Relation between electromotive force (E), internal resistance (r) and potential difference (V) in a circuit

Relation between electromotive force $(E)$, internal resistance $(r)$ and potential difference $(V)$:

Let us consider:

The cell having electro-motive force = $E$

The cell having internal resistance = $r$

The external resistance of the circuit = $R$

The potential difference between the external resistance of the circuit = $V$

The current in circuit = $i$

So, The emf of the cell from the given circuit in the figure above

$E=iR+ir$

$E = V+ir$

$V=E-ir$