Charge and Current




The concept of electric charge is the underlying principle for explaining all electrical phenomena. Also, the most basic quantity in an electric circuit is the electric charge. We all experience the effect of electric charge when we try to remove our wool sweater and have it stick to  our body or walk across a carpet and receive a shock.

Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C).

We know from elementary physics that all matter is made of fundamental building blocks known as atoms and that each atom consists of electrons, protons, and neutrons. We also know that the charge e on an electron is negative and equal in magnitude to C, while a proton carries a positive charge of the same magnitude as the electron. The presence ofequal numbers of protons and electrons leaves an atom neutrally charged.
The following points should be noted about electric charge:

  1. The coulomb is a large unit for charges. In 1 C of charge, there are: electrones.  Thus realistic or laboratory values of charges are on the order of pC, nC, or µC

  2. According to experimental observations, the only charges that occur in nature are integral multiples of the electronic charge 
  3. The law of conservation of charge states that charge can neither be created nor destroyed, only transferred. Thus the algebraic sum of the electric charges in a system does not change.  

We now consider the flow of electric charges. A unique feature of
electric charge or electricity is the fact that it is mobile; that is, it can
be transferred from one place to another, where it can be converted to another form of energy.
When a conducting wire (consisting of several atoms) is connected to a battery (a source of electromotive force), the charges are
compelled to move; positive charges move in one direction while negative charges move in the opposite direction. This motion of charges creates electric current. It is conventional to take the current flow as the movement of positive charges. That is, opposite to the flow of negative charges, as Fig. 1.3 illustrates. This convention was introduced by Benjamin Franklin (1706–1790), the American scientist and inventor. Although we now know that current in metallic conductors is due to negatively charged electrons, we will follow the universally accepted convention that current is the net flow of positive charges. Thus,  

Electric current is the time rate of change of charge, measured in amperes (A).


Mathematically, the relationship between current i, charge q, and time t is :


where current is measured in amperes (A), and

 1 ampere = 1 coulomb/second


The charge transferred between time and t is obtained by integrating both sides of Eq. (1.1). We obtain 

The way we define current as i in Eq. (1.1) suggests that current need
not be a constant-valued function. As many of the examples and problems in this chapter and subsequent chapters suggest, there can be several types of current; that is, charge can vary with time in several ways. If the current does not change with time, but remains constant, we call it a direct current (dc).

A direct current (dc) is a current that remains constant with time.

By convention the symbol I is used to represent such a constant current. A time-varying current is represented by the symbol i. A common form of time-varying current is the sinusoidal current or alternating current (ac).

An alternating current (ac) is a current that varies sinusoidally with time.

Such current is used in your household to run the air conditioner, refrigerator, washing machine, and other electric appliances. Figure 1.4