Electric Current
Category : JEE Main & Advanced
(1) The time rate of flow of charge through any cross-section is called current. \[i=\underset{\text{ }\!\!\Delta\!\!\text{ }t\to 0}{\mathop{Lim}}\,\frac{\text{ }\!\!\Delta\!\!\text{ }Q}{\text{ }\!\!\Delta\!\!\text{ }t}=\frac{dQ}{dt}\]. If flow is uniform then \[i=\frac{Q}{t}\]. Current is a scalar quantity. It's S.I. unit is ampere (A) and C.G.S. unit is emu and is called biot (Bi), or ab ampere. 1A = (1/10) Bi (ab amp.)
(2) Ampere of current means the flow of \[6.25\times {{10}^{18}}\] electrons/sec through any cross-section of the conductor.
(3) The conventional direction of current is taken to be the direction of flow of positive charge, i.e. field and is opposite to the direction of flow of negative charge as shown below.
(4) The net charge in a current carrying conductor is zero.
(5) For a given conductor current does not change with change in cross-sectional area. In the following figure \[{{i}_{1}}={{i}_{2}}={{i}_{3}}\]
(6) Current due to translatory motion of charge : If n particle each having a charge q, pass through a given area in time t then \[i=\frac{nq}{t}\]
If n particles each having a charge q pass per second per unit area, the current associated with cross-sectional area A is \[i=nqA\]
If there are n particle per unit volume each having a charge q and moving with velocity v, the current thorough, cross section A is \[i=nqvA\]
Types of current
| Alternating current (ac) | Direct current (dc) |
|
(i) Magnitude and direction both varies with time
\[ac\to \text{Rectifier}\to dc\] |
(i) (Pulsating dc)
(Constant dc)
\[dc\to \text{Inverter}\to ac\] |
| (ii) Shows heating effect only | (ii) Shows heating effect, chemical effect and magnetic effect of current |
|
(iii) It's symbol is
|
(iii) It's symbol is
|
(7) Current due to rotatory motion of charge : If a point charge q is moving in a circle of radius r with speed v (frequency n, angular speed w and time period T) then corresponding current \[i=q\nu =\frac{q}{T}=\frac{qv}{\mathbf{2}\pi r}=\frac{q\omega }{\mathbf{2}\pi }\]
(8) Current carriers : The charged particles whose flow in a definite direction constitutes the electric current are called current carriers. In different situation current carriers are different.
(i) Solids : In solid conductors like metals current carriers are free electrons.
(ii) Liquids : In liquids current carriers are positive and negative ions.
(iii) Gases : In gases current carriers are positive ions and free electrons.
(iv) Semi conductor : In semi conductors current carriers are holes and free electrons.
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