A voltmeter of resistances `400(Omega)`is used to measure the potential difference across the `100(Omega)`resistor in the circuit shown in figure.(a) What will be the reading of the voltmeter ?(b) What was the potential difference across `100(Omega)`before the voltmeter was connected ?

Explanation

(a) `R_eff = ((100 xx 400)/500) + 200 = 280 `
` i=84/280 = 0.3 `
` 100i = (0.3 - i) 400 `
` rArr i=1.2 - 4i `
` rArr 5i = 1.2 rArr i=0.24`
Voltage measured by the voltameter
= 100 xx 0.24 = 24 V.
(b) If voltameter is not connected
` R_eff = (200 + 100) `
` = 300 Omega `
` i = 84/300 = 0.28 A `
` Voltage across 100 Omega `
` = (0.28 xx 100 ) = 28V `.