The ammeter your physics instructor uses for in-class demonstrations has a resistance R_g and requires a current of 1.0 mA for full-scale deflection.The same ammeter can be used to measure currents of much greater values by wiring a shunt resistor of relatively small resistance,R_p ,in parallel with the ammeter.The shunt resistance needed to produce a full-scale current of 12 A is 3.5 m$\varOmega$.What is the resistance of the ammeter,R_g? A)75 $\varOmega$. B)35 $\varOmega$. C)42 $\varOmega$. D)28 $\varOmega$. E)60 $\varOmega$.

Question

Quizplus · Accepted Answer

The full-scale current of the ammeter is 1.0 mA and it has a resistance of RS. The shunt resistor has a resistance of 3.5 mΩ and it is wired in parallel with the ammeter to measure currents up to 12 A. When the full-scale current of 12 A flows through the shunt resistor, the voltage across it will be:
V = IR = (12 A)(3.5 mΩ) = 0.042 V
This voltage is also the voltage across the ammeter. The current through the ammeter is:
I = V/RS
For full-scale deflection of the ammeter, I = 1.0 mA, so we have:
1.0 mA = 0.042 V/RS
RS = 42 Ω. Therefore, the answer is (C) 42 Ω.

The Ammeter Your Physics Instructor Uses for In-Class Demonstrations Has Ω\varOmegaΩ

The Ammeter Your Physics Instructor Uses for In-Class Demonstrations Has $\varOmega$