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  4. A galvanometer gives full scale deflection with 0.006 A current. By connecting it to a 4990 Ω resistance, it can be converted into a voltmeter of range 0-30 V. If connected to a (2 n /249) Ω resistance, it becomes an ammeter of range 0-1.5 A. The value of n is

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Q. A galvanometer gives full scale deflection with $0.006\, A$ current. By connecting it to a $4990 \,\Omega$ resistance, it can be converted into a voltmeter of range $0-30\, V$. If connected to a $\frac{2 n }{249} \,\Omega$ resistance, it becomes an ammeter of range $0-1.5 \,A$. The value of $n$ is

JEE AdvancedJEE Advanced 2014Current Electricity

Solution:

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$i =\frac{ V }{ R }$
$0.006=\frac{30}{4990+ R }$
$R =10$
$i _{ RG }=0.006 $
$i _{ RS }=1.494$
Since $R_{G}$ and $R_{S}$ are in parallel,
$i_{G} R_{G}=i_{S} R_{S}$
$0.006 \,R =1.494\left(\frac{2 n }{249}\right)$
$\therefore n =5$