State ohm's law.
In the circuit shown in figure, a voltmeter reads 30 volts when it is connected across 400 ohm resistance. Calculate what the same voltmeter will read when it is connected across the 300 ohm resistance.

Answer

B

Explanation

Potential difference across the 400 Omega resistance = 30 V. Therefore, potential difference across the 300 Omega resistance `=60-30V = 30V.` Let R be the resistance of the voltmeter. As the voltmeter is in the parallel with the 400 Omega resistance, their combined resistance is
`R' = (400R)/(400 + R)`
As the potential difference of 60 V is equally shared between the `300 Omega and 400 Omega` resistance. `R'`should be equal to `300 Omega`.
Thus,
`300 = (400R)/(400 + R)`
which gives `R = 1200 Omega,` is the resistance of the voltmeter. When the voltmeter is connected across the `300 Omega` resistance,
their conbined resistance is
`R' = (300R)/(300 + R) = (300 xx 1200)/(300 + 1200) = 240 Omega`
`:. Total resistance in the circuit = 400 + 240 = 640 W`
`:. Current in the circuit is
`I = (60V)/(640Omega) = 3/(32)A `
`:. Voltmeter reading`
`=Potential difference across 240 Omega resistance`
`= 3/32 xx 240 = 22.5V` .