What makes the electric charge to flow? let us consider the of flow of water. Charges do not flow in a copper wire by themselves, just as water in a perfectly horizontal tube does not flow. If one end in the tube is connected to a tank of water kept at a higher level, such that there is a pressure difference between the two ends of the tube, water flows out of the other end of the tube.
for flow of charges in a conducting metallic wire, the gravity, of course, has no role to play the electrons move only if there is a difference of electric pressure called the potential difference along the conductor. This difference of potential may be produced by a battery consisting of one and more electric cells. The chemical action inside a cell generates the potential difference across the terminals of the cells, even when no current is drawn from it. When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current. In order to maintain the current in a given electric circuit, the cell has to expend its chemical energy stored in it.
We define the electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to another.
The SI unit of electric potential difference is volt (V), named after Alessandro Volta, an Italian physicists. One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one points to the other.
The potential difference is measured by instrument called the voltmeter. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.