A voltmeter reads the potential difference across the terminals of an old battery as `1.40 V`, while a potentiometer reads its voltage to be `1.55 V`. The voltmeter resistance is `280 Omega`.

Options

A. `60 Omega`
B. `45 Omega`
C. `35 Omega`
D. `30 Omega`

Answer

D

Explanation

Volt meter reading
`1.4 = l xx 280`

`I = 1.4/280 = 5 xx 10^(-3) A`
Now, `I = 1.55/(R + r) rArr 5 xx 10^(-3) = 1.55/(280 + r)`
`rArr 1.4 + r xx 5 xx 10^(-3) = 1.55 rArr r = 30 Omega`