While performing an experiment to determine the equivalent resistance of two resistors connected in parallel, a student observes the pointer of voltmeter at position X when the key is 'off' and the same pointer at position Y when the key is 'on'.

The correct voltmeter reading is :
(1) 0.2 V
(2) 2 V
(3) 2.2 V
(4) 1.8 V

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Solution

Total number of divisions on the main scale of the voltmeter = 30

Least count of the given scale $= \frac{3 V}{30} = 0.1 V$

Main scale reading $= 20 \times 0.1 V=2.0 V$

Zero error = $2 \times 0.1 V = 0.2 V$

True reading of the ammeter = Main scale reading - Zero error = 2.0 V $-$ 0.2 V = 1.8 V

Hence, the correct answer is option 4.

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Q. A student while performing the experiment to find the resultant resistance of two resistors connected in series observed the ammeter pointer at position P when the key is 'off' and the same pointer at position Q when the key is 'on'.

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(2) 2.0 V
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(4) 2.4 V

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Q. In the circuit shown in figure, an ideal ammeter and an ideal voltmeter are used. When the key is open, the voltmeter reads 1.54 V. When the key is closed, the ammeter reads 1.0 A and the voltmeter reads 1.03 V. The resistance R is

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Total number of divisions on the main scale of the voltmeter = 30 Least count of the given scale =3 V30=0.1 V Main scale reading =20×0.1 V=2.0 V Zero error = 2×0.1 V=0.2 V True reading of the ammeter = Main scale reading - Zero error = 2.0 V - 0.2 V = 1.8 V Hence, the correct answer is option 4.