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Question

# A voltmeter is connected in parallel with a variable resistance are which is in series and an ammeter and call. For one value of R, metres read 0.3 A and 0.9 volt. For another value of R the readings are 0.25 ampere and 1.0 volt. What is the internal resistance of the cell ?

A
0.5Ω
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B
2Ω
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C
1.2Ω
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D
1Ω
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Solution

## The correct option is B 2ΩLet E be the emf of the cell emf r be its internal resistance.In the circuit the voltage equation can be written as,E−Ir−IR=0, where I=ammeter readingbut IR=v=voltmeter reading⇒E−Ir−V=0⇒E=Ir+VFrom case i,E=0.3r+0.9From case ii,E=0.25r+1Equating both the equation,0.3r+0.9=0.25r+1⇒0.05r=0.1⇒r=0.10.05=2ohmsHence the internal resistance of the cell is 2 ohms.

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