose you’re
given only the voltage and the resistance. Remember the Ohm’s Law formula for ob-
2
taining current: I E/R. Therefore, P E(E/R) E /R. Take the voltage, multiply it
by itself, and divide by the resistance.
Stated all together, these power formulas are:
2
2
P EI I R E /R
Now you are ready to do some problems in power calculations. Refer once again to
Fig. 4-7.