NEB Lecture Notes: DC Circuits

Current Electricity:

The branch of physics which deals with the study of charges in motion is called the current electricity.

Electric Current:

The flow of charge in a definite direction in a conductor is called electric current. The electric current is measured by the flow of charge through any cross section of the conductor in a unit time. i.e., the rate of flow of electric charge.

If q is the steady charge flowing through any cross section of the conductor in time t, then

Electric current (I) = [latex]\frac{q}{t}[/latex]

If the charge flowing through a conductor is not steady but varying with time, i.e. if dq charge flowing in small time dt, then electric current at any time (instant) is instantaneous current and is given by,

Instantaneous current (i) = [latex]\frac{dq}{dt}[/latex]

Unit of Electric Current:

Electric current (I) = [latex]\frac{q}{t}[/latex]

If q = 1 Coulomb (C) and t = 1 second, then [latex]I = \frac{1C}{1S} = 1Cs^{-1} = 1[/latex] ampere (A)

Therefore, the SI unit of current is ampere (A).

One ampere of current is said to flow through a wire if at any cross – section, one coulomb of charge flows in one second.

One coulomb (1C) is equal to the charge of [latex]6.25 \times 10^{18}[/latex] electrons. Thus, 1A of current through a wire means that [latex]6.25 \times 10^{18}[/latex] electrons per second pass any cross – section of the wire.