Basics:DC Circuits

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Electric Circuit

Figure 1 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.


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.

Electric Potential

Law of conservation of energy states that energy can neither be created nor destroyed. However, its form can be converted from one form of energy to another. In all these cases the function of each of these sources of electric energy is the same in terms of energy and charge i.e., the energy is spent as work for transporting charge from one point to another in a circuit. The movement of charges contribute to current and the amount of work done per unit charge is the potential difference between the two points. The electronic charges flow from a lower potential to a higher potential and these contribute to electronic current, whereas the conventional current is considered to flow from higher potential to lower potential.

If a differential charge dq is given a differential energy dw, the rise in potential of the charge

The units of potential or potential difference as derived from equation are joule/coulomb, watt-sec/amp-sec or watt/amp and usually is termed as volts.