CBSE Class 12 · 2020 · Magnetic Effects of CurrentShort Answer

(a) Define current sensitivity of a galvanometer. Write its expression. (b) A galvanometer has resistance

G

and shows full-scale deflection for current

I_{g}

. (i) How can it be converted into an ammeter to measure current up to

I_{0} (I_{0} > I_{g})

? (ii) What is the effective resistance of this ammeter?

Solution

(a) \textbf{Current sensitivity} is the deflection per unit current,

\frac{θ}{I} = \frac{N B A}{k}

. (b)(i) Connect a low shunt

R_{s}

in parallel so that it carries

(I_{0} - I_{g})

while the coil carries

I_{g}

(I_{0} - I_{g}) R_{s} = I_{g} G \Rightarrow R_{s} = \frac{I _{g}}{I _{0} - I _{g}} G .

(ii) The coil and shunt are in parallel, so

\frac{1}{R _{A}} = \frac{1}{G} + \frac{1}{R _{s}} \Rightarrow R_{A} = \frac{R _{s} G}{R _{s} + G} .

CBSE Class 12 Board · 2020 · Subjective