Question

A milliammeter of range $10mA$ and resistance $9\Omega$ is joined in a circuit as shown. The meter gives full-scale deflection for current $I$ when $A$ and $B$ are used as its terminals, i.e. current enters at $A$ and leaves at $B$ ($C$ is left isolated). The value of $I$ is

• A. $100mA$
• B. $900mA$
• C. $1A$
• D. $1.1A$

Answer 1

The correct option is C $1A$

Voltage across ammeter is

$V=10\times 9\times {10}^{-3}=9\times {10}^{-2}V$

Voltage between terminals A and B is $9\times {10}^{-2}V$

$9\Omega$ and $0.9\Omega$ are in series

Hence , $0.1\times i=10\times {10}^{-3}\times (9+0.9)$

$i=990mA$

Current through the ammeter is $10mA$

Hence , total current $I=990+10=1000mA=1A$