Summary
NCERT Class 12 Physics Chapter 3, Current Electricity, covers the flow of electric charges through conductors, Ohm's law (V = RI), resistivity, drift velocity of electrons, Kirchhoff's rules, and circuit analysis tools such as the Wheatstone bridge.
Chapter 3 of NCERT Class 12 Physics Part I introduces the fundamentals of steady electric current. It explains how moving charges constitute current (I = Q/t), how free electrons in metallic conductors drift under an applied electric field, and derives Ohm's law (V = RI) from first principles. The chapter covers resistivity and its temperature dependence, electrical power dissipation (P = IV = I²R = V²/R), EMF and internal resistance of cells, series and parallel combinations of cells, Kirchhoff's junction and loop rules, and the Wheatstone bridge as a method for measuring unknown resistances.
Key points & formulas
- 01Electric current is defined as the net charge flowing per unit time across a cross-section: I = Q/t (steady) or I = lim(ΔQ/Δt) as Δt→0 (instantaneous).
- 02Ohm's law states V = RI, where resistance R = ρl/A; resistivity ρ depends on material and temperature but not on the conductor's dimensions.
- 03Drift velocity of electrons under an electric field E is vd = –eEτ/m, where τ is the relaxation (average collision) time, giving conductivity σ = ne²τ/m.
- 04Resistivity of metals increases with temperature (ρT = ρ0[1 + α(T – T0)]), while resistivity of semiconductors decreases with increasing temperature.
- 05For a cell of EMF ε and internal resistance r connected to external resistance R, current I = ε/(R + r) and terminal voltage V = ε – Ir.
- 06Kirchhoff's rules — Junction Rule (sum of currents entering a junction equals sum leaving) and Loop Rule (algebraic sum of potential changes around any closed loop is zero) — enable analysis of complex circuits; the Wheatstone bridge balance condition is R1/R2 = R3/R4.
Frequently asked questions
01What is the drift velocity of electrons and why is it so small yet currents are large?
Drift velocity is the small net average velocity (vd = eEτ/m) that electrons acquire opposite to the electric field, superposed on their large random thermal velocities. It is typically ~10⁻³ m/s for ordinary currents, yet currents can be large because the free electron number density in metals is enormous (~10²⁸–10²⁹ per m³), so even a tiny drift moves a great deal of charge per second.
02What is the difference between EMF and terminal voltage of a cell?
EMF (ε) is the potential difference between the positive and negative terminals of a cell when no current flows (open circuit). Terminal voltage V = ε – Ir is the actual voltage across the terminals when current I flows, reduced by the voltage drop Ir across the cell's internal resistance r.
03Under what conditions does Ohm's law fail?
Ohm's law fails when (a) V is not proportional to I (e.g., resistance increases with current), (b) the V–I relationship depends on the sign of V, as in a diode (rectifier), or (c) the relationship between V and I is non-unique, as seen in GaAs where multiple voltage values correspond to the same current.
04Is the NCERT Class 12 Physics Chapter 3 PDF free to download?
Yes, the NCERT Class 12 Physics Part I Chapter 3 PDF is completely free to download on cbseprepmaster.com.
More chapters in Physics Part I
This is the complete Physics Part I Chapter 3 as published by NCERT — every diagram, solved example, and exercise included, free. Browse all CBSE Class 12 textbooks.
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