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CBSE Physics class 12 Ohm’s Law and Resistance (Theory)

Physics class 12 Ohm's law and resistance,12th physics

CBSE Physics class 12 Ohm’s Law and Resistance

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Ohm’s Law

Ohm’s law:  It states that “the current flowing through a conductor is directly proportional to potential difference between the ends of the conductor provided physical conditions like temperature, pressure etc.  remain the same”.

i.e.   VαI

or V = IR

Here V is apotential difference,I is current and R is Resistance of the conductor.

Resistance: It is the ability of a conductor to oppose the flow of electric current.

SI unit of resistance is ohm.

Factors on which resistance of a conductor depends:

  • Nature of the material of the conductor
  • Length of the conductor ‘l’
  • Area of cross-section ‘A’
  • Temperature

ie R ∝l

R ∝ 1/A

R= ?l/A

Where ? is resistivity of the material of the conductor.

What to study thesame topic in Hindi? click on Physics in Hindi

The resistivity of thematerial of a conductor:


If we have A=1m2 and l=1m

So ?=R

The resistivity of the material of a conductor is defined as the resistance of the conductor having unit length and unit area of cross section. S.I. unit is ohm-meter  (Ω – m).

For the practice of Current Electricity problems & more,  NCERT Physics Class 12.

Ohmic conductors: Ohmic conductors are the conductors which obey ohms law and for which V-I graph is linear.Eg: Metallic conductor.

Physics class 12 Ohm's Law and resistance

Non-Ohmic conductors are the conductors which do not obey Ohm’s law and for which V-I graph is nonlinear.Eg:junction diodes and transistors.

class 12 Ohm's Law

Ques:Why copper is used as for making connecting wires?

Ans:BecauseCopper has low resistivity.

Ques:Why Nichrome is used as aheating element of electrical devices?

Ans: BecauseNichrome has

  1. i) High resistivity
  2. ii) High melting point.

Ques:Why aluminium wires are preferred for overhead power cables?

Ans: Aluminium has low resistivity. It is cheaper and lighter.

Ques:Ohm’s law is not a universal law. Explain.

Ans: All materials do not obey Ohm’s law. Metals obey ohms law while semiconductors, electrolytes, diodes etc. do not obey Ohm’s law. So Ohm’s law is not a universal law.

Click on Physics notes Class 12 for notes on various chapters and NCERT Solutions.

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