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Ch -5 Electric Resistance and Ohm's Law, Important stuff - Coggle…
Ch -5 Electric Resistance and Ohm's Law
Current density
At a point of a current carrying conductor, current desity is defined as the current flowing through unit area of cross section drawn through the point perpendicular to the flow of current
Represented by j
Its SI unit is A m-2
Mean free path
The average distance travelled by an electron between two succesive collisions is called mean free path of the electron.
Relaxation time
The time interval between two succesive collisions is called Relaxation time of the electron.
Drift velocity
Drift velocity is defined as the average velocity with which the free electrons drift towards the positive end of the conductor under the influence of an externally applied electric field.
Mobility of electrons
The mobility of an electron is numerically equal to the magnitude of drift velocity imparted by a uniform electric field of 1 V m-1
Its SI unit is m2 V-1 s-1
Ohm's law
If there is no change in the physical state of a conductor, the ratio of the potential difference applied across its ends and the current flowing through it is constant.
Exceptions
No measurable current in the beginning. (In water voltmeter, measurable current staart to flow only after a certain voltage is applied.)
For vaccuum tubes, p-n junction diodes, thermistors, transistors and thermionic valves, Ohm's law does not hold even for small currents.
Change in nature due to flow of current(Change in temperature)
Specific resistance ( ρ )
The ratio of the intensity of electric field E at any point within the coductor and the current-density at thet point is called specific resistance of of the conductor.
Effect of temperature on resistors
On Alloys : Temperature
↑
Resistance
↑
(lesser)
On Metals : Temperature
↑
Resistance
↑
On Electrolytes : Temperature
↑
Resistance
↓
On Semiconductors : Temperature
↑
Resistance
↓
Important stuff
Derivations
Derivation of Ohm's law
Series and paralell combinations of resistances
Drift velocity (Mean Free Path)