Practical ElectricityCopy

Cheat Sheet: Practical Electricity – Uses, Power, Safety (O Level / IGCSE Physics)

4.4.1 Uses of Electricity

1. Common Applications of Electricity

• Heating: electric heaters, ovens, kettles
• Lighting: bulbs, LED lamps
• Battery charging: phones, laptops, electric vehicles
• Powering motors: fans, fridges, washing machines
• Electronic systems: computers, TVs, calculators

2. Advantages of Lamps in Parallel

• Each lamp gets full voltage
• If one lamp fails, others stay on
• Individual control of each lamp possible

3. Power Equation

• Power (P) is the rate of energy transfer
• Equation:
  P = I × V
  P = power (W), I = current (A), V = voltage (V)

4. Electrical Energy Equation

• Energy transferred = current × voltage × time
• Equation:
  E = I × V × t
  E = energy (J), t = time (s)

5. Kilowatt-hour (kWh) and Cost Calculation

• 1 kWh = energy used by a 1 kW device in 1 hour
  = 1000 W × 3600 s = 3.6 × 10⁶ J
• Cost = energy used (kWh) × cost per kWh

4.4.2 Electrical Safety

1. Electrical Hazards

• (a) Damaged insulation: exposes live wires → risk of electric shock
• (b) Overheating cables: due to high current → risk of fire
• (c) Damp conditions: water conducts electricity → increased shock risk
• (d) Excess current: caused by overloading sockets or extension leads

2. Trip Switches and Fuses

• Fuse: melts and breaks circuit if current exceeds safe limit
  • Common ratings: 3 A, 5 A, 13 A
  • Choose closest higher fuse rating to device current
• Circuit breaker (trip switch):
  • Automatically switches off circuit when current is too high
  • Reusable (unlike fuse)

3. Live Wire Touches Metal Case

• If the live wire touches an earthed metal case:
  → Large current flows through earth wire
  → Fuse blows or circuit breaker trips
  → Prevents electric shock

4. Double Insulation or Earthing

• Metal case must be either:
  • Double insulated: outer case is non-conducting, no earth needed
  • OR Earthed: connected to earth wire for safety

5. Mains Circuit Wiring

• Live wire (brown): carries current into device
• Neutral wire (blue): returns current
• Earth wire (green/yellow): safety path for current in case of fault
• Switch must be in live wire: ensures entire device is disconnected from supply when switched off

6. Fuses and Circuit Breakers in Live Wire

• Always connected to live wire
• Ensures supply is cut off before current can flow through user or appliance
• Prevents shock or fire risk from energised internal parts