Practical Electricity (Amos) (1)

Practical Electric Circuit

Uses of Electricity

Heating

Lighting

Motor

Uses of Electricity >> Heating

Radiator

ElectricKettle

Electric Iron

ElectricOven

Heating element positioned at the bottom

Electric current flows through the heating element and this generates heat

Water around the element is heated first by conduction

Thermal energy is transferred through the water by convection•Warm water around heating element expands•Its density decreases and floats•Displacing cooler water at the top•Continuous cycle forms a convection current that warms the entire liquidUses of Electricity >> Heating >> Electric Kettle

QuestionExplain how the heating element at the bottomof an electric kettle transfer thermal energythroughout the whole liquid.

Thermal energy spreads evenly over a large heat-conducting base

Safety MeasureBase insulated from the heating element

Design Features•Contains a thermostat which switches current off when they get too hot •Thermostat switches current on again when the iron cools below the temperature selected

Uses of Electricity >> Heating >> Electric Iron

Air is drawn in through the bottom and heated by the heating element as current flows through it to generate heat.

The warm air then rises in the room. •Warm air around heater expands•Its density decreases and floats•Displacing cooler air at the top of the room•Continuous cycle forms a convection current that warms the entire room

Design Features•The heating elements are often shaped like metal fins to increase surface area•Some are filled with fluid to maximise thermal energy retention

Heating element

Uses of Electricity >> Heating >> Electric Heater (Radiator)

QuestionExplain how a heater draws air from its surrounding to warm up the room in a temperateclimate.

This design helps to increase surface area to maximise transfer of heat to surrounding air by conduction.

Heating element

Uses of Electricity >> Heating >> Electric Heater (Radiator)

QuestionExplain why the heating elements are oftenshaped like metal fins.

The fluid helps to maximise thermal energy retention because of its high heat capacity.

QuestionSome models of radiators are found to fill theirmetal fins with fluid. Explain how this helps toincrease its effectiveness.

Explain how is thermal energy transferred within the electric oven.

Uses of Electricity >> Heating >> Electric Oven

How does an electric current produce a heating effect ?


Act

ivit

y 1

• The heating element is usually made of nichrome wire coiled round an insulating fire-proof material.

• Nichrome is chosen because of its high resistivity and high melting point.

• It also does not oxidise easily even at high temperatures.

• When the electric current flows through the wire, it heats up by means of conduction.

• The rate at which the electrical energy is converted to thermal energy depends on the size of the current and the resistance of the element


Uses of Electricity >> Lighting

Lighting

• Filament is made of tungsten because of its high resistivity and melting point.

• The filament is very thin & long (coiled), giving it a higher resistance (R=l ∕A)

• When an electric current flows through the filament, the tungsten gets extremely hot and generates light.

Uses of Electricity >> Lighting >> Filament Lamp

QuestionExplain how tungsten is used as a material for filament ina bulb as shown in the diagram.

Uses of Electricity >> Motor

Motor

Use electromagnetic effectsto convert electrical energy to rotational kinetic energy.

Uses of Electricity >> Motor

Electric fan

Food mixer

Hard drive

Uses of Electricity >> Calculating Electrical Power


Electrical Power / W

Electrical Energy / J

P = IV E = Ivt

P = I2R E = I2Rt

P = V2/R E = (V2/R)t

E = Pt


Work Book p.153 Topic 19A1, 2, 3, 4

Uses of Electricity >> Power Rating

Power ratings show the correct working voltage and power consumption needed to use them.

An electricity meter

Uses of Electricity >> Electricity Consumption

Unit of Measurement

kWh

Alternative to Joule (J)

How many Joules (J)are there in 1 kWh?


Tariff

Cost = (228 x $0.2258) + (137 x $0.2728)


Appliance Power Rating / W

Duration of use per day / h

Cost / $

Phone charger 5 W 8

Laptop 0.065 kW

Air-conditioning 1.5 kW

Lamp 36 W

Water heater 2000 W

Electric fan 72 W

Total Cost


Workbook p.153Topic 19A5 – 10, 11

Uses of Electricity >> Hazards of Electricity

"Laurence [A History of Capital Punishment] reports the common technique: 2000-2200 volts at 7-12 amperes for 60-90 seconds, possibly lowered and reapplied at various intervals until death."


