Question

A network shown in the figure consists of a battery and five unknown resistors. When an ideal ammeter is connected between the terminals $A$ and $B$, its reading is $4A$ and when a resistance of $3\Omega$ is connected in series with the ammeter its reading becomes $2A$. Now the ammeter and the resistance are disconnected and an ideal voltmeter is connected between the terminals $A$ and $B$. What would the voltmeter read?

• A. $6V$
• B. $10V$
• C. $12V$
• D. $18V$

Answer 1

The correct option is C $12V$

Let the potential difference between A and B is V and Req between A & B is R.

When an ideal ammeter is connected-

$V=\left(4A\right)R$

$V=4R$ ----eq.1

When a $3\Omega$ is connected in series with the ammeter:

$V=\left(3\Omega \right)\times \left(2A\right)+\left(2A\right)R$

$V=6+2R$ ----eq.2

Form eq.1 and eq.2:

$4R=6+2R$

$2R=6$

$R=3\Omega$

Form eq.1:

$V=4\ast 3\Omega$

$V=12V$

When only voltmeter is connected between the terminals, it will read $12V$

Hence option 'C' is connected.