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Current Electricity

Ohm's law, Kirchhoff's laws, Wheatstone bridge

Key Concepts
  • Ohm's law: V = IR (valid for ohmic conductors at constant temperature)
  • Resistivity depends on material and temperature; ρ = ρ₀(1 + αΔT)
  • Kirchhoff's current law (KCL): sum of currents at junction = 0
  • Kirchhoff's voltage law (KVL): sum of EMFs = sum of IR drops in a loop
  • Wheatstone bridge: balanced when R₁/R₂ = R₃/R₄
Important Formulae
Ohm's law V = IR
Resistance R = ρL/A
Power P = VI = I²R = V²/R
EMF with internal resistance V = E − Ir
Resistors in series R_s = R₁ + R₂ + ...
Resistors in parallel 1/R_p = 1/R₁ + 1/R₂ + ...
Quick Tips
  • Maximum power transferred to external resistance when R_ext = r (internal resistance).
  • Drift velocity: v_d = I/(nAe); typically very small (~mm/s) but electrons signal at ~c.
  • Colour code for resistors: BBROYGBVGW (0–9): Black Brown Red Orange Yellow Green Blue Violet Grey White.
Sample Practice Questions

  1. Cells connected in series: total EMF and internal resistance:

    • EMF adds, resistance adds
    • EMF averages, resistance adds
    • EMF adds, resistance averages
    • Both average
    Show answer

    Answer: EMF adds, resistance adds

  2. Kirchhoff's voltage law states that algebraic sum of all voltages in a closed loop:

    • Is maximum
    • Is zero
    • Equals current
    • Equals resistance
    Show answer

    Answer: Is zero

  3. SI unit of resistivity:

    • Ω
    • Ω/m
    • Ω·m
    • Ω·m²
    Show answer

    Answer: Ω·m

  4. Power dissipated in a resistor R with current I:

    • IR
    • I²R
    • IR²
    • I/R
    Show answer

    Answer: I²R

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Practice Questions

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