Damaged insulationor frayed electrical cable

Overloading

Overheating of cables

Wet condition


Damaged insulationor frayed electrical cable

• Damaged insulation between the live and neutral wires may result in a short circuit

• The current flowing through will then suddenly become very large

• This can produce enough heat to start a fire

Remedy:Replace the cable

QuestionExplain cables with damaged insulationcan be a hazard. Suggest how this can beremedied.

• Fitting wires or cables with unsuitable power rating to appliances.

• Too much current flowing through thin wires of higher resistance can cause overheating.

• This can cause a fire.

RemedyFit wires of correct power rating to

appliances.


Overheating of cablesQuestionExplain how electric cables can become overheated and what can be done to prevent it.

• Connecting too many electrical appliances into the same power outlet, through the use of multi-way adaptors.

• If all of these electrical appliances were used at the same time, the total current drawn by them through the electrical cable from the mains supply may be so large that the cable becomes overloaded and overheated.

• This may result in a fire.

RemedyDistribute connection of appliances across

multiple power outlets.

Overloading


QuestionExplain what is overloading and how this can bean electrical hazard.

• Using electrical appliance under wet or damp conditions.

• Dry skin has a resistance of 100 000 Ω or more.

• When skin is wet, resistance falls drastically to just a few hundred ohms.

• Since water can conduct electric current, the user may get an electric shock.

Remedy• Never operate an electrical appliance with

wet hands• Switches, plugs, sockets and connecting

wires should always be kept in a dry condition

Wet condition


QuestionExplain why it is potentially hazardous to handle electrical applianceunder wet conditions.

Uses of Electricity >> Safety Measures

A lighting circuit with two-way switches

Uses of Electricity >> Safety Measures >> Lighting Circuit

• A parallel circuit, using mainly thin wires, connected to lighting components only

• Cables used are quite thin because they are cheaper and also because current flowing through are seldom more than 3 A

• Current carried by the circuit do not generate a lot of heat• However, larger currents will cause appreciable heating• Hence, high-powered appliances should never be used in lighting circuit

QuestionWhat is a lighting circuit? Explain why are high-powered appliances not used in this circuit commonly.

Uses of Electricity >> Safety Measures >> Fuse

FuseA thin wire with low melting point. When a highcurrent flows through the live wire, excess heat generated will melt the wire, thereby isolating the appliance from the live current.

A lighting circuit with two-way switches

Uses of Electricity >> Safety Measures >> Fuse

A B C

Each lamp in the Lighting Circuit draws different amount of current. What should be the fuse rating ofA, B and C?Hint: 1.Fuses are usually selected with ratings slightly higher than the required operating current.2.Common fuse ratings are 1A, 3A, 5A, 13A

Uses of Electricity >> Safety Measures >> Power Circuit

live wire: brown (maintained at high voltage 240 V)

neutral wire: blue (maintained at zero voltage)

earth wire: yellow and green (or just green) (zero voltage for safety purpose)

Note that only the live and neutral wires are required to create a p.d. for current to flow through an appliance

live wire: brown (maintained at high voltage 240 V)

neutral wire: blue (maintained at zero voltage)

earth wire: yellow and green (or just green) (zero voltage for safety purpose)

Note that only the live and neutral wires are required to create a p.d. for current to flow through an appliance

live

earth



Live Neutral Earth

P.D. 240 V 0 V 240 V

Current I I 0

Since our bodies are usually at earth potential ( 0 V ), if someone touches a live wire, the large p.d. will cause a large current to flow through his body.

A fuse must be installed on the Live wire to isolate appliance from the current.

If a person touches the neutral wire , the person should not get an electric shock because the neutral wire is at the same potential as the body, so there will be no current flowing through the body.

What is an Earth wire?Connect metal casing of an appliance to the ground. If the live wire of the appliance touches the metal casing, a large current will flow to the Earth and blow the fuse in the live wire. In this way, the case is prevented from being ‘live’ and users will avoid an electric shock.

If the live wire accidentally comes in contact with the metal casing of the electric iron, the casing will become ‘live’ with a potential of 240 V.

Current will flow from the high-voltage wire through the person touching the metal casing and to the Earth.


QuestionHow could the absence of an earth wire in the powercircuit be a potential electrical hazard to users?


Question ADraw a diagram to show how a power circuit can include a fuse, an earth wire and an electrical appliance to prevent potential electrical hazard due to a user touching a live metal casing.

• As soon as the live wire touches the casing, there will be a short circuit.

• Current will flow from the live wire through the casing to the Earth via the earth wire.

• The large surge of current will melt the fuse, break the live wire connection and isolate the appliance.

• As long as the metal casing is properly earthed and a fuse installed on the live wire, the person will not get an electric shock should he or she touch the faulty appliance


Question BFrom the diagram you drew in (A), explain how the fuse andthe earth wire prevent the appliance from becoming live andthe user from getting electrocuted.

• In the event of a short circuit, a large surge of current will still flow through the live wire into the appliance.

• The fuse that eventually blows on the neutral or earth wire will not isolate the appliance from the live current.

• Users who accidentally touch the metal casing can still get electrocuted.


Question CExplain what happens if the fuse is installed on the neutral orearth wire instead.

Uses of Electricity >> Safety Measures >> Ring Circuit

• Earth pin is longer than the other two so that it makes contact first with the socket and opens the safety shutters that cover the slots for the live and neutral pins

• This feature ensures that the electrical device is properly earthed before it is connected to the mains supply with the live and neutral wires

Uses of Electricity >> Safety Measures >> 3-pin plugs

QuestionElectrical appliances are connected to power circuits through the use of three-pin plugs. Explain why the earth pin is designed to be longer than the live and neutral pins.

QuestionOther than a longer earth pin, explain how the fuse and the cord grip inside the 3-pin plug address potential electrical hazards.

Uses of Electricity >> Safety Measures >> 3-pin plugs

• The cord grip holds the cables in place firmly to prevent any chanceof a short circuit.

• The fuse is installed on the live wire to cut off circuit and isolate appliance when a large surge of current flows through it.

A wall switch

• A switch is used to turn an electrical appliance on and off

• In the case of an accident where someone touches a live wire and gets an electric shock, a switch can be used as a safety device to cut off the flow of current from the main power supply

Uses of Electricity >> Safety Measures >> Switch

QuestionExplain how a switch can be used as a safety device.

Two-way switches

• Two switches can sometimes be used for control, for example, a single lamp at the top and bottom of a staircase or at opposite ends of a corridor

• For this type of arrangement, two-way switches have to be used

Uses of Electricity >> Safety Measures >> Switch

• In these appliances, the casing is made of electrical insulator and also the wire cannot touch the casing.

• These appliances have only a 2-pin plug, using only the live and neutral wires

Uses of Electricity >> Safety Measures >> Double Isolation

Earth leakage circuit breaker

The earth leakage circuit breaker (ELCB) will disconnect all circuits in the house from the mains supply in a very short time (less than 25 ms) if an earth leakage current of more than 25 mA occurs

Uses of Electricity >> Safety Measures >> Circuit Breaker

QuestionExplain how an earth leakage circuit breaker shownin the diagram below serves as a safety measure inmost houses.

Uses of Electricity >> Safety Measures >> Circuit Breaker

Miniature Circuit Breaker (MCB)can “trip” or disconnect faultyappliances from power supplywithout having the need to replace its fuse.

• Switches, fuses or circuit breakers must always be inserted along the live wires of the circuit so that when the switch is off or the fuse has blown, the live wire is disconnected from the high potential of 240 V

• The fuse must be connected before a switch from where the live current is flowing so that current can also isolate the switch when it blows.

Uses of Electricity >> Safety Measures >> Installation of switches, fuses and circuit breakers

AssignmentsType of Assignment Submission & Checking

Theory Workbook Topic A – DSubmit relevant pages from workbook

• Submission on 23 Mar (