In the circuit, a voltmeter reads `30 V` when it is connected across 400 `Omega` resistance. Calculate what the same voltmeter will read when it is connected across the `300Omega` resistance?

Explanation

The cicuit is shown in figure.
Let G be the resistance of voltmeter, connected in parallel wich `400Omega`, their effective resistance becomes,
`R_p=(400xxG)/(G+400)`
`:.` Current in the circuit,
`I=(60)/(300+400G//(G+400))`
`=(60(G+400))/(300(G+400)+400G)`
Pot. diff. across A and B `=IR_p=30V`
or `(60(G+400))/(300(G+400)+400G)xx(400G)/((G+400))=30`
On solving, `G=1200Omega`
When this voltmeter is connected in parallel with the resistance of `300Omega`, the effective resistance
becomes, `R_p^'=(1200xx300)/(1200+300)=240Omega`
Total resistance of the circuit
`=240+400=640Omega`
`:.` Current, `I^'=(60)/(640)=(3)/(32)A`
`:.` Reading of voltmeter=pot. diff. across
`R_p^'=I^'R_p^()'=(3)/(32)xx240=22*5V`