Electrical Power and Household Circuits Short notes and Selected PYQs

Electric Power: The rate at which electrical energy is consumed or work is done. P = W/t = VI.
SI Unit: Watt (W). 1 Watt = 1 Joule/second.
Alternative Formulas: P = I²R = V²/R (from Ohm's law).
Commercial Unit: Kilowatt-hour (kWh). 1 kWh = 3.6 × 10⁶ J = 1 unit of electricity.
Rating of Appliances: Given as power and voltage (e.g., 100 W, 220 V).

Three Wire System: Live wire (red/brown), neutral wire (black/blue), earth wire (green/yellow).
Live Wire: Carries current at high voltage (220 V in India). Connected to positive terminal of supply.
Neutral Wire: Completes the circuit. Connected to negative terminal of supply, at zero potential.
Earth Wire: Safety wire connected to the earth. Protects from electric shocks by providing low-resistance path.
Two-Pin and Three-Pin Plugs: Two-pin for low-power devices; three-pin for high-power devices with earth connection.

Fuse: Safety device with high-resistance wire that melts when excess current flows. Rating in Amperes.
MCB (Miniature Circuit Breaker): Automatic switch that trips when excess current flows. Can be reset.
ELCB (Earth Leakage Circuit Breaker): Detects leakage current to earth and trips the circuit.
Advantages of MCB: Reusable, more sensitive, faster response, no replacement needed.
Fuse Placement: Always in live wire, never in neutral or earth wire.

Why Parallel?: Each appliance gets full voltage, works independently, failure of one doesn't affect others.
Voltage: All appliances receive same voltage (220 V in India).
Current: Total current is sum of individual currents.
Safety: Lower total resistance, circuit breaker protects all appliances.
Switchboards: Have individual switches for each appliance for independent control.

Energy Consumed: E = P × t (in kWh or units).
Cost: Cost = Energy consumed (kWh) × Rate per unit.
Meter Reading: Difference between current and previous reading gives units consumed.
Power Factor: In AC circuits, actual power consumed may be less than apparent power.
Energy Conservation: Use LED bulbs, star-rated appliances, switch off unused devices.

Electrical Power and Household Circuits