Consider a cell having terminal voltage $V'$ and EMF, $V$ having internal resistance, $r$ and current, $I$ flows through the circuit in which an external resistance, $R$ is connected.
Potential drop across internal resistance= Ir
$$V= V' + Ir = IR + Ir$$
Now if the cell is in an open circuit then $I=0$ and hence,
But we know that the Emf of the cell is equal to the Terminal Voltage of the cell in an open circuit, i.e.,
Why such a contradiction in my concept?
I know how to derive $V=V'$. I don't need that. I want to know why using this concept makes Emf, $V= 0$ rather becoming $V=V'$.