Electricity in the Home

1. Electricity in the Home Unit D: Topic 7

2. Transmission of Electricity • Electricity starts as a power generating station where it passes through two different kinds of transformers: • Step-up Transformers increase voltage so that the current speeds up and electricity is able to travel farther. • Step-down Transformers decrease voltage so that levels are less dangerous for home.

3. From the Power line to the Home • Distribution lines connect the power line to the power metre located outside the house. It then goes into the fuse box in the house. • From the service panel branch circuits move to the separate outlets in the home. These branch circuits have low resistance.

4. Branch Circuit • Made up of three separate wires: • 1. Neutral Wire – A ‘live wire’ insulated by white cable. • 2. Hot Wire – The other live wire insulated by black cable. • 3. Ground Wire – Channels excess electricity away from load to prevent electrical fires.

5. Fuses and Breakers • Occasional electric surges send in too much electricity that can ruin appliances and start fires. There are two solutions. • 1. Fuses – older solution. Thin wire melts when there’s a surge breaking the circuit. Downside: you have to replace fuse every time. • 2. Circuit Breakers – new solution. Switch bends away from circuit when too much voltage, breaking the circuit. You just move the switch back after.

6. Digital Devices • Use micro switches called transistors set in integrated circuits which make up microprocessors. • Transistors, and microprocessors use a binary code: Either they are turned on (1) or off (0). All digital devices from watches to cd players rely on transistors.

7. Power Review • Power = Energy/time • Calculating Electric Power: P = I*V • Units = Watt (W) (so I W = 1 J/s) • So P = E/t = I*V • Units are Watts (W), 1 Watt = 1 Joule/second. • I = P/V V = P/I

8. Cost Review • Cost of energy is measured in kilowatt-hours (kWh), a non-standard unit of energy. • A kWh is equal to the total electrical energy needed to supply a 1000 W device for one hour. • So if the going rate is 10¢ per kWh, you measure the total energy used by the household in kWh and multiply that number by 10 ¢ (or $0.10) • When reading an electric metre you round the value down to the nearest whole number (see fig. 4.47 on p. 324) • Challenge: How many joules is 1 kWh?

9. Power Ratings • Energuide ratings state how many energy in kWh an appliance will use in a year of typical use. • The bar with the triangle indicates how efficient the appliance is compared to similar products.

10. Efficiency of Devices • Efficiency = Useful Energy Output÷Total Energy going in * 100% • Remember Energy = Power * Time (Measured in Joules)

11. Light Bulbs • Incandescent – Most common lights. Only 5% efficient, other 95% of energy is lost as heat. • Halogen – Filled with high pressure gas. 15% efficient. Last two-six times longer than incandescent. • Fluorescent – Contains mercury vapour which excites powder causing it to glow. It’s 20% Efficient. They last 10-13X longer but are more expensive to buy.