Resistance: R = ρL/A (ρ = resistivity, L = length, A = area)

Conductance G = 1/R, conductivity σ = 1/ρ.

Temperature dependence: R = R₀(1 + αΔT), ρ = ρ₀(1 + αΔT).

Series: R_eq = R₁ + R₂ + ... ; same current, voltage divides.

Parallel: 1/R_eq = 1/R₁ + 1/R₂ + ... ; same voltage, current divides.

Wheatstone bridge: Balanced when P/Q = R/S ⇒ no current through galvanometer.

Used to measure unknown resistance.

Meter bridge: A practical form; balance point l gives unknown R = S × (l/(100–l)).

Comparison of EMFs: E₁/E₂ = l₁/l₂ (balance lengths).

Internal resistance: r = R (l₁/l₂ – 1).

Cells in series: ε_eq = ε₁ + ε₂ + ... , r_eq = r₁ + r₂ + ...

Cells in parallel (identical): ε_eq = ε, r_eq = r/n.

💡 NEET TIPS & SHORTCUTS

• For maximum power transfer, load resistance = internal resistance of source.
• In series combination, the highest resistor gets the most voltage; in parallel, the smallest resistor gets the most current.
• Kirchhoff's laws are fundamental – practice node and loop analysis.
• Potentiometer gives accurate measurement because it uses null method.

⚠️ COMMON MISTAKES

• Using V = IR for non-ohmic devices like semiconductors.
• Forgetting internal resistance when calculating terminal voltage.
• Applying Wheatstone bridge formula without checking balance condition.
• Confusing series and parallel combination of cells.

Ohm's law: V = IR

Resistivity: R = ρL/A

Power: P = VI = I²R = V²/R

Series: R_eq = ΣR

Parallel: 1/R_eq = Σ(1/R)

Wheatstone balance: P/Q = R/S

Potentiometer EMF: E₁/E₂ = l₁/l₂