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?

Answer

A, B

Explanation

Voltmeter reads `30 V`, half of `60 V`. Hence, resistance of `400 Omega` and voltmeter is also equal to `300 Omega`

`:. 300=((400xxR)/(400+R))`
where R = resistance of voltmeter,
Solving the above equation we get
`R=1200 Omega`
In the new situation
`R_("net")=400=((300)(1200))/(300+1200)=640Omega`
`:.i=60/640=0.09375A`
Now voltage drop across
Voltmeter =60-potential drop across `400 Omega` resistor
`=60-(400)i`
`=60-(400)(0.09375)`
`=22.5V`