JEE Advanced 2019 · Paper 1 · Physics Q12

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PhysicsMultiple correct+4 / -1Question 12 of 54

Two identical moving coil galvanometers have 10 Ω resistance and full scale deflection at 2 μA current. One of them is converted into a voltmeter of 100 mV full scale reading and the other into an Ammeter of 1 mA full scale current using appropriate resistors. These are then used to measure the voltage and current in the Ohm's law experiment with $R = 1000$ Ω resistor by using an ideal cell. Which of the following statement(s) is/are correct?

The resistance of the Voltmeter will be 100 kΩ

The resistance of the Ammeter will be 0.02 Ω (round off to 2nd decimal place)

The measured value of

R

will be

978\ \Omega < R < 982\ \Omega

If the ideal cell is replaced by a cell having internal resistance of 5 Ω then the measured value of

R

will be more than 1000 Ω

Answer and worked solution

Official answer: B, C

(A) Voltmeter.
$R_V=\frac{100\text{ mV}}{2\ \mu\text{A}}=50\ \text{k}\Omega,$
not 100 kΩ. False.

(B) Ammeter. The shunt is
$S=\frac{I_gG}{I-I_g}=\frac{2\times10^{-6}\times10}{998\times10^{-6}}\approx0.02\ \Omega,$
so the ammeter's resistance $G\parallel S$ is about 0.02 Ω. True.

(C) Measured value. With the voltmeter across $R$ and the ammeter in series, the ratio $V/I$ measures $R\parallel R_V$ :
$\frac{1000\times50\,000}{51\,000}\approx980\ \Omega,$
which lies between 978 and 982 Ω. True.

(D) An internal resistance in the cell scales $V$ and $I$ together, so $V/I$ is still about 980 Ω, less than 1000 Ω. False.

(B), (C)

Topics:Ammeters and voltmeters Moving coil galvanometer Metre bridge and Ohm's law experiments

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JEE Advanced 2019 · Paper 1, Physics question 12