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JEE MainPhysicsCurrent Electricity

A 12 V ideal battery is connected in series with an unknown resistor R and a 40 , resistor. A voltmeter having a resistance of 120 , is connected across the 40 , resistor. If the voltmeter reads 8 V , the value of R is _____ .

Correct answer

15

Step-by-step solution

The voltmeter is connected in parallel with the 40 , resistor. The equivalent resistance of this parallel combination is: R_p = 40 120 40 + 120 = 4800 160 = 30 , The voltage across this parallel combination is given as 8 V . The total current in the circuit is: I = V_p R_p = 8 30 = 4 15 A The voltage drop across the unknown series resistor R is the remaining voltage from the battery: V_R = 12 V - 8 V = 4 V Using Ohm's law for the resistor R : R = V_R I = 4 4/15 = 15 , Answer: 15

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