Arrangement of Cells

Cells in Series    emf = 1.5 V
Two cells in series with positive terminal of first cell connect to negative terminal of next cell.
=>emf = 2 * 1.5 = 3.0V
Two cells in series with positive terminal of first cell connect to positive terminal of next cell.
=>emf = 1.5 – 1.5 = 0V
Two cells in series with positive terminal of first cell connect to positive terminal of next cell.
=>emf = 2.0 – 1.5 = 0.5V

Note that in this case, current will flow anticlockwise, which is determined by bigger power source.
Cells in Parallel     Two cells in parallel with positive terminal of first cell connect to positive terminal of next cell and negative to negative terminal.
=>emf = 1.5V for the circuit but it will last longer as compare to only one cell of 1.5V in the circuit.
    Two cells in parallel with positive terminal of first cell connect to negative terminal of next cell and negative to positive terminal.
=>No emf supplied to the circuit.
Note that we cannot connect two cells with different e.m.f in parallel as they should be the same in parallel circuit, which will be discussed in D.C Circuit.
Excel@Physics