Combining emf
When we join a few cells together, we get a battery. A cell is the smallest unit that can produce an emf.
One way to join cells together is in series. This means joining positive to negative pole. The purpose of doing this is to get a larger emf. For example, if each cell has an emf of 1.5 V, then joining two identical cells in series gives an emf of 1.5 + 1.5 = 3 V.
Consider 1 coulomb of charge starting at A, going round the circuit and coming back to A. This 1 C of charge goes through one cell and then the next cell. Each cell contributes 1.5 J of work. Together, the two cells do 1.5 + 1.5 = 3 J of work to move 1 C of charge around the circuit once.
Another way to combine the cells is in parallel. This means joining positive to positive pole, and negative to negative pole.
The two cells form a closed circuit among themselves. We would expect current to flow around this loop in between A and D. In fact, no current flows because the cells have the same emf in this case. So within the loop between A and D, the emf cancels.
If we connect a light bulb to A and D, current flows through the bulb. The combined emf is the same as the emf of a single battery.
To understand this, suppose that the emf of each cell is 1.5 V. Suppose 2 coulombs of charge start from A and go round the circuit. It goes through the light bulb, arrives at D and split equally into two parts. 1 C goes through one cell, and 1 C goes through the other cell.
Each cell contribute 1.5 J over work to the 1 C of charge. The total work is 1.5 + 1.5 = 3 J on the 2 C of charge.
So the combined emf = work done / charge = 3 / 2 = 1.5 V. the same as that of one cell.