19.2 Domestic electricity Paying for electrical energy  Most of the electrical appliances in your home use mains electricity.  Mains electricity: alternating current (a.c.)  Electricity from batteries: direct current (d.c.) What is the difference between mains electricity and electricity from batteries? Symbol for an a.c. voltage source

19.2 Domestic electricity  In Hong Kong, the mains electricity is at 220 V and is charged for the electrical energy consumed.  Energy consumed: Energy = power  time or E = Pt  Substituting P = VI,, and into E = Pt, we have

19.2 Domestic electricity  Kilowatt-hour meter measures the electrical energy supplied to your home.  SI unit of electrical energy: kilowatt-hours (kW h).  1 kW h = energy supplied to a 1000 W electrical appliance in an hour = 1 kW  1 h = 1000 W  60  60 s = 3.6  10 6 J = 3.6 MJ

19.2 Domestic electricity  This electricity bill shows that the amount of electrical energy supplied is 449 kW h.

19.2 Domestic electricity Neutral wire is connected to the Earth. –a zero voltage and provides a return path for the electric current. Household wiring How is electricity supplied by the electricity company distributed around your home? Electricity company Your home mains cable Live wire carries a voltage.

19.2 Domestic electricity main fuse neutral wire live wire A household wiring plan kilowatt- hour meter consumer unit circuit breaker air conditioner electric water heater lighting circuit ring circuit

19.2 Domestic electricity main fuse neutral wire live wire kilowatt- hour meter consumer unit circuit breaker air conditioner electric water heater ring circuit lighting circuit  The mains cable is connected to a kilowatt-hour meter which measures the electricity consumed.

19.2 Domestic electricity main fuse neutral wire live wire kilowatt- hour meter consumer unit ring circuit lighting circuit electric water heater air conditioner  It has a main switch which can be switched off manually in case of an emergency. circuit breaker  The mains cable inside branches into a number of parallel circuits.

19.2 Domestic electricity main fuse neutral wire live wire Kilowatt- hour meter consumer unit ring circuit lighting circuit electric water heater air conditioner circuit breaker  It delivers electricity to ceiling lamps.

19.2 Domestic electricity main fuse neutral wire live wire Kilowatt- hour meter consumer unit ring circuit lighting circuit electric water heater air conditioner circuit breaker  It delivers electricity to mains sockets.  Colour coded wires are used.  The cables used in the lighting circuit and the ring circuits have an earth wire for safety reasons.

19.2 Domestic electricity Lighting circuit  The lamps are connected in parallel. Any faulty lamp will not affect the lamps in other branches. to consumer unit  The switches are connected to the live wire – The lamp sockets carry no voltage when the switches are off.

19.2 Domestic electricity  Sometimes, a pair of two-way switches is used. Switch on the lamp at the right of the corridor. The lamp is on when we are in the corridor. The switches are both up or both down. Switch off the lamp at the other end of the corridor.

19.2 Domestic electricity Ring circuit  Current flows from the consumer unit to the sockets via two paths. consumer unit to electrical appliance  Each of path carries half of the current.

19.2 Domestic electricity What are the benefits of having two paths from the consumer unit to the mains sockets?  The chance of overloading can be reduced.  Thinner and cheaper wires can be used.  If one of the paths does not work due to socket or cable damage, the other path can still carry the current.

19.2 Domestic electricity  The sockets in the ring circuit are connected in parallel to each other.  The branches operate independently. N L

19.2 Domestic electricity Other circuits  Some appliances draw very large current.  They are connected directly to the consumer unit.  Thicker wires are used.  For example: electric heater air conditioner

19.2 Domestic electricity Connecting an electrical appliance earth (E) live (L) neutral (N) An electrical appliance is connected to the ring circuit by plugging a three-pin plug into a mains socket.

19.2 Domestic electricity E N L Mains socket The neutral and live sockets are closed to avoid children from putting their fingers into the sockets. How can the neutral and live sockets be opened? When the longer earth pin is plugged into the earth socket.

19.2 Domestic electricity Inside a three-pin plug green and yellow earth wire blue neutral wire brown live wire

19.2 Domestic electricity That’s the end of Section 19.2 Check Point Key Ideas Previous Page Section 19.3 Exit