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JEE MainPhysicsCurrent Electricity

A galvanometer having a coil resistance of 240 and full-scale deflection current of 25 mA is converted into a voltmeter of range 0 to 15 V . This voltmeter is then used to measure the potential difference across a 120 resistor in a circuit where a 30 V ideal battery is connected in series with a 150 resistor and the 120 resistor. The reading of the voltmeter will be:

Options

  1. A13.3 V
  2. B11.25 V
  3. C10.4 V
  4. D12 V

Correct answer

D. 12 V

Step-by-step solution

First, find the total resistance of the converted voltmeter. The resistance of the voltmeter R_v is given by R_v = V I_g R_v = 15 25 10⁻³ = 600 The voltmeter is connected in parallel with the 120 resistor. The equivalent resistance of this parallel combination is: R_p = 120 600 120 + 600 = 72000 720 = 100 The total resistance of the circuit is the sum of the series resistor and the parallel combination: R_ total = 150 + 100 = 250 The main current drawn from the battery is: I = V_ battery R_ total = 30 250 = 0.12 A

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