The rate of flow of charge per unit time is called electric current. The materials having large number of free electrons develop strong electric currents, when an electric field is applied. They are called as conductors. In solid conductors, large numbers of free electrons are responsible for electric current. In liquid conductors, the positively and negatively charged ions are responsible for electric current.
Conventional current is the current in which the direction of flow of positive charge gives the direction of current. Practically, the direction of flow of electrons gives the direction of electronic current. The direction of electrons is opposite to that of conventional current.
The free electrons attain a velocity in the presence of an electric filed called the drift velocity. Drift velocity is defined as the average velocity with which the free electrons get drifted towards the positive end of the conductor under the influence of applied external electric field. The magnitude of the drift velocity per unit electric field is termed as mobility and the current flowing per unit area is called current density.
Ohm’s law states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) across the ends of the conductor, provided physical conditions of the conductor such as temperature, mechanical strain etc. are kept constant. The graph plotted for Ohm’s law, between potential difference and current through a conductor, is a straight line. Non-ohmic conductors are the substances that do not obey Ohm’s law.
On the basis of resistance offered to the flow of current, substances are classified as conductors, insulators and semi-conductors. The resistance of the conductor depends on the length and area of the conductor. For a unit length and unit area, the resistance is defined as the resistivity of the material. Resistivity is temperature dependent and is different for different materials.
Electric power is the rate at which electric work is done by the source of e.m.f in maintaining the current in an electric circuit. The total electric work done or energy supplied by the source of e.m.f in maintaining the current in an electric circuit for a given time is the electric energy consumed in the circuit.
Resistances and cells are connected in either series or in parallel or in combination of both in a circuit. Simple circuits are easy to analyse but for complex circuits Kirchhoff’s junction and loop rules are very useful.
Wheat stone bridge is an application of Kirchhoff’s rules. Wheatstone bridge and its balance condition provide a practical method for determination of an unknown resistance. Meter bridge or slide wire bridge is a practical form of Wheatstone bridge.
An instrument called potentiometer is used for measuring the potential (voltage) in a circuit. The measuring method involves a condition of no current flow. It is also used to measure internal resistance of a cell and compare emf’s of two sources.