Question

The resistance of an ammeter is $13\Omega$ and its scale is made for a current upto 100 amp. After an additional shunt has been connected to this ammeter it becomes possible to measure current upto 750 amp.. The value of shunt resisance is

• A. $2\Omega$
• B. $0.2\Omega$
• C. $20\Omega$
• D. $200\Omega$

Answer 1

The correct option is A $2\Omega$

As ammeter can measure upto 100 amp. So, when a resistance is connected in parallel then also max. current that can flow through this ammeter is 100 Amp $i.e$ $I_a=100A$
But, as a resistance is connected in parallel (called shunt) then the voltage in both ammeter and in shunt is same so,
$V_s=V_a$
$\begin{array}{}{I}_s{R}_s=I_a{R}_a& \text{(i)}\end{array}$
From KCL, $I=I_s+I_a$
$750=I_s+100$
$I_s=650A$
Put all values in equation (i)
$650\times R_s=100\times 13$
$R_s=2\Omega$