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MOVING CHARGES AND MAGNETISM — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsMOVING CHARGES AND MAGNETISM2 Marks
Question
A galvanometer cannot be used directly to measure current - why ? Explain the solution of this problem.
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State the difficulties occured and their solution when galvanometer is directly used as ammeter.
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Answer

→There are two reasons why a galvanometer cannot be used directly as an ammeter

Image
(i) Galvanometer is a very sensitive device. It also shows the full scale deflection for a current of the order of $\mu A$.
(ii) To measure current, a galvanometer has to be connected in series but its resistance is high, so that it changes the value of the current flowing in the circuit.
→To overcome these difficulties, a small resistance called a shunt is connected to the galvanometer.
→The shunt is connected in parallel with the galvanometer. Due to this, most of the current passes through the shunt.
→The resistance of this arrangement is $=\frac{R_{ G } r_{ S }}{ R _{ G }+r_{ S }}$ But $R _{ G } \gg r_{ S }$
So, we can neglect $r_{ S }$ as compared to $R _{ G }$
So, $\frac{ R _{ G } r_{ S }}{ R _{ G }} \approx r_{ S }$
→Since, the value of $r_{ S }$ very small, the original current does not change and the true value of current can be measured.

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