14.2 POTENTIAL DIFFERENCE
When one end A of a conductor is connected to the positive terminal and its other end B is connected to the negative terminal of the battery, then the potential at A becomes higher than the potential at B (Fig.14.6). This causes a potential difference between the two ends of the conductor. The flow of current continues as long as there is a potential difference. The agency which provides the potential difference for the steady flow of current in the copper wire is the battery. As the current flows from higher potential to the lower potential through the conductor, the electrical energy (due to current) is converted into other forms (heat and light etc.).
When current flows through the conductor, it experiences a resistance in the conductor by collisions with atoms of the conductor. The energy supplied by the battery is utilized in overcoming this resistance and is dissipated as heat and other forms of energy. The dissipation of this energy is accounted for by the potential difference across the two ends of the light bulb. Thus
Potential difference across the two ends of a conductor causes the dissipation of electrical energy into other forms of energy as charges flow through the circuit.
SI unit of potential difference is volt. A potential difference of 1 V across a bulb means that each coulomb of charge or 1 ampere of current that passes through the bulb consumes 1 joule of energy. When a bulb is lit, the energy is taken from the current and is transformed into light and heat energy.
14.3 ELECTROMOTIVE FORCE (e.m.f)
A source of electromotive force (e.m.f.) converts non- electrical energy (chemical, thermal, mechanical etc.) into electrical energy. Examples of sources of e.m.f. are batteries, thermocouples and generators. When a conductor is connected to a battery, current flows through it due to potential difference.
For the continuous flow of current through a wire, battery supplies energy to the charges. The positive charge leaves the positive terminal of the battery, passes through the conductor and reaches the negative terminal of the battery. As a positive charge enters the battery at its lower potential point (negative terminal), the battery must supply energy, say to the positive charge to drive it to a point of higher potential i.e., positive terminal. Now we define e.m.f. of the source as: