Could you explain about what are Cells emf and internal resistance please?

Internal Resistance

When there is current present in the device or the electrical circuit and there’s a voltage drop in source voltage or source battery is internal resistance. It is caused due to electrolytic material in batteries or other voltage sources.

Internal Resistance (r) = (E – V)/I

Where E is the emf of the device; V is the potential difference between the device; I is the current in the device. Internal Resistance is the result of the resistance in the battery or the accumulation in the battery. the equation used to derive this is as follows:

V = (E – Ir)