Definition of Electric Current :
Electric current is defined as the amount of electric charge flowing through any cross-section of a Conductor per unit time.
I = Q/t
If n electrons , each of charge e (= 1.6×10-19) 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 = Q/t = ne/t
If the net charge ∆Q crosses the shaded cross - sectional area in a time ∆t , then the instantaneous current across the conductor is given by
I = lim ∆Q / ∆t = dQ/dt
The charge that passes through the given cross-sectional area in a time interval from 0 to t is given by
Q = dQ = I(t)dt
Where ,I varies with time t.
Direction of electric current :
The direction of flow of positive charge gives the direction of electric current. This current is known as conventional current.
However , in a conductor , the flow of electrons ( negative charge ) constitute electric current. The direction of flow of electrons is opposite to the direction of conventional Current.
Units of Electric current :
The S.I unit of current is Ampere (A).
1 ampere = 1 coulomb(C)/ 1 second(s)
= 1 C /s
Definition of 1 ampere :
Electric current through a conductor is said to be one ampere if charge of one flows through any cross - section of the conductor in one second.
Commonly used smaller units of electric current are :
milliampere ( 1 mA = 10-3 A ) and microampere (1 uA = 10-6 A)
Note :
- The current in human nerve is of the order of microampere (uA).
- Average current during lightning is of the order of tens of thousands of ampere.
- Passage of electric current more than 0.015 A through human body can be fatal.
Nature of Electric current :
Electric current is a Scalar quantity. Although electric current has magnitude as well as direction yet it is not a vector quantity. It does not require vector laws and the laws of addition of simple algebra are used to find net electric current.
Example : Comsider three metallic wires A, B and C meeting at a point or junction O . Let 2 A and 3 A currents flow through the wires A and B respectively. Then the current in third wire C is 5 A ( = 2 A + 3 A ) .
Type of Electric Current :
Different types of electric current are
(1) Steady Direct current:
An electric current is said to be steady ( i.e constant or regular ) direct current if it's magnitude and direction do not change with time.
(2) Varying or variable Direct current :
An electric current is said to be varying ( i.e changing ) direct current if it's magnitude changes with time and polarity remains same.
(3) Alternating Current ( A.C) = An electric current is said to be alternating current if it's magnitude changes with time and polarity (i.e + or - ) reverse periodically