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Friday, 3 March 2017

Current Electricity - II


Resistivity:

Resistivity r is defined to be reciprocal of conductivity
It is measured in ohm-metre ( Qm) .

Resistivity as a function of temperature:

It is given as,

Where a is the temperature coefficient of resistivity and rT is the resistivity of the material at temperature T.

Ranges of Resistivity:

a) Metals have low resistivity: Range of r  varies from 10–8 Ω m to 10–6 Ω m.

b) Insulators like glass and rubber have high resistivity: Range of r  varies from 1022 to
1024 times greater than that of metals.

c) Semiconductors like Si and Ge lie roughly in the middle range of resistivity on a logarithmic scale.

Ranges of Resistivity:


a) Metals have low resistivity: Range of r varies from 10–8 Ω m to 10–6 Ω m.

b) Insulators like glass and rubber have high resistivity: Range of r varies from 1022 to
1024

c) Semiconductors like Si and Ge lie roughly in the middle range of resistivity on a logarithmic scale.

Total resistance in Series and in Parallel

(a) Total resistance R of n resistors connected in series is given by R = R1 + R2 + … + Rn

(b) Total resistance R of n resistors connected in parallel is given by

If the mass of a charge carrier is large, then for a given field
 , its acceleration will be small and will contribute very little to the electric current.


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