Question

A resistance R is connected across a cell of emf ε  and internal resistance r. A potentiometer now measures the potential difference between the terminals of the cell as V. Write the expression for r in terms of ε, V and R.

Answer 1

ε = l(R +r) and V = IR

∴ ε/V = (R + r/R)

We get, r = (ε/V -1)R.