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

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

Takshila Learning provides online CBSE exams coaching to the students so that they can easily score good marks in any CBSE or State Board exams. We always consider the latest pattern of question papers and syllabus, before designing the study material under the guidance of our best faculties. Here, we will discuss the topic ‘Ohm’s Law’ of CBSE Class 12 Physics. We offer a complete package that includes Class 12 Physics, Biology, and Chemistry video lectures, a question bank, virtual experiments, NCERT solutions, and sample papers with performance analysis.

## 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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August 22, 2019