Physics XII: Current Electricity

Chapter 03: Current Electricity

Practice Current Electricity Class 12 MCQs. Cover Ohm's law, Kirchhoff's rules, resistivity, emf, and circuits with solved questions for exam success.

Score:
0
Time:
05:00

Quick Revision: Electric Charges and Field

  • Electric Current: I = Q/t; rate of flow of charge with SI unit ampere (A).
  • Current Density: J = I/A; current per unit area with SI unit A/m².
  • Drift Velocity: v_d = (eEτ)/m; average velocity of electrons in conductor.
  • Ohm's Law: V = IR; voltage directly proportional to current for constant temperature.
  • Resistance: R = V/I; opposition to current flow with SI unit ohm (Ω).
  • Resistivity: ρ = RA/l; material property independent of dimensions with SI unit Ω·m.
  • Conductivity: σ = 1/ρ; reciprocal of resistivity with SI unit siemens/m (S/m).
  • Temperature Dependence: R_T = R_0[1 + α(T - T_0)]; α is temperature coefficient.
  • Resistors in Series: R_eq = R₁ + R₂ + ...; current same, voltage divides.
  • Resistors in Parallel: 1/R_eq = 1/R₁ + 1/R₂ + ...; voltage same, current divides.
  • Kirchhoff's Current Law (KCL): ΣI_in = ΣI_out; conservation of charge at junction.
  • Kirchhoff's Voltage Law (KVL): ΣV = 0 in closed loop; conservation of energy.
  • Wheatstone Bridge: Balanced when P/Q = R/S; used for precise resistance measurement.
  • Meter Bridge: Based on Wheatstone principle; uses null point for resistance comparison.
  • Potentiometer: Measures emf without drawing current; more accurate than voltmeter.
  • EMF of Cell: Maximum potential difference when no current flows; SI unit volt (V).
  • Internal Resistance: r = (E - V)/I; opposition within cell to current flow.
  • Electrical Power: P = VI = I²R = V²/R; rate of energy consumption with SI unit watt (W).
  • Joule's Law: H = I²Rt; heat produced in conductor with resistance.

Chapter Summary:

Current Electricity deals with the motion of electric charges in conductors and the associated phenomena. The chapter begins with fundamental concepts of electric current as the flow of charge carriers, introducing drift velocity and its relationship with current density. The microscopic origin of resistance is explained through electron scattering in conductors.

Ohm's Law forms the cornerstone of this chapter, establishing the linear relationship between voltage and current for ohmic conductors. The concepts of resistivity and conductivity are introduced as material properties that determine how well a substance conducts electricity. Temperature effects on resistance are analyzed, distinguishing between conductors, semiconductors, and insulators based on their temperature coefficients.

Circuit analysis techniques include series and parallel combinations of resistors, with detailed coverage of Kirchhoff's laws for complex circuits. Practical applications like Wheatstone bridge for precise resistance measurement, meter bridge for comparison of resistances, and potentiometer for accurate emf measurement are thoroughly explained. The chapter also covers electrical energy, power calculations, and heating effects described by Joule's law.

This chapter provides the essential foundation for understanding DC circuits, electrical measurements, and power distribution systems, connecting theoretical concepts with practical electrical engineering applications.