Power calculations

Chapter Power calculations

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

  • Problem 4-7If the voltmeter reads 24 V and the ammeter shows 3.0 A, what is the value of the po-tentiometer?Use the formula RE/I and plug in the values directly, because they are expressedin volts and amperes: R24/3.08. 0 .Note that you can specify this value to two significant figures, the eight and thezero, rather than saying simply 8 . This is because you are given both the voltage andthe current to two significant figures. If the ammeter reading had been given as 3 A(meaning some value between 21/2 A and 31/2 A), you would only be entitled to expressthe answer as 8 (somewhere between 71/2 and 81/2 ). A zero can be a significant fig-ure, just as well as the digits 1 through 9.Problem 4-8What 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 I0.018 A and E0.229 V. Then plug into the equation RE/I0.229/0.01813. You’re justified ingiving your answer to two significant figures, because the current is only given to thatmany digits.Problem 4-9Suppose 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 E2330 V. Then pluginto the formula: R2330/0.00005245,000,00045 M .Power calculationsYou can calculate the power, in watts, in a dc circuit such as that shown in Fig. 4-7, bythe formula PEI 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 currentand the resistance.Remember the Ohm’s Law formula for obtaining voltage: EIR. If you know I andR, but don’t know E, you can get the power P by means of the formula P(IR)II 2R.That is, you take the current in amperes, multiply this figure by itself, and then multiplythe result by the resistance in ohms.You can also get the power if you aren’t given the current directly. Suppose you’regiven only the voltage and the resistance. Remember the Ohm’s Law formula for ob-taining current: IE /R. Therefore, PE (E/R) E 2/R. Take the voltage, multiply itby itself, and divide by the resistance.Stated all together, these power formulas are:PEII 2RE 2/RNow you are ready to do some problems in power calculations. Refer once again toFig. 4-7.72 Basic dc circuits