Determine the EMF of a source of electrical energy with an internal resistance of 0.25 Ohm

Determine the EMF of a source of electrical energy with an internal resistance of 0.25 Ohm, if, when it is closed with an iron conductor 0.5 m long and with a cross section of 0.2 mm, a current equal to 0.5 A.

Data: r (internal resistance of the source) = 0.25 Ohm; l (length of the iron conductor) = 0.5 m; S (conductor section) = 0.2 mm2; I (current that occurred when closing) = 0.5 A.
Constants: ρ (specific electrical resistance of iron) = 0.1 Ohm * mm2 / m.
1) Determine the external resistance of the source (conductor resistance) = R = ρ * l / S = 0.1 * 5 / 0.2 = 2.5 Ohm.
2) We calculate the EMF of the source: Ei = I * (R + r) = 0.5 * (2.5 + 0.25) = 1.375 V.
Answer: EMF of the source of electrical energy is 1.375 V.