Answer to Question 6 |
An analogue voltmeter is known to be accurately calibrated. A digital voltmeter of infinite resistance measures the voltage across a 150 kW resistor as 3.5 V, but when the analogue meter is used, the voltage is measured as 1.4 V. Calculate the resistance of the meter. |
Work out the current with the perfect voltmeter: I = 3.5 V ÷ 150 × 10^{3} W = 23.3 × 10^{-6 }A
Treat the voltmeter as a perfect voltmeter in parallel with an unknown resistor, R. Assume that the current to the parallel resistors remains constant. The current through the 150 kW resistor is now: I = 1.4 V ÷ 150 × 10^{3} W = 9.33 × 10^{-6 }A^{ }
The current through the voltmeter resistor = 23.3 × 10^{-6 }A - 9.33 × 10^{-6 }A^{ } = 13.9 × 10^{-6} A
R = 1.4 V ÷ 13.9 × 10^{-6} A = 100 000 W |