What is Electrical Resistance? Definition and Units.

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Resistance is the property of a substance due to which it opposes the flow of electrons (current) through it.

It varies from substance to substance, some substance have low resistance like class A Metals like Silver, Copper and Aluminium. These metals offers very less resistance to flow of electricity through them when we applied potential difference(voltage) across them, so these metals are called good conductors of Electricity.

                                    Those substances which offer relatively greater resistance or hindrance to the flow of electrons through them are called poor conductor or one can say Insulators of electricity like bakelite, Mica, glass, rubber P.V.C. (polyvinyl chloride) and wood etc. These substances are bad conductor of electricity and does allow electric charge to flow through them.

Resistance is donated by symbol R.

Units of Resistance   

SI Unit of Resistance is ohm.

A conductor contain one ohm resistance if it permits one ampere current to flow through it when one volt of potential difference is applied across it.

Bigger units of Resistance are Mega-ohm = 10⁶ ohm , Kilo-ohm = 10³ohm used for insulators having high resistance.

Smaller Units are Milli-ohm = 10ˉ³ohm and Micro-ohm=10ˉ⁶ ohm used for good conductors having low resistance.

Symbol used to donate ohm is Ω.

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Resistance Laws:-

Resistance R offered by a conductor depends on:

  1. R is directly proportional to length L of conductor.
  2. R is inversely proportional to cross sectional area A of the conductor.
  3. R depends on the nature of material.
  4. R depends on the nature of material.
  5. R also depends on the temperature of conductor.

Neglecting last two factor we get:-

R = ρΙ/A

Where ρ is constant called specific resistance or resistivity.

l is length of conductor.

A is cross sectional area of conductor.

If length l =1metre and area A= 1 metre² then R =ρ

Units of Resistivity

Since R = ρΙ/A

Then from above equation

ρ=AR/l = A metre² * R ohm/ l metre

ρ = ohm-metre

Hence Unit of resistivity is ohm-metre (Ω-m)

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