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JUNE 2024 — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsJUNE 20242 Marks
Question
State the two difficulties in using the galvanometer directly as an ammeter to measure the value of the electric current. What should be done to overcome these difficulties?
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Answer

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