Basics:DC Circuits
Electric Circuit
An electric circuit is a closed path consisting of active and passive elements all interconnected and the current flow is confined to the closed path. Fig. l shows a typical circuit consisting of one active and two passive elements. An active element is one which supplies energy to the circuit where as a passive element is one which receives energy and then this energy is converted into heat (resistor) or stores it in an electric (capacitor) or magnetic field (inductor). The battery is the active element in Fig. l.
Electric Current
The concept of charge is based on atomic theory. An atom has positive charges (protons) in its nucleus and an equal number of electrons (negative charges) surround the nucleus making the atom neutral. Removal of an electron leaves the atom positive charged and addition of an elec tron makes the atom negatively charged. The basic unit of charge is the charge on an electron. The mks unit of charge is coulomb. An electron has a charge of 1.062 x l0-19 C.
When a charge is transferred from one point in the circuit to another point is constitutes what is known as electric current. An electric current is defined as the time rate of flow of charge through a certain section. Its unit is ampere. A current is said to be of one ampere when a charge of 1 coulomb flows through a section per second.
Mathematically,
If charge q is expressed in coulomb and time in second, 1 amp flow of current through a section is equivalent to approx. flow of 6.24 x 1018 electrons per second through the section.
Yet another method of defining electric current (1 amp) is as the constant electric current in two infinite parallel conductors separated from each other by 1 m, experience a force of 2 x 10-9 N/m.
