Current Electricity | Class 12 Physics Notes
By ConceptScroll Team · Published on 17 July 2026 · 2 min read
Current Electricity – this guide gives you a concise, exam-ready overview of Current Electricity from Class 12 Physics, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
3.4 OHM'S LAW
Ohm's law, discovered by Georg Simon Ohm in 1828, states that the potential difference V across a conductor is directly proportional to the current I flowing through it, expressed as V = R × I, where R is the resistance of the conductor. Resistance depends on the material and the dimensions of the conductor. Experimentally, resistance is found to be proportional to the length l and inversely proportional to the cross-sectional area A of the conductor, leading to the relation R = ρ (l/A), where ρ is the resistivity, a material property independent of dimensions. The current density j is defined as current per unit area (I/A), and the electric field E inside the conductor relates to the potential difference by V = E × l. Combining these, Ohm's law can be expressed in vector form as E = ρ j or equivalently j = σ E, where σ = 1/ρ is the conductivity. This formulation connects macroscopic quantities like voltage and current to microscopic properties of materials.
📊 Diagram: Figures 3.2(a), (b), and (c) illustrate the dependence of resistance on length and cross-sectional area using rectangular slabs of conductor.
🔗 Connection: This section leads to the microscopic explanation of resistivity and drift velocity in the next section.
Frequently asked questions
One coulomb per second is equal to
1 ampere
Heat sensitive resistors are called
Thermistors
The resistance of a wire is 10 ohm. What will be the new resistance, if it is stretched uniformly 8 times its original length?
640 ohm
There are n number of resistors with resistance as R ohm. What is the ratio of effective resistances of series and parallel combinations?
n 2 :1
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