Last Updated: April 2026
Current Electricity is one of the most important chapters in NEET Physics, contributing 3–5 questions every year from Class 12. In NEET 2025, there were 4 questions directly from this chapter — totalling 16 marks. The chapter covers fundamental electrical concepts that form the basis of electronics, circuits, and medical imaging devices tested in NEET.
Electric Current and Drift Velocity
Electric current is the rate of flow of electric charge: I = Q/t (SI unit: Ampere)
Drift velocity (v_d) is the average velocity of free electrons in a conductor under an electric field:
- v_d = eE τ/m (where τ = relaxation time, m = electron mass)
- Drift velocity is very small: ~10⁻³ m/s (≈ 1 mm/s)
- This is why it takes time for a new appliance to “warm up” despite the signal travelling at ~c
Current density: J = I/A = nev_d (where n = number density of free electrons)
Ohm’s Law and Resistance
Ohm’s Law: V = IR (at constant temperature)
Current is directly proportional to voltage; resistance opposes current flow.
Resistance Formula
R = ρL/A where:
- ρ = resistivity (Ω·m) — material property
- L = length of conductor
- A = cross-sectional area
| Material | Resistivity (Ω·m) at 20°C | Type |
|---|---|---|
| Silver | 1.6 × 10⁻⁸ | Conductor |
| Copper | 1.7 × 10⁻⁸ | Conductor |
| Nichrome | 100 × 10⁻⁸ | Alloy/Resistor |
| Silicon | 640 | Semiconductor |
| Glass | 10¹⁰–10¹⁴ | Insulator |
Temperature Dependence of Resistance
- Metals (conductors): Resistance INCREASES with temperature (positive temperature coefficient)
- Semiconductors: Resistance DECREASES with temperature (negative temperature coefficient)
- Formula: R_T = R₀(1 + αΔT) where α = temperature coefficient
Combination of Resistors
Series Combination
- R_eq = R₁ + R₂ + R₃ + …
- Same current flows through all resistors
- Total voltage = sum of individual voltages
- R_eq is ALWAYS greater than the largest individual resistance
Parallel Combination
- 1/R_eq = 1/R₁ + 1/R₂ + 1/R₃ + …
- Same voltage across all resistors
- Total current = sum of individual currents
- R_eq is ALWAYS less than the smallest individual resistance
Two resistors in parallel: R_eq = R₁R₂/(R₁+R₂)
Kirchhoff’s Laws
Kirchhoff’s Current Law (KCL) — Junction Rule
At any junction, the algebraic sum of currents = 0
ΣI_in = ΣI_out (current entering = current leaving)
Based on conservation of charge.
Kirchhoff’s Voltage Law (KVL) — Loop Rule
Around any closed loop, the algebraic sum of all EMFs and voltage drops = 0
ΣEMF = ΣIR
Based on conservation of energy.
EMF and Internal Resistance
A battery has an EMF (ε) and internal resistance (r).
- Terminal voltage: V = ε − Ir (during discharge)
- Terminal voltage: V = ε + Ir (during charging)
- When I = 0 (open circuit): V = ε
- Short circuit current: I_sc = ε/r
Wheatstone Bridge
Four resistors P, Q, R, S arranged in a diamond. Galvanometer in the middle branch. Balanced condition: P/Q = R/S → no current through galvanometer → S = QR/P (unknown resistance found).
Meter Bridge (Slide Wire Bridge)
Uses Wheatstone bridge principle on a 1m wire. R/X = l/(100−l) where l = balance length. Used to find unknown resistance X.
Potentiometer
A potentiometer is an instrument that compares EMFs or finds internal resistance. Advantages over voltmeter: does not draw current during measurement → more accurate.
- Principle: V ∝ l (voltage proportional to length of wire)
- EMF comparison: E₁/E₂ = l₁/l₂
- Internal resistance: r = R(l₁−l₂)/l₂
Electric Power and Energy
| Formula | When to use |
|---|---|
| P = VI | General formula |
| P = I²R | When current and resistance known |
| P = V²/R | When voltage and resistance known |
| Energy = Pt (Joules) | Time in seconds |
| Energy = Pt/3600000 (kWh) | Electrical billing (1 unit = 1 kWh) |
Important Devices
- Ammeter: measures current — connected in SERIES; ideal ammeter has zero resistance
- Voltmeter: measures voltage — connected in PARALLEL; ideal voltmeter has infinite resistance
- Galvanometer to ammeter: add low resistance (shunt) in parallel
- Galvanometer to voltmeter: add high resistance in series
Practice MCQs — NEET Current Electricity 2027
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Frequently Asked Questions (FAQ)
What is the difference between EMF and terminal voltage?
EMF (electromotive force) is the maximum potential difference a battery can provide when no current is flowing (open circuit). Terminal voltage is the actual voltage across the battery terminals when current flows. Terminal voltage = EMF − (current × internal resistance) during discharge. The difference equals the voltage drop across internal resistance.
Why is the drift velocity of electrons so slow but electric signals travel so fast?
Drift velocity is the average speed of electrons in a conductor — about 1 mm/s. The electric signal (the electromagnetic field that pushes electrons) propagates at close to the speed of light (~3×10⁸ m/s). When you switch on a light, the field propagates instantly throughout the wire, causing all electrons everywhere to start moving together — the signal speed is the field speed, not the electron speed.
How do I solve complex circuit problems in NEET?
Step 1: Identify all nodes and loops. Step 2: Assign current directions (choose any — KCL will self-correct). Step 3: Apply KCL at each node (sum of currents entering = sum leaving). Step 4: Apply KVL around each independent loop. Step 5: Solve the system of equations. For NEET, most circuits can be simplified using series/parallel combination first before applying Kirchhoff’s laws.
Aiming for 350+ in NEET Physics? Practice daily with NEET Gurukul’s Physics chapter-wise MCQ bank — 500+ Current Electricity problems, detailed solutions and full-length NEET mock tests.