Measurements

Explanation of question 29

The DC power to a load is to be calculated using a voltmeter and ammeter. When correctly connected the voltmeter reads 10V and the ammeter 10A. If both instruments readings are subject to a tolerance of ±10%, the power will be approximately

1. 100W ± 20%
2. 100W ± 15%
3. 100W ± 10%
4. 100W ± 5%

The answer is :- 1

If both instruments accurate, power = lOV x 1OA = 100W

Highest possible value of voltage = 10 V x 1.1 = 11 V
Highest possible value of current = 10A x 1.1 = 11 A
Highest possible value of power = 11 V x 11 A = 121 W
Lowest possible value of voltage = 10 V x 0.9 = 9 V
Lowest possible value of current = 10 A x 0.9 = 9 A
Lowest possible value of power = 9 V x 9 A = 81 W

The highest and lowest values of power are therefore 100W ± 20% (approximately).