Voltage is a force! A force that can move electrons, and make them flow. The flow of electrons is called Current.
Voltage is also called Potential Difference, which is a nice scientific term but may be confusing. When we call it "difference", it implies that we are comparing it with something (by finding difference). But with what it is being compared is not clear. Generally any voltage/potential is alway with compared to zero volts when the reference point is not explicitly specified, the zero volts is also called the ground.
For example, consider the above diagram in which two cells of 1.5 volts are connected in series. It is clear that potential at point B is 1.5V more than that of point A, hence potential difference between point B with respect to point A is 1.5V. But note that potential difference at point A with respect to B is -1.5V. The reference point is very important.
Similarly, potential difference at C with respect to point B is 1.5V and potential difference at B with respect to C is -1.5V.
Now, can you tell what is potential at C with respect to A and vice-versa ?, Yes it is 3V and -3V. (by this we have observed one important property of voltage i.e VOLTAGE IN SERIES GETS ADDED).
Now analyze Fig-2 and try to give answers to the questions below it. Please see the way cells are connected.
Potential at point B with respect to A ?
Potential at point C with respect to A ?
Potential at point D with respect to A ?
Potential at point C with respect to B ?
Potential at point C with respect to D ?
Potential at point A with respect to B ?
Potential at point A with respect to C ?
Potential at point A with respect to D ?
Potential at point B with respect to C ?
Potential at point D with respect to C ?
Now, lets try to analyze the combination shown in FIG-3.
Carefully see how 3rd cell is connected.
Potential at point B with respect to A ?
Potential at point C with respect to A ?
Potential at point D with respect to A ?
Potential at point C with respect to B ?
Potential at point C with respect to D ?
Potential at point A with respect to B ?
Potential at point A with respect to C ?
Potential at point A with respect to D ?
Potential at point B with respect to C ?
Potential at point D with respect to C ?