Question

What is the total e.m.f. when three cells of voltages $1V,1.5V$ and $2V$ are connected in (A) series and (B) parallel.

Answer 1

$Emf=2V$

$r=?$

$R=5\Omega$

terminal voltage$=5V$

Let internal resistance be r ohm

the effective resistance of each three cells in parallel, $r=\frac{r}{3}$

total resistance $=\frac{r}{3}+R$

$V=emf\times \frac{R}{r}+R$

terminal voltage $V=emf\times \frac{R}{\left(\frac{r}{3}\right)}+R$

$1.5=2\times \frac{5}{\left(\frac{r}{3}\right)}+5$

$r=5\Omega$