NCERT & CBSE Physics class 12 Ohm’s law and resistance Theory
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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”.
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’
ie R ∝l
R ∝ 1/A
Where ? is the resistivity of the material of the conductor.
The resistivity of the material of a conductor:
If we have A=1m2 and l=1m
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
- i) High resistivity
- 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.