NCERT & CBSE Physics class 12 Ohm’s law and resistance Theory

NCERT & CBSE Physics class 12 Ohm’s law and resistance Theory

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Ohm’s Law

Ohm’s law:  It states that “the current flowing through a conductor is directly proportional to the 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 a potential difference, I is current and R is the 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 the resistivity of the material of the conductor.

The resistivity of the material of a conductor:

?=RA/l

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 Solutions for Class 12 Physics .

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

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.

Ques: Why copper is used for making connecting wires?

Ans: BecauseCopper has low resistivity.

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

Ans: BecauseNichrome has

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

Ques: Why aluminum 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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