What is the mathematical expression of Ohm's law of the whole circuit?

What is the mathematical expression of Ohm's law of the whole circuit?

I = E / (R + R) u-voltage volt (V) r-resistance ohm (Ω) i-current ampere (a) where e is electromotive force, R is external circuit resistance, R is internal resistance of power supply, internal voltage uine = IR, e = uine + uexternal scope of application: energy conversion in closed circuit of pure resistance circuit: e = u + IR EI = UI + IRP release = EIP output = UI pure resistance electricity