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19.2 Domestic electricity
Paying for electrical energy
 Most of the electrical appliances in your home
use mains electricity.
What is the difference
between mains electricity and
electricity from batteries?
 Mains electricity:
alternating current (a.c.)
 Electricity from batteries:
direct current (d.c.)
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
2
V
 Substituting P = VI, P  I 2 R , and P 
into
R
E = Pt, we have
2
V
t
2
E  VIt, E  I Rt and E 
R
19.2 Domestic electricity
 Kilowatt-hour meter
measures the electrical energy
supplied to your home.
 SI unit of electrical energy:
kilowatt-hours (kW h).
energy supplied to a
 1 kW h = 1000 W electrical
appliance in an hour
 1 kW h = 1 kW 1 h = 1000 W  60  60 s
= 3.6  106 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
Household wiring
How is electricity supplied by
the electricity company
distributed around your home?
Electricity
company
mains cable
Your
home
Live wire carries a voltage.
Neutral wire is connected to the Earth.
– a zero voltage and provides a return path for
the electric current.
19.2 Domestic electricity
A household wiring plan
live wire
neutral
wire
kilowattmain hour meter
fuse
consumer unit
circuit breaker
electric air
water conditioner
heater
lighting circuit
ring circuit
19.2 Domestic electricity
kilowatthour
main meter
live wire
neutral
wire
fuse
consumer unit
circuit breaker
electric air
water conditioner
heater
 The mains cable is
connected to a
kilowatt-hour meter
which measures the
electricity consumed.
lighting circuit
ring circuit
19.2 Domestic electricity
live wire
neutral
wire
kilowattmain hour meter
fuse
consumer unit
 It has a main switch
which can be switched
off manually in case of
an emergency.
 The mains cable inside
branches into a number
lighting circuit
of parallel circuits.
circuit breaker
electric air
water conditioner
heater
ring circuit
19.2 Domestic electricity
live wire
neutral
wire
Kilowattmain hour meter
fuse
consumer unit
circuit breaker
electric air
water conditioner
heater
It delivers
electricity to
ceiling lamps.
lighting circuit
ring circuit
19.2 Domestic electricity
 It delivers
Kilowatt- electricity
hour meter
main
to
mains
sockets.
fuse
live wire
neutral Colour coded wires
wire
are used.
 The cables used in
the lighting circuit
and the ring circuits
have an earth wire
for safety reasons.
lighting circuit
consumer unit
circuit breaker
electric air
water conditioner
heater
ring circuit
19.2 Domestic electricity
Lighting circuit
 The lamps are
connected in parallel.
to
consumer
unit
 Any faulty lamp will
not affect the lamps
in other branches.
 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
N
L
 The sockets in the
ring circuit are
connected in
parallel to each
other.
 The branches
operate
independently.
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
An electrical appliance is connected to the ring
circuit by plugging a three-pin plug into a mains
socket.
earth (E)
live (L)
neutral (N)
19.2 Domestic electricity
Mains socket
The neutral and live
sockets are closed to avoid
children from putting their
fingers into the sockets.
E
N
L
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
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Section 19.3
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