In the given circuit, an ideal voltmeter connected across the 10Ω resistance reads 2V . The internal resistance r, of each cell is
A
1.5Ω
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B
0.5Ω
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C
1Ω
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D
0Ω
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Solution
The correct option is B0.5Ω Given the two cells are in series
Equivalent emf V=1.5V+1.5V=3V
Equivalent resistance of the circuit is Req=Rp+2Ω+2r
(series combination )
Where parallel combination
Rp=15Ω×10Ω15Ω+10Ω
Rp=6Ω
Req=8+2r
Let the current flowing in the circuit be i. ⇒i=VReq
i=3(8+2r)
(ohm’s law )
It is given that the voltage across 10Ω is 2V
(as measured by ideal voltmeter) this is same as voltage across the parallel combination Rp, current through RP is i