THEORY OF ELECTRIC CURRENT
ELECTRIC CURRENT through a conductor is defined as the rate of flow of electric charge through any cross-section of that conductor.
The direction opposite to the motion of electrons is taken as the direction of conventional current, even though charge is carried across due to the flow of electrons. Hence the direction of electronic current is opposite to that of conventional current.
SI unit: 1 ampere = 1 Coulomb / second
Definition: Current through a conductor is 1 ampere, if 1 coulomb of charge flows through it in 1 sec.
Ohm’s law: It states that, physical conditions remaining unchanged, the current through a conductor is directly proportional to the potential difference across its two ends.
Cause of resistance: When a PD is applied across the ends of a conductor, free electrons are accelerated opposite to the direction of the electric field. As they move, they collide against the atoms and hence their motion is opposed due to which they are slowed down. This opposition of a conductor to the flow of charge through it is the cause of electrical resistance.
RESISTANCE of a conductor is said to be 1 ohm, if 1 ampere of current flows through it when a potential difference of 1 volt is applied across its ends.
RESISTIVITY of the material of a conductor is the resistance offered by a wire made of that material having unit length and unit area of cross-section.
COLOUR CODE OF CARBON RESISTORS
A carbon resistor usually has four rings or bands on it. They are of different colours and are denoted by A,B,C and D. The colour of the first two bands A and B give the first two significant digits of resistance in ohm. The colour of the third band C gives the decimal multiplier. The colour of the fourth band D gives its tolerance.
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Black Brown Red Orange Yellow Green Blue Violet Grey White
0 1 2 3 4 5 6 7 8 9
CURRENT DENSITY in a current carrying conductor is defined as the current flowing through unit area of it when the area is held normally to the direction of current. It is a vector quantity.
MOBILITY is defined as the drift velocity per unit electric field.
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