Charges in motion constitute electric current. This chapter will introduce you to current electricity and related phenomena such as conventional current, Ohm's law, electric power, Joule's heating effect, hazards of electricity and safety measures. We will also learn how current or voltage is measured in a circuit by electrical devices.
14.1 ELECTRIC CURRENT
Most of the electric charge around us is bound in neutral atoms. It is not easy to overcome the electrostatic force of attraction between the nuclei and electrons in an atom. However, in metals some electrons are not tightly bound to nuclei and are free to move around randomly. They have weak force between them and the nucleus. Similarly, in solutions some positive and negative charges can freely move around randomly. When such free charges are exposed to an external electric field, they move in a specific direction, and thus constitute current.
Electric current is produced due to the flow of either positive charge or negative charge or both of charges at the same time. In metals, the current is produced only due to the flow of free electrons i.e., negative charges. In case of electrolyte its molecules in aqueous solution dissociate among positive and negative ions. So the current in electrolyte is produced due to the flow of both positive and negative charges.
The rate of flow of electric charge through any cross-sectional area is called current.
SI unit of current is ampere (A). If a charge of one coulomb passes through a cross- sectional area in one second, then current is one ampere. Smaller Units of current are milli ampere (mA), micro ampere (μA), which are defined below as:
Battery is one of the sources of current. The electrochemical reaction inside a battery separates positive and negative electric charges (Fig.14.1). This separation of charges sets up potential difference between the terminals of the battery. When we connect a conducting wire across the terminals of the battery, the charges can move from one terminal to the other due to the potential difference. The chemical energy of the battery changes to electrical potential energy. The electrical potential energy decreases as the charges move around the circuit. This electrical potential energy can be converted to other useful forms of energy (heat, light, sound etc.). It is only the energy which changes form but the number of charge carriers and the charge on each carrier always remains the same (i.e., charge carriers are not used up). Instead of electrical potential energy we use the term electric potential which is potential energy per unit charge.