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Moving Charges and Magnetism — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsMoving Charges and Magnetism5 Marks
Question
A multirange voltmeter can be constructed by using a galvanometer circuit as shown in Fig. We want to construct a voltmeter that can measure 2V, 20V and 200V using a galvanometer of resistance 10Ω and that produces maximum deflection for current of 1mA. Find R1, R2 and R3 that have to be used.

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Answer

Key concept: The galvanometer can also be used as a voltmeter to measure the voltage across a given section of the circuit. For this a very high resistance wire is to be connected in series with galvanometer. The relationship is given by Ig(G + R) where Ig is the range of galvanometer, G is the resistance of galvanometer and R is the resistance of wire connected in series with galvanometer.

Applying expression in different situations

For Ig(G + R1) = 2 for 2V range

For Ig(G + R1 + R2) = 20 for 20V range

And For Ig(G + R1 + R2 + R3) = 200 for 200V range

By solving, we get

R1 = 1990Ω, R2 = 18kΩ and R3 = 180kΩ.

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