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

Question

5] A circuit you're building needs an ammeter that goes from 0 mA to a full-scale reading of 50 mA. Unfortunately, the only ammeter in the storeroom goes from 0 to a full-scale reading of only 500 Fortunately, you've just finished a physics class, and you realize that you can make this ammeter work by putting a resistor in parallel with it, as shown in the figure below. You've measured that the resistance of the ammeter is 50.0 , not the 0 of an ideal ammeter -Your ammeter .500 A ammeter a. What value of R must you use so that the meter will go to full scale when the current lis 50 mA? b. What is the effective resistance of your ammeter?

Explanation / Answer

Use current division rule We want I to be 40mA and the current through the ammeter equal to 500A If the current

through the ammeter is 500A.

then the current through the resistor R must be

R = 40 mA - 500A

R = 0.0395 A

from current division rule

0.0395 = 50 / ((R + 50) x 0.05 A

R = [(50 x 0.05) / 0.0395] x 0.05 A

R = 3.165 ohm

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