A circuit you\'re building needs an ammeter that goes from 0 mA to a full-scale

Question

A circuit you're building needs an ammeter that goes from 0 mA to a full-scale reading of 60.0 mA . Unfortunately, the only ammeter in the storeroom goes from 0 A to a full-scale reading of only 500 A. Fortunately, you can make this ammeter work by putting it in a measuring circuit, as shown in the figure. This lets a certain fraction of the current pass through the meter; knowing this value, you can deduce the total current. Assume that the ammeter is ideal. https://session.masteringphysics.com/problemAsset/1811566/1/jfk.Figure.23.P57.jpg What value of R must you use so that the meter will go to full scale when the current I is 60.0 mA ? Hint: When I= 60.0 mA , the ammeter is reading its maximum value.

What is the equivalent resistance of your measuring circuit?

Explanation / Answer

let,

resistance r=50 ohms

imax=60 mA

current in ammeter is, ir=500uA

resistance is R

use,

current in ammeter is,

ir=(R/R+r)*imax

500*10^-6=(R/(R+50))*60*10^-3

R=0.42 ohms

now,

net resistance Rnet=50*0.42/(50+0.42)

Rnet=0.42 ohms

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