According to Ohm's Law the potential difference across a conductor is directly proportional to the electric current , provided the temperature is kept constant. Mathematically ohm's law can be written ad V proportional to I , at a constant temperature.The constant of proportionality or the voltage to current ratio is termed resistance of the conductor.resistance is a scalar quantity, Its SI unit is ohm.
Limitation of Ohm's Law
If the above experiment is performed using a nichrome wire and for very large current , the graph will not be a straight line. Similarly If a diode is used then the graph obtained will be parabolic and when the polarity of cell is interchanged , practically no current flows through the diode. Thus Ohm's law is not a universal law.
Conductors which obey Ohm's law are known as ohmic conductors and those which do not obey Ohm's law are known as non ohmic conductors All metallic conductors are ohmic conductors. Example for non ohmic conductors are semiconductors.
Factors Affecting Resistance of Resistor
A given material resistance is directly proportional to the length and inversely proportional to the area of cross-section R=p(L/A).
Where p is the constant of proportionality called the resistivity of the material.
Resistivity of a Material
Resistivity or coefficient of specific resistance is defined as the resistance offered by a resistor of unit length and unit area of cross-section.
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