Current Electricity Revision Notes for NEET Physics PDF Download
⚡ CURRENT ELECTRICITY · Circuits & Measurements
Ohm's law • Resistance • Kirchhoff's laws • Wheatstone bridge • Potentiometer • Cells & EMF • NEET problems
📏 OHM'S LAW & RESISTANCE
Ohm's law: V = IR (for ohmic conductors).
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).
📈 V-I characteristic for ohmic & non-ohmic
🔗 RESISTOR COMBINATIONS
Series: Req = R₁ + R₂ + ... ; same current, voltage divides.
Parallel: 1/Req = 1/R₁ + 1/R₂ + ... ; same voltage, current divides.
🔌 Series & parallel circuits
📜 KIRCHHOFF'S LAWS
KCL (Junction Law)
Σ Iin = Σ Iout (charge conservation).
KVL (Loop Law)
Σ V = 0 around any closed loop (energy conservation).
Sign convention: traverse loop, + if moving from – to + of battery, –IR if current in direction of traversal.
⚖️ WHEATSTONE BRIDGE & METER BRIDGE
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)).
🖍️ Wheatstone bridge circuit
🎚️ POTENTIOMETER
Used to compare EMFs, measure internal resistance, and measure potential difference without drawing current.
Comparison of EMFs: E₁/E₂ = l₁/l₂ (balance lengths).
Internal resistance: r = R (l₁/l₂ – 1).
🔋 CELLS & INTERNAL RESISTANCE
EMF (ε): maximum potential difference when no current flows.
Terminal voltage: V = ε – Ir (when discharging), V = ε + Ir (when charging).
Cells in series: εeq = ε₁ + ε₂ + ... , req = r₁ + r₂ + ...
Cells in parallel (identical): εeq = ε, req = r/n.
💡 ELECTRICAL POWER & HEATING
Power: P = VI = I²R = V²/R (watts). Energy: E = Pt (joules).
Heating effect: Joule's law H = I²Rt (calories if divided by 4.18).
💡 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.
📌 QUICK REVISION CARD
Ohm's law: V = IR
Resistivity: R = ρL/A
Power: P = VI = I²R = V²/R
Series: Req = ΣR
Parallel: 1/Req = Σ(1/R)
Wheatstone balance: P/Q = R/S
Potentiometer EMF: E₁/E₂ = l₁/l₂