The algebraic sum of the currents at a junction in a closed circuit is zero. . 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 gotta 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 .