In a series circuit there is only one path for the current to take from one terminal of the source through the components and back to the other terminal.There is a single closed loop and the current is the same in every position i.e. the rate of flow of charge is the same in every position.The e.m.f. is shared between the components in the ratio of their resistances.If the sources are connected with different polarities, the total e.m.f. is the algebraic sum of their e.m.f.s (opposing cells subtract)