Ch 19 using electricity 2-e

Using ElectricityUsing Electricity

Chapter 19

Chapter 19

(Part 1)(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


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

b) Overloading


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

c) OverheatingDangers of Electricity

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.

d) Damp ConditionsDangers of Electricity

wet skin lowers resistance of human body.

water especially with sweat, contains salts of good conductibility make it easier for electricity to pass through.

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.

d) Poor or loose connections


Safe Use of Electricity at home

fuse

Three-pin plugs

safe use of electricity in the home

live wire (brown)

neutral wire (blue)

earth wire (yellow and green stripes)

cable grip

wiring a 3-pin plug

live wire: brown

neutral wire: blue

earth wire: yellow and green (or just green)

coloured wires should be connected correctly to their respective pins

ensure that a fuse of the correct value is fitted

live

neutral

earth

Three-pin plugs


test pen is a specially-made

screwdriver used to distinguish a live wire from a neutral wire

metal cap

transparent plastic handleneon tube

metal contact

Three-pin plugs


remove the insulating plastic from the three

wires

twist the wires and fold the ends into the hole of each

terminal

wiring a 3-pin plug

Three-pin plugs


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

appliance

tighten the screws

wiring a 3-pin plug

Three-pin plugs


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

the cover

wiring a 3-pin plug

Three-pin plugs


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

the cover

wiring a 3-pin plug

Three-pin plugs


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

cartridge fusefuse wire in a fuse

metal contacts

ceramic tube

fuse wire soldered to ends

plastic base

ceramic insulator

5A fuse wire

Fuse


Working principle of a fuseWhen 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 ratingIn practice, cartridge fuses are made with the following ratings:

2A, 3A, 5A, 10A and 13A.

Choosing a fuse ratingA 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.

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 kettle is not earthed

if someone happens to touch the kettle, current can flow through his body to the earth, giving him an electric shock

Working PrincipleWhen 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 wiring system.

Earth wire


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

• 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.

LN

Heating element

Earth wire

Earthing an appliance is only useful when the appliance has a metal casing.

fuse

fault causes live wire to be in electrical contact

with the metal case

heating element

Earth wire


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.

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


symbol for double insulation

consumer unit with circuit breakers for each circuit in the house

Consumer unit


• A consumer unit has four main safety devices:

Consumer unit

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.

• 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.

3. Residual Current Circuit Breaker (RCCB)

4. Fuse


Using ElectricityUsing Electricity

Chapter 19

Chapter 19

(Part 2)(Part 2)

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

heating effect of an electric current

electric power and energy

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


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

heating elements

electric iron


heat produced must be 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 below the temperature selected

chromium-plated

metal baseheating element

thermostat control

electric kettle


insulated heating element enclosed

contact pins

earth pin

when a current flows through the heating element, water around the element is heated first by conduction

heat is spread through the water by convection

electric cookers

flat hotplate heating element wound inside tubular

spirals


two types of heating elements used on electric cookers:

heating element wound inside tubular spirals

heating element set inside the body of the hotplate

other common electrical appliances


immersion heater soldering

iron hair dryer

toaster

common electrical appliances with heating elements of different shapes:

heating element

heating element

heating element

heating element

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

electric meter

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

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

Business

solve simple problems

pin plugs wiring

pin plug check

electrical energy consumption4

natural resources

live wire

double insulation

electrical hazards