Question

The figure shows a source (a battery) with an emf $E$ of $12V$ with an internal resistance $r$ of $2\Omega$ and an external resistance of $4\Omega$ is added to complete the circuit. What are the voltmeter and ammeter readings?

Answer 1

An ideal voltmeter has infinite resistance, hence no current flows through it. An ideal ammeter has zero resistance. The current $i$ through the resistor $R$.
$i=\frac{E}{R+r}=\frac{12}{4+2}=2A$
The ammeter will read $i=2A$
As there is no potential difference between points $a$ and $p$ or between points $b$ and $q$, i.e., $V_{ab}=V_{pq}$,we find $V_a$ by considering $a$ and $b$ either as the terminals of the resistor or as the terminals of the source
$V_{pq}=IR=2\times 4=8V$
Considering them as the terminals of the source, we have
$V_{ab}=E-ir=12-2\times 2=8V$
Either way, we conclude that the voltmeter reads $V_{ab}=8V$