Resistivity: Resistivity of any material is defined as amount of resistance shown by that material between two opposite faces of a unit cube of that material. 1) R ∝ L (resistance is proportional to length) 2) R ∝ 1 / A (resistance is inversely proportional to cross section area) Combining both proportionalities gives: R ∝ L / A R = ρ (L / A) ρ = R (A / L) Unit of the resistivity is ohm-meter Temperature coefficient of resistance: Temperature coefficient of resistance of any material is the value that measure of change in electrical resistance of any substance per degree change of temperature. Sl. No. Material/Substances Chemical Symbol/Chemical composition Temperature coefficient of resistance / o C (at 20 o C) 1 Silver Ag 0.0038 2 Copper Cu 0.00386 3 Gold Au 0.0034 4 Aluminum Al 0.00429 5 Tungsten W 0.0045 6 Iron Fe 0.00651 7 Platinum Pt 0.003927 8 Manganin Cu = 84% + Mn = 12% + Ni = 4% 0.0000
Ohm's law: Definition : Temperature and other physical quantity is remain constant the electric current flowing through the conductor is directly proportional to the potential difference between two ends of the conductor. I ∝ V I=GV Where, V is Voltage in volts (V) G is conductance in siemens ( ℧ ) I is Current in Ampere (A) R is Resistance in ohm (Ω) G=1/R So, V=IR Limitation of ohm's law: Ohm’s law is not applicable for unilateral electrical elements like diodes and transistors as they allow the current to flow through in one direction only. For non-linear electrical elements with parameters like capacitance, resistance etc the voltage and current won’t be constant with respect to time making it difficult to use Ohm’s law. Definition of some important terms: 1) Unilateral Circuits This is the particular componen
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