Four identical cells each of emf 2V, are joined in parallel providing supply of current to external circuit consisting of two `15 Omega` resistors joined in parallel. The terminal voltage of the cells ae read by an ideal voltmeter is 1.6V. Calculate the internal resistance of each cell.

Explanation

The four cells are connected in parallel to the parallel combination of two `15 Omega` resistors as shown in figure

Let r be internal resistance of each cell. As cells are in parallel, the effective emf of all the cells, `epsilon`= emf of one cell=2 V.
Terminal voltage, V = 1.6V,
Total internal resistance r' of four cells in parallel is obtained from
`1/r = 1/r + 1/r +1/r+1/t = 4/r or r' = r/4`
Total external resistance, `R = (15 xx 15)/(15+15)=7.5 Omega`
Internal resistance r' of the parallel combination of cells is given by
`r' = ((epsilon- V)/(V)) R=((2-1.6)/(1.6)) xx 7.5=7.5/4`
or `r/4 = 7.5/4` or `r=7.5 Omega`