1、Electricity in the Home 1Electricity in the Home (A) Electrical Power and Energy(1) Electrical PowerElectrical power is the rate of electrical energy consumer per unit time by an electrical appliance. It is given by the equationP = ,VItEwhere V and I are the voltage across it and the current though
2、it respectively.For an electrical appliance with resistance R, it is also given by P = =RI2VThe SI unit of electrical power is watt (W). 1 W = 1 J s-1The total power can be calculated by using the equivalent resistance or by adding the individuals. The rated value of an electrical appliance gives us
3、 its power rating, the electrical energy given to it per unit time when operated at the rated voltage.(2) Electrical EnergyElectrical energy supplied to, or consumed by or dissipated by it in a time interval t is given by E = Pt = VItFor an electrical appliance of resistance R, the electrical energy
4、 is also given by E = Pt = t= tRI2V2The SI unit of electrical energy is joule (J).Electric company measures the electrical energy supplied to consumers in kilowatt-hour (kW h). 1 kilowatt-hour of energy is the amount of energy consumed by an electrical appliance of electrical power of 1 kW in one ho
5、ur. Also, 1 kWh = 3600000 J.Electricity in the Home 2(B) Mains electricity and household wiring(1) Direct current and alternating currentAlternating current (a.c.) is current that reverses its direction at a certain frequency.Direct current (d.c.) is current that never reverses its direction. Howeve
6、r, the value of a d.c may vary with time. Hong Kong uses 220 V a.c. at 50 Hz. A 220-V appliance cannot function normally when connected to a 110-V supply. Since P is proportional to V2, the output power is much smaller than the rated value.(2) Plug and socketThe live wire goes +V and -V alternately,
7、 i.e. the voltage between the live wire and the earth changes from +V to V alternatively.The neutral wire has zero voltage (earthed), i.e. the voltage between the neutral wire and the earth is zero.The earth connects the metal case to the Earth and prevents the body of appliance from becoming live i
8、f a fault develops.The earth pin is long (i) to ensure the appliance is earthed before connected to the live wire;Electricity in the Home 3(ii) to open shutters on the socket holes that prevent accidental insertion of metal objects. The switch is fitted in live wire. Otherwise, the appliance is stil
9、l live when the switch is off. The fuse is fitted in live wire and melts when the current is larger than the fuse value. If the live wire touches the metal case, a short circuit will be set up between the live wire and the earth. The fuse will blow.If the neutral wire touches the metal case, the cir
10、cuit can be still completed, and the appliance will continue to work.Double insulation: The live part of an appliance is completely covered by an insulating plastic case.Safety rules should be observed when using mains-operated appliances. Electric hazards include electric shock, burns and fire.(3)
11、Household wiringIn a consumer unit, each circuit is fitted with a circuit breaker which acts like a fuse. Electrical energy is supplied to the home by a cable containing a live and a neutral wire. These wires are connected to the consumer unit via a kilowatt-hour meter. They then branch out into parallel circuits.Electricity in the Home 4Mains sockets are connected to a ring main which loops around the house as shown. In the ring main all the sockets are connected in parallel, so that a fault in any socket will not affect the others. In a ring main, thinner and cheaper cables can be used.
