Definition : The time rate of low of charge through any cross-section is called current. 
. If flow is uniform then i = Q/t. Current is a scalar value. Its 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.)
Ampere: of current means the flow of 6.25*108 electrons/sec through any cross-section of the conductor.
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The conventional direction of current is taken to be the direction of flow of positive charged particle, i.e. field and is opposite to the direction of flow of negative charge as shown below.
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The total charge in a current carrying conductor is zero.
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For a given conductor current does not change with change in cross-sectional area. In the given diagram
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Current due to translatory motion of charge: If n particle each occupy a charge q, goes through a given area in time t then
If n particles each having a charge q pass per second per unit area, the current related with cross-sectional area A is i = nqA
If there are n atoms per unit volume each occupy a charge q and moving with velocity v, the current thorough, cross-section A is i = nqvA.
Types of current
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Alternating Current (ac)
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Direct Current (dc)
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(i) Magnitude and direction both changes with time
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(i)
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(ii) Shows heating effect only
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(ii) Shows chemical effect, heating effect and magnetic effect of current
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(iii) It's symbol is
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(iii) It's symbol is
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Current due to rotatory motion of charge: If a point charge q is moving in a circle of radius r with speed v then related current
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Current carriers: The charged particles whose flow in a definite direction constitutes the electric current are called current carriers. In separate positions current carriers are different.
(i) Solids: In metal conductors current carriers are free electrons.
(ii) Liquids: In liquids current goes are positive and negative ions.
(iii) Gases: In gases current takes are positive ions and free electrons.
(iv) Semi conductor: In semi conductors current occupy are holes and free electrons.
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