The range of voltmeter is 10V and its internal resistance is 50Ω. To convert it to a voltmeter of range 15V, how much resistance is to be added?

Options

(a) Add 25Ω resistor is parallel
(b) Add 25Ω resistor is series
(c) Add 125Ω resistor in parallel
(d) Add 125Ω resistor in series

Correct Answer:

Add 25Ω resistor is series

Explanation:

The current in case of voltmeter range 10 V and resistance 50 Ω is I = 10 / 50 —–(i)

Let X is the resistance conneced in order to make a voltmeter with range 15 V is I = 15 / X —-(ii)

Equating equation (i) and (ii), we get

15 / X = 10 / 50 ⇒ X = (15 × 50) = 75 Ω

Then, R = 75 – 50 = 25 Ω must be connected in series, because V ∝ R when current is constant.