Question

a galvanometer of resistance 500ohm gives current of 4ma what resistance should be used to convert it into voltmeter of range 5v​

Answer 1

Answer:

Here, galvanometer resistance G=30Ω and full scale deflection current  

IP g=2mA

(a) To convert the galvanometer into an ammeter of range 0.3 ampere, a resistance of value S is connected in parallel with it such that the current through G should not be more than Ig =0.3A and (I−Ig ) should pass through S (I−Ig )S=Ig G

∴S=        IgG           =  2×10−3  ×30    =0.2Ω

            (I-Ig)               (0.3−2×10−3 )

Hence, to convert the galvanometer into an ammeter of the desired range a shunt resistance (a small valued resistance) of 0.2Ω is connected parallel to the meter. This shunt resistance gives us a low resistance insturment with a deflection current Ia=0.3 ampere, while the current through the galvanometer is 2mA.

(b) To convert the galvanometer into a voltmeter of range 0.2 volt, a resistance R is connected in series with it such that

​V=Ig (R+G) 0.2=2×10  −3 (30+R)  

or

R=100−30−70ohms

Thus, to convert the galvanometer into a voltmeter of the desired range, a high resistance (Rs) is connected in series with the galvanometer.

The equivalent meter resistance is  

Req =30+70=100ohm ; In this case, most of the voltage appears across the series resistor. The current through the voltmeter is 2mA.

Hope this helps !

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