Two resistances R1=60Ω and R2=70Ω are connected with an ideal battery of emf ε=120V. When a voltmeter is connected across R1, it reads 36 V. What will be its reading in volts, if it is connected across R2?

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84V

88V

94V

No Optionsne of these

answer is A.

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Detailed Solution

Concept: Current in the circuit will not flow through the voltmeter because the resistance of the voltmeter is very high. Apply Kirchhoff’s law to express the potential across each component in the circuit. Substitute the voltage across R1 and emf of the battery and find out the voltage across R2.
Ohm’s law, V=IR, where, V is the voltage, I is the current and R is the resistance.
We know that the voltmeter has high resistance. Therefore, the current in the circuit will not flow through the voltmeter. Thus, the direction of current will be from the positive terminal of the battery, R1, R2
and the negative terminal of the battery. We have given that the voltage across the resistor R1 is 36 V and we want to determine the voltage across the resistor C.
Applying Kirchhoff’s voltage law in the above closed circuit, we get,
V1+V2=ε
Here, V1 is the voltage across the resistor R1 and V2 is the voltage across the resistor R2 Substituting 36 V for V1 and 120 V for ε
in the above equation, we get,
36+V2=120
⇒V2=120−36
∴V2=84V
Hence, option 1 is correct.

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