Two identical cells of emf 1.5 V each joined in paralllel provide supply to an external circuit consisting of two resistances of `7 Omega` each joined in parallel. A very high resistance voltmeter reads the terminal voltage of cells to be 1.4 V. Calculate the internal resistance of each cell.

Explanation

Let the internal resistance of each cell be .r.. The equivalent simplified circuit can be drawn as shown in the figure (b).
Current, `I=(1.5)/((r)/(2)+8.5)`
Terminal potential under the given condition will be
`V=epsilon-Ir`
`1.4=1.5-(1.5)/(((r)/(2)+8.5))(r)/(2)`
`(1.5r)/(r+17)=0.1`
`15r=r+17`
`14r=17`
`r=(17)/(14)=1.21Omega`