No current flows in a copper wire by itself, just as water in a horizontal tube does not flow. If one end of the tube is connected to a tank with water such that there is a pressure difference between the two ends of the horizontal tube, water flows out of the other end at a steady rate. The rate at which water flows out depends on the pressure difference, for a given tube. If the flow rate (current) is to be kept constant, the water flowing out for instance has to be put back into the jar to maintain the pressure head. This requires work to be done by an external agency. The above analogy brings out several features of electrical current flow. An electric current flows across a conductor only if there is an electric potential difference between its two ends. To maintain a steady current flow, one needs an agency, which does work on the charges. This agency is called the electromotive force or emf.
In the case of water flow, the agency is the pump 'P' which does work at a steady rate in putting the water back into the tank. Just as for a given tube, the current of fluid flowing out depends on its viscosity, the electrical current flowing for a given potential difference depends on the electrical resistance of the conductor.
The above circuit diagram shows the flow of charges. A steady electric current 'I' flows through the resistance R, from A to B. That is, positive charges flow from higher potential (A) to lower potential (B). The potential drop from A and B is V. The source P of emf does work on these charges, as they come through at B because it has to take the positive charges from lower to higher potential. The charge is transferred from one end of the source of emf to the other and 'qV' work is done on the charge. The source of emf by doing work on the electric charges, maintains a potential difference V between its terminals.
