Define electric current. What is the direction of electric current? State and define S.I. unit of electric current. What is the nature of electric current? Give types of current.
ELECTRIC CURRENT :
Electric current: Electric current is defined as the amount of electric charges flowing through any cross-section of a conductor per unit time.
Let charge $Q$ crosses through a cross-section of a conductor in time $t$, then
$I = \frac{\text{Total charge flowing (Q)}}{\text{Time taken (t)}}$
$I = \frac{Q}{t} \quad \dots (1)$
If $n$ electrons, each of charge $e$ ($= 1.6 \times 10^{-19}$), cross through a cross-section of a conductor in time $t$, then total charge passing through a cross-section of the conductor is given by, $Q = ne$.
Hence,
$ I = \frac{Q}{t} = \frac{ne}{t} \quad \dots(2) $
Electric current defined by eqn. (1) is known as steady current.
If the net charge $\Delta Q$ crosses the shaded cross-sectional area of the conductor in a time $\Delta t$, then the average electric current in the conductor is given by
$$ I_{av} = \frac{\Delta Q}{\Delta t} \quad \dots(3) $$
If the rate at which charge flows varies in time, then the instantaneous current in the conductor is given by
$$ I(t) = \lim_{\Delta t \to 0} \frac{\Delta Q}{\Delta t}$$
$$ I(t) = \frac{dQ}{dt} \quad \dots(4) $$
Direction of electric current:
The direction of flow of positive charge through a material gives the direction of electric current . This current is known as conventional current .
The direction of flow of electrons ( negative charges ) is opposite to the direction of conventional current
Units of Electric Current:
The S.I. unit of current is ampere (A).
1 ampere (A) = $\frac{1 \text{ coulomb (C)}}{1 \text{ second (s)}} = 1 \text{ C s}^{-1}$
Definition of ampere :
Electric current through a conductor is said to be one ampere if charge of one ampere flows through any cross - section of the conductor in one second .
Smaller units of electric current are : milliampere ( 1 mA = 10$^{-3}$ A ) and microampere ( 1 µA = 10$^{-6}$ A ) .
Nature of Electric Current
Electric current is a scalar quanity . Although electric current has magnitude as well as direction yet it is not a vector quantity . It is because the laws of addition of simple algebra are used to find net electric current in the circuit .
For example :
Let 2A and 3A currents flow through the wires A and B respectively . Then the current in third wire C is 5A ( = 2A + 3A ) .
Thus , electric currents are added algebraically .