In the circuit shown in figure when switch S1 is closed and S2 is open, the ideal voltmeter shows a reading of 18V. When switch S2 is closed and S1 is open, the reading of the voltmeter id 24V. When S1 and S2 both are closed the voltmeter reading will be
14.4 V
V=E=ir=E−(ER+r)r=ERR+r
Hence, 18=6E6+r⋯(i)
And 24=12E12+r⋯(ii)
Solving Eqs. (i) and (ii), we get
r=6Ω and E=36 V
When both S1 and S2 are closed external resistance in the circuit will be
R=(6)(12)6+12=4Ω
∴V=(36)(4)4+6=14.4V