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Resistance

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The opposition offered by a substance to the flow of electric current through them is called resistance
All materials oppose the flow of electric current through them i,e they have resistance
Since the current is the flow of free electrons, resistance is the opposition offered by the substance to the flow of free electrons.this opposition occures because atoms and molecules of the substance obstruct the flow of these electrons .certain substances  offer very little opposition to the flow of electric current are called conductors e,g copper ,aluminium ,silver etc
On the otherhand substances which offers high opposition to the flow of electric current  are called insulators e,g glass, rubver mica etc
The resistance is the friction offered by the substance and causes production of heat with the flow of current.the moving electrons collide with the atoms or molecules of the substance, each collision resulting in the liberationof minute quantity of heat
The unit of resistance is ohm
      1ohm=1v/1A
a wire is said to be a resistance of 1 ohm if a p.d of 1 v across its ends  causes 1 ampere to the flow of current through it
FACTORS ON WHICH RESISTANCE DEPENDS
1,the resistance of a conductor depends upon the  nature of  material
2,it depends upon the temperature
 Resistance of conductor is directly proportional to its length i,e
R directly proportional to L
And is inversely proportional to its area of cross section

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