JEE MAIN - Physics (2019 - 11th January Evening Slot - No. 13)
A galvanometer having a resistance of 20 $$\Omega $$ and 30 divisions on both sides has figure of merit 0.005 ampere/division. The resistance that should be connected in series such that it can be used as a voltmeter upto 15 volt, is:
120 $$\Omega $$
125 $$\Omega $$
80 $$\Omega $$
100 $$\Omega $$
Explanation
Rg = 20$$\Omega $$
NL = NR = N = 30
FOM = $${1 \over \phi }$$ = 0.005 A/Div.
Current sentivity = CS = $$\left( {{1 \over {0.005}}} \right)$$ = $${\phi \over {\rm I}}$$
$${\rm I}$$gmax = 0.005 $$ \times $$ 30
= 15 $$ \times $$ 10$$-$$2 = 0.15
15 = 0.15 [20 + R]
100 = 20 + R
R = 80
NL = NR = N = 30
FOM = $${1 \over \phi }$$ = 0.005 A/Div.
Current sentivity = CS = $$\left( {{1 \over {0.005}}} \right)$$ = $${\phi \over {\rm I}}$$
$${\rm I}$$gmax = 0.005 $$ \times $$ 30
= 15 $$ \times $$ 10$$-$$2 = 0.15
15 = 0.15 [20 + R]
100 = 20 + R
R = 80
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