Okay so are EMF and Potential Difference the same thing ? I know they are now but I am not able to understand how ? I came through this Derivation today if someone could explain it to me it would be great The figure shows a battery connected to a resistor V(A) - V(B) = V(b) = ε (emf) what the Derivation says is if we Remove the resistor R the resistance becomes infinitely large (HOW?) . So R = ∞ therefore the current I = V/R = 0 So the current flowing would be zero Now , the derivation says that there is always an internal resistance (r(i)) present inside the battery . So now if we again Connect the Resistor R so ε = I(R+r(i)) (HOW? IS OHM's law used here ?) Further it says , now V(b) = IR = ε - I(i) (Why is V(b) not I(R+r(i)) ? ) So the ε is greater than the V(b) . Please someone explain this Derivation .