IP Physics Notes (Upper Secondary, Year 3-4): 13) Practical Electricity
Download printable cheat-sheet (CC-BY 4.0)30 Sep 2025, 00:00 Z
Quick recap -- Treat electrical appliances as energy converters. Compute how much power they draw, how long they run, and ensure safety with proper wiring, fuses, and earthing.
Energy Sources Snapshot
| Source | Type | Energy chain | Pros | Cons |
| Fossil fuels | Non-renewable | Chemical -> thermal -> steam -> turbine -> generator | Reliable, high output | CO, finite, pollution |
| Nuclear fission | Non-renewable | Nuclear -> thermal -> steam -> turbine | Very high energy density, low CO | Radioactive waste, high capital |
| Hydroelectric | Renewable | GPE of water -> kinetic -> turbine | Clean, quick start-up | Site dependent, ecosystem impact |
| Solar PV | Renewable | Light -> electric | Silent, low running cost | Intermittent, high setup cost |
| Wind | Renewable | Kinetic -> turbine | Clean, free fuel | Needs steady wind, visual/noise concerns |
| Geothermal | Renewable | Earth heat -> steam -> turbine | Stable, low emissions | Limited locations |
| Biomass | Renewable | Chemical -> thermal -> turbine | Uses waste, dispatchable | Emits CO, land use |
Power & Energy Calculations
- Electrical power: \[ P = V I = I^2 R = \frac{V^2}{R} \]
- Energy used: \[ E = P t = V I t \]
- \( E \) in joules for ( t ) seconds, or kilowatt-hours (kWh) for tariffs.
- Cost of electricity: \( \text{cost} = E_\text{(kWh)} \times \text{tariff} \).
Worked Example: Monthly Bill Contribution
An air-conditioner rated \( \pu{1.5 kW} \) operates \( \pu{8 h} \) daily for 30 days. \[ E = 1.5 \times 8 \times 30 = \pu{360 kWh}. \] At \( \pu{0.30} \) per kWh, cost = \( 360 \times 0.30 = \pu{108} \). Sum contributions from all appliances plus fixed charges for full bill.
Household Mains Basics (Singapore)
- Supply: \( \pu{230 V} \), \( \pu{50 Hz} \) AC.
- Three-pin plug wiring:
- Live (brown): delivers current from supply.
- Neutral (blue): returns current to supply.
- Earth (green/yellow): safety path connected to metal casing.
- Switches, fuses, and circuit breakers must be on the live conductor so fault disconnection removes dangerous voltage.
Safety Measures
- Fuses: thin wire melts when current exceeds rating. Select fuse slightly above working current (e.g., \( I = P/V \)).
- Circuit breakers: resettable switches using thermal (overload) and magnetic (short circuit) trip mechanisms.
- Earthing: provides low-resistance path for fault current, ensuring fuse/breaker operates quickly.
- Double insulation: plastic casing and no exposed metal; no earth wire required (two-core flex).
- Residual current devices (RCD/ELCB): detect imbalance between live and neutral currents and disconnect rapidly to prevent shocks.
Hazards to Recognise
- Overloaded sockets causing overheating.
- Damaged insulation exposing live wire.
- Wet environments lowering body resistance (higher shock risk).
- Incorrect fuse rating preventing timely disconnection.
Key Takeaways
- Convert appliance ratings into energy and cost with \( P = V I \) and kWh billing.
- Identify energy source trade-offs: availability, cost per kWh, environmental impact.
- Always reference correct conductor colours and placement of protective devices.
- Matching fuse/breaker ratings to normal operating current is critical for safety compliance.
