Domestic Electro – Essential Electrics Study Notes

Power

  • Purpose of any electrical circuit: convert electrical energy into another energy form (heat, light, magnetism, chemical energy).
  • The extent of work done by the current is called power.
    • Symbol: P
    • Unit: watt (W)
  • Practical meaning: measures the rate at which heat is dissipated when electrical energy converts to heat.
  • Fundamental formulae (equivalent ways to state the same relationship):
    • P=V×IP = V \times I
    • W=V×AW = V \times A (older notation: W for watts, A for amperes)
  • Worked example (electric fire):
    • Supply voltage V=230  VV = 230\;\text{V}
    • Current drawn I=10  AI = 10\;\text{A}
    • Power dissipated P=230 V×10 A=2300 WP = 230\,\text{V} \times 10\,\text{A} = 2300\,\text{W}
  • Rating plates/labels on appliances list:
    • Wattage (power rating)
    • Working voltage
    • Model number
  • Kilowatt (kW) conversion because watt is small:
    • 1 kW=1000 W1\,\text{kW} = 1000\,\text{W}
    • For the example: 2300 W÷1000=2.3 kW2300\,\text{W} \div 1000 = 2.3\,\text{kW}

Energy (Work Done by Electricity)

  • Electrical energy E combines power and the time for which the power is used.
  • Standard commercial units:
    • Power in kilowatts (kW)
    • Time in hours (h)
    • Derived energy unit: kilowatt-hour (kWh)
  • Formulae (all equivalent):
    • E=P×tE = P \times t
    • kWh=kW×h\text{kWh} = \text{kW} \times h
  • Example with the 2.3 kW fire used for 3 h:
    • E=2.3 kW×3 h=6.9 kWhE = 2.3\,\text{kW} \times 3\,\text{h} = 6.9\,\text{kWh}

Cost Calculation for Electricity Usage

  • Supply companies set a price per unit (per kWh) to bill customers.
  • General cost equation:
    Cost=(Number of kWh)×(Price per kWh)\text{Cost} = (\text{Number of kWh}) \times (\text{Price per kWh})
  • Example (unit price = 10 pence):
    • Units used: 6.9 kWh6.9\,\text{kWh}
    • Cost: 6.9 kWh×10 p=69 p6.9\,\text{kWh} \times 10\,\text{p} = 69\,\text{p}
  • Practical approach to estimating running cost:
    1. Read power rating (kW) from rating plate.
    2. Determine intended operating time (h).
    3. Multiply to get kWh.
    4. Multiply by tariff (price per unit) from electricity bill.

Power Loss, Voltage Drop & Overheating

  • Heat in conductors is often undesirable, causing energy waste and equipment damage.
  • Causes:
    • All conductors have resistance (R).
    • Current passing through resistance produces heat (Joule effect).
  • Consequences along a cable:
    • Power loss (heat dissipated within the cable itself).
    • Associated voltage drop (reduction in the voltage available to the load).
  • Minimisation strategies:
    • Use conductors with larger cross-sectional area (CSA) → lower resistance.
    • Keep cable runs as short as practical.
  • Hazards of undersized or overly long cables:
    • Elevated power loss → higher voltage drop.
    • Excessive temperature rise → insulation damage, potential fire risk, damage to adjacent materials.

Quick Reference Formulas & Conversions

  • Power: P=V×IP = V \times I
  • Energy: E=P×tE = P \times t (when P in kW and t in h, result is kWh)
  • Conversion: 1 kW=1000 W1\,\text{kW} = 1000\,\text{W}
  • Cost: Cost=kWh×tariff\text{Cost} = \text{kWh} \times \text{tariff}

Practical & Real-World Connections

  • Rating plates allow homeowners and installers to quickly estimate energy consumption and costs.
  • Selecting appropriate cable size is not only an efficiency concern but also a critical safety measure.
  • Energy-cost calculations empower consumers to manage usage and compare appliance efficiency.
  • Voltage drop considerations are mandatory in electrical installation standards and regulations; excessive drop can impair equipment performance (e.g., dim lights, slow motors).