Current Electricity
~11 min read
- Ohm's Law: V = IR. Resistance R = ρL/A (ρ = resistivity, L = length, A = area).
- Power: P = VI = I²R = V²/R. SI: Watt.
- Household: Single-phase AC at 230 V, 50 Hz in India. Earthing for safety. Fuses and circuit breakers protect against overload.
Current electricity is one of the most-tested NDA Physics topics. NDA tests Ohm's law, resistance in series/parallel, power calculations, and household wiring safety.
Current and Ohm's Law
- Electric current (I): Rate of flow of charge. I = Q/t. SI: Ampere (A) = 1 coulomb/second.
- Conventional current direction = direction of positive charge flow (opposite to electron flow in metals).
- Ohm's Law (V = IR): Current through a conductor is directly proportional to applied voltage, at constant temperature.
- Resistance: Property opposing current. SI: Ohm (Ω) = V/A.
- Resistivity (ρ): Material property. R = ρL/A. SI: Ω·m.
- Good conductors: silver, copper, gold (low ρ). Insulators: rubber, glass (high ρ).
Resistors in Series and Parallel
- Series: Same current through each. R_total = R₁ + R₂ + R₃ ... Voltage divides.
- Parallel: Same voltage across each. 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ ... Current divides.
- Parallel R_total is always less than smallest individual R.
- Household wiring uses parallel — so each appliance gets full voltage; one failure doesn't disable others.
Electric Power and Energy
- Power (P) = Voltage × Current. P = VI.
- Combined: P = I²R = V²/R.
- SI unit: Watt (W).
- Energy: Power × time. Domestic energy billed in kilowatt-hour (kWh).
- 1 kWh = 1 unit = 3.6 × 10⁶ J.
- Electric bulb (60 W) running 5 hours = 300 Wh = 0.3 units.
Kirchhoff's Laws
- Junction Rule (Current Law): Sum of currents entering a junction = sum leaving. (Charge conservation.)
- Loop Rule (Voltage Law): Sum of voltage drops around any closed loop = 0. (Energy conservation.)
- Wheatstone Bridge: Used to measure unknown resistance. Balance condition: R₁/R₂ = R₃/R₄.
- Potentiometer: Measures EMF and potential difference accurately. No current drawn from cell.
Household Electricity
- India: 230 V, 50 Hz alternating current.
- Three-wire system: Live (red/brown, has voltage), Neutral (black/blue, at ground potential), Earth (green/yellow, safety).
- Earthing: Conductive connection to ground. Prevents shocks if appliance casing becomes live.
- Fuse: Thin wire that melts on overload — breaks circuit. Single-use.
- Circuit breaker (MCB): Resettable; trips on overload.
- ELCB (Earth Leakage Circuit Breaker)/RCD: Detects current imbalance between live and neutral — trips on leakage to ground (potential shock).
- Bulb filament: Tungsten — high melting point, high resistance.
NDA PYQ Examples
Q: The SI unit of electric current is:
(a) Volt (b) Watt (c) Ampere (d) Ohm
Answer: (c) Ampere.
Q: Ohm's law states:
(a) V is proportional to R (b) V = IR (c) I = V × R (d) P = VI
Answer: (b) V = IR.
Q: In a parallel circuit, voltage:
(a) Divides equally (b) Is same across all elements (c) Decreases (d) Increases
Answer: (b) Same across all elements.
Q: In India, household AC voltage is:
(a) 110 V (b) 220-230 V (c) 400 V (d) 1000 V
Answer: (b) 220-230 V at 50 Hz.
Q: A 100 W bulb runs for 10 hours. Energy consumed:
(a) 0.1 kWh (b) 1 kWh (c) 10 kWh (d) 100 kWh
Answer: (b) 1 kWh = 1 unit.
Drill Current Electricity for NDA
NDA-pattern items on Current Electricity with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is household wiring done in parallel?
Each appliance receives the full 230 V regardless of others. One appliance failing doesn't disable others. Individual switches can control each. Total current draw is the sum of individual currents.
Why is the third pin (earth) of a plug thicker and longer?
Thicker because earth pin must carry potentially large fault currents. Longer so it makes contact first when inserting plug — ensuring earthing before live circuits are made.
What happens during an electric shock?
Current flows through the body to ground. Even 10 mA can cause painful shock; 50 mA can be lethal if it passes through the heart. Wet skin lowers body resistance, increasing danger. Always switch off mains before working on circuits.
How does a fuse protect a circuit?
Has a thin wire that melts at a specific current (e.g., 5 A, 15 A). On overload, the wire melts and breaks the circuit before wiring or appliances are damaged. After fixing the fault, the fuse must be replaced (or use a resettable MCB).
Why are AC voltages used for power transmission?
AC can be easily stepped up to very high voltage (using transformers) for long-distance transmission, then stepped down for use. High voltage = low current = low I²R loss in transmission lines. DC transformation is harder.