First slide

voltmeter

Question

In the adjoining circuit, the emf of the cell is 2 volt and the internal resistance is negligible. The resistance of the voltmeter is 80 ohm. The reading of the voltmeter will be

Moderate

A

0.80 volt
B

1.60 volt
C

1.33 volt
D

2.00 volt

Solution

Total resistance of the circuit $= \frac{80}{2} + 20 = 60 Ω$
$\Rightarrow Main current, i = \frac{2}{60} = \frac{1}{30} A$
Combination of voltmeter and $80 Ω$ resistance is connected in series with $20 Ω$ so current through $20 Ω$ and this combination will be same, i.e., $\frac{1}{30} A$ . Since the resistance of voltmeter is also $80 Ω$ , so this current is equally distributed in $80 Ω$ resistance and voltmeter $(i . e ., \frac{1}{60} A through each)$
Potential difference across $80 Ω$ resistance $= \frac{1}{60} \times 80 = 1 .33 V$

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