Electricity – Current Electricity – Basics
* Electricity current is uniform flow of charges in a particular direction.
* The rate of flow of charge o uniform motion of electrons:
(I = Q/t)
* Potential difference is the work done in moving a unit charge between two points in an electric field against the direction of force:
(v = W/Q)
* Electric resistance is the opposition to the flow of electrons by the conductor.
Factors affecting electric resistance are:
* The length of the conductor
* The cross sectional area of the conductor
* The resistivity of conductor material
* The temperature of the conductor
Thus, ( R = p (1/A))
* Resistivity of a material is the resistance offered by a conductor having unit length and unit area of cross section.
Ohm’s law states that the current passing through a conductor is :
* Direct proportional to the voltage in the conductor and
* Inversely proportional to the resistance of the conductor.
Thus, I = v/r
Electricity – Current Electricity – Circuits
In parallel circuits:
V = V1 = V2 = V3
Itotal = I1 + I2 + I3
1/Req = 1/R1 + 1/R2 + 1/R3
Verification of OHM’s Law
* Ohm’s law can be experimentally verified by plotting a V-I graph.
* Conductors that obey Ohmic law are called ohmic resistance or ohmic conductors or linear conductors.
* Conductors that do not obey ohms are called non – ohmic conductors or linear conductors.
* At a given constant temperature, the resistance of a conductor is directly proportional to its length and inversely proportional to its cross- sectional area.
* The resistivity or specific resistance of a material is the resistance of a conductor is a wire of that material of unit length and unit cross- sectional area is represented by ρ.
* Resistivity depends on the nature of the material and the temperature of the conductor.
* Conductivity is the reciprocal of resistivity. It is represented by σ.
Electromotive Force
* The flow of positive charges from the positive terminal to the negative terminal of a cell through an external circuit is termed as electric current.
* Positive charges move through the external circuit from higher potential point to lower potential point.
* The amount of energy required to drive a unit positive charge through the external circuit is called electromotive force, EMF.
* When no current id drawn from a cell, the potential difference across its terminals is equal to its EMF
* The resistance offered by a cell in bringing a charge from lower to higher potential within itself is called its internal resistance.
* The internal resistance of a cell depends on the area of the electrodes, the distance between them, and the nature, concentration and temperature of the electrolyte.