Ch 19 using electricity 2-e

Chapter
19

Using
Electricity
(Part 1)

In this chapter, you will learn to:
1. state some electrical hazards and precautionary
measures to ensure the safe use of electricity at home

2. explain what is meant by power and state its units

3. solve simple problems on the cost of using electrical
appliances using kilowatt-hour as a unit of electrical
energy consumption

4. show an appreciation that Singapore has no natural
resources of her own, and thus has to conserve energy

5. discuss the importance of reducing electrical energy
wastage

Dangers of Electricity
Electrocution is one of the electrical hazards that
may be caused by misuse of electricity.
When it happens,
1. Do not touch the victim or you may be
electrocuted as well
2. Cut off all power supply by switching off the
mains.
3. Use an insulator like a wooden stick to move
the victim away from all electrical contact
points.
4. Seek medical help. Perform CPR if it is safe to
touch the victim.

Electrical hazards caused by improper use of
electricity are:
 Burns
 Electrocutions
 Electrical fires
 Explosions

a) Damaged insulation
The insulation of a wire protects us because:
 an exposed live wire can
electrocute someone
who accidentally touches
it.
 live and neutral wires
may come into contact in
a short circuit, causing a
large current to flow and
ignite a fire. exposed electrical wire

b) Overloading

 if too many electrical appliances
were used at the same time, the
total power drawn by them
through the electric cable from
the mains supply may be very
large
 cable becomes overloaded and
overheated, which may result in
a fire

c) Overheating
 as resistance is inversely proportional
to the cross-sectional area, a thin wire
possesses high resistance
 thin wires produce more heat which
heat up the wires faster
 thin wires damage the wire insulations
and may cause a fire
 thin wires are used for electrical
appliances which need low power e.g.
lamps and radios
 thick wires are used for appliances
which require high power e.g. electric
irons and kettles

d) Damp Conditions

 dry skin has a resistance of 100 000  or more.
 size of current which flows through a human body
depends on the voltage and the electrical resistance of
the body.

 wet skin lowers resistance of
human body.
 water especially with
sweat, contains salts of good
conductibility make it easier for
electricity to pass through.

d) Poor or loose connections

 May cause short circuits.
 May have gaps that increase
the resistance of the
circuits, leading to
overheating.
 High voltage may cause
electric charges to jump
across the contacts and
produce electric sparks.

Safe Use of Electricity at
home

safe use of electricity in the home

Three-pin plugs

earth wire (yellow
and green stripes)

fuse

neutral wire
(blue)
live wire
cable grip (brown)


Three-pin plugs

wiring a 3-pin plug neutral

 live wire: brown
 neutral wire: blue
 earth wire: yellow and
green (or just green) earth live

 coloured wires should be
connected correctly to
their respective pins
 ensure that a fuse of the
correct value is fitted


Three-pin plugs

metal cap
test pen
 is a specially-made
screwdriver used to transparent
plastic
distinguish a live wire handle
from a neutral wire neon
tube

metal
contact


Three-pin plugs
wiring a 3-pin plug

remove the insulating
plastic from the three
wires
twist the wires and fold the
ends into the hole of each
terminal


Three-pin plugs
wiring a 3-pin plug

tighten the screws

replace the fuse: check that it
is of the right rating for the
appliance


Three-pin plugs
wiring a 3-pin plug

check there are no loose strands
anywhere in the plug before replacing
the cover


Fuse
 protects electrical appliances from damage when
excessive current flows through
 fuse ratings: choose a fuse which can take a current
slightly larger than the maximum current that can pass
through the electrical appliance before it overheats
ceramic metal contacts
insulator

ceramic
5A fuse
plastic tube
wire
base
fuse wire
soldered
to ends

fuse wire in a fuse cartridge fuse

Working principle of a fuse
When the current flowing through the fuse exceeds a
certain value, the fuse melts and breaks. This breaks the
electric circuit, stops the wires from overheating and
prevents fire.
Fuse rating
In practice, cartridge fuses are made
with the following ratings:
2A, 3A, 5A, 10A and 13A.
Choosing a fuse rating
A suitable fuse is the one that can take a current which is
slightly larger than the maximum current that can be
taken by the electrical appliance before it overheats.


Earth wire
 an earth wire serves as a safety
device incorporated in the power
circuit to prevent electric shocks
 joined to the earthing case
(metal case or chassis of an
appliance) through an error, the
metal case of the
Working Principle kettle is not earthed
When there is short a circuit due to
faulty heating element such that the
live wire is in contact with the metal
case, the large surge of current flowing
to the earth will melt the fuse and
break the live wire connection, thereby
protecting the user as well as the if someone happens to touch the
wiring system. kettle, current can flow through
his body to the earth, giving him
an electric shock

Earth wire
• For the sake of safety, an earth wire connects the metal
body of an electrical appliances directly o the earth.

N L

Heating
element
• This is because, if there is an damaged insulation, such
that the metal casing is in contact with a live wire, the
metal casing will become “live”.
• Thus, if user accidentally comes into contact with the
metal casing, part of the conducting path for current
will be formed to complete its circuit. As a result, user
may be electrocuted.
Earthing an appliance is only useful when
the appliance has a metal casing.


Earth wire

fault causes live wire to
be in electrical contact
with the metal case

fuse

heating
element

Double insulation
• Some appliances have casing made of insulating materials ,
such as plastics. Thus, they have double insulation.
• Hand-held electrical appliance, such as hair dryer, is
protected by double insulation.
symbol for
double insulation

• Such appliances do not have metal casing or conducting part in
contact with the user. Hence, no earth wire connection is
provided.


Consumer unit

consumer unit with circuit breakers
for each circuit in the house


Consumer unit
• A consumer unit has four main safety devices:

1. Main switch
• It is connected to the live wire from the main supply, so it connects
or cuts off the electricity supply to a whole building or a house.
2. Miniature Circuit Breaker
(MCB)
• It is a small electromagnetic switch.
• It operates like a fuse but it breaks the circuit by „tripping‟ its
switch when the current exceeds its rating.
3. Residual Current Circuit Breaker (RCCB)
• It contains an electromagnetic switch that breaks the circuit when
current flowing through the earth wire exceeds 30 mA or when
there is faulty equipments being used.
4. Fuse

Chapter
19

Using
Electricity
(Part 2)

electric power and energy
heating effect of an electric current

chemical
energy

 in a closed electrical circuit, chemical energy in a cell
generates electrical charge at a higher potential
 when electric charge flows from a higher to a lower
potential, energy is released in the form of heat through a
resistor

heating elements
 found in electric irons, kettles, cookers and
immersion heaters
 contains heating elements usually made of
nichrome wire (which has a high resistance
and high melting point) coiled round an
insulating fire-proof material (e.g.fire-
clay, silica or mica)
 nichrome heats up when current flows
through the wire

electric iron
thermostat
 heat produced must be control
spread evenly over a
large metal base surface
 contains a thermostat
which switches the
current off when the iron
is too hot and on again
when the iron cools chromium-
plated
below the temperature metal base
heating
selected element

electric kettle
 when a current flows
through the heating
element, water around
the element is heated
first by conduction
earth
pin  heat is spread through
the water by
contact insulated convection
pins heating
element
enclosed

electric cookers
 two types of heating elements used on
electric cookers:
heating element set inside heating element wound
the body of the hotplate inside tubular spirals

heating element
flat hotplate
wound inside tubular
spirals

other common electrical appliances
 common electrical appliances with heating
elements of different shapes:
heating heating heating
element element element

immersion
heater soldering
iron hair
dryer

heating
element

toaster

Power
• The power of a circuit component tells you how
much electrical energy is being converted per
second by the component.
• Power of an electrical component =
Energy converted
Time
• The S.I. unit of power is the watt (W). When the
rate of energy is converted to one joule per
second, the power is said to be one watt.

Energy usage in a home
• We can calculate the electrical energy
by an electrical appliance if we know
its power rating.
• A device whose power rating is one
kilowatt, when used for one hour, has
used up a unit of electrical energy
known as the kilowatt-hour (kWh).

Measuring electricity consumption

 has a unit of kilowatt-hour (kWh)
 1 kWh is the energy used by 1 kW electrical
appliance in 1 hour

Electrical energy used
(in kWh) =
Power Rating of
appliance (kW)
x time (h)
electric meter

Energy usage in a home
Example 1
An oven rated 1.2kW is switched on for 2
hours. Calculate the electrical energy used
by it.
Solution
Electrical energy used = Power  Time
= 1.2 kW  2 h
= 2.4 kWh

Example 2 :
If 500W are consumed by an electrical appliance
in 5 hours, what is the number of kilowatt-hour
consumed ?

Solution :
E (kWh) = P (kW) x t (h)
= (500 / 1000) x 5
= 2.5 kWh

1 kW = 1000 W
1 W = 0.001 or 1/1000 kW

Example 3
If 1 kWh of energy costs 15 cents, how much
would it cost to operate a 750 W electric iron for 4
hours?

Solution :
Energy = Power x time
= 0.75 kW x 4 h
= 3 kWh

Cost = 3 kWh x 15 cents
= 45 cents

Example 4
How much would you have to pay the Public Utilities
Board if you used two 40W lamps and a 120 W
television for 5 hours a day for the month of April ?
(Assume the cost of 1 kWh of energy to be 13 cents.)

Solution :
Total Energy used = power x time
= 2 x 40/1000 x 5 x 30 + 120/1000 x 5 x 30
= 30 kWh

cost = 30 x $0.130 = $3.90

Example 5
If you watched a 120 W television for 2 hours and
used a 20 W table lamp for 4 hours every day for
30 days, how much would you have to pay at the
end of the 30 days, assuming that electrical energy
costs 15 cents per kWh?

Answer: $1.44 or 144 cents

Reducing wastage of electricity
• It is important for us to minimise electricity
wastage because:
– Singapore does not have natural energy
resources such as fossil fuels. The electrical
energy we need comes from imported fuels.
– Our economic performance and survival
depends on our careful management of
limited energy resources.

• We need to conserve resources so that
the depleting fuel reserves can last longer.
• We can minimise wastage by:
– Turning off electrical appliances when they
are not in use.
– Ensuring that all your electrical appliances are
in good working condition.

• We can minimise wastage by:
– Using energy efficient electrical appliances
(e.g. fluorescent lamps instead of
conventional filament lamps.)
Flourescent lamps –
produces light by passing
electric current through
mercury vapors and using
phosphor lining. More
energy saving and energy
efficient as compared to
filament lamps.

Filament lamps – produces bright light through
a heated wire of high thermal and electrical
resistance. Less energy efficient as they draw a
large amount of electricity.

Ch 19 using electricity 2-e

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