Question

A battery of emf 12 V and internal resistance $2\Omega$ is connected to a $4\Omega$ resistor as shown in the figure.
(a) Show that a voltmeter when placed across the cell and across the resistor, in turn, gives the same reading.
(b) To record the voltage and the current in the circuit, why is the voltmeter placed in parallel and the ammeter in series in the circuit ?

Answer 1

We know that,
E = V + Ir
V = E - Ir
(a) Current, $I=E/(R+r)=12/(4+2)=12/6=2A$
Terminal potential difference across the cell can be calculated as,

$V=E=12-2\times 2=12-4=8V$
Also, Potential difference across 4$\Omega$ resistor can be calculated as

$V=IR=2\times 4=8V$
So, a voltmeter when placed across the cell and across the resistor, gives the same reading.
(b) An ammeter is connected in series because it has very low resistance. So, when, an ammeter is connected in series, then there is not much increase in the resistance of the circuit and hence the current through the circuit is unchanged.
A voltmeter is connected in parallel because it has very high resistance. So, it draws a very small current from the circuit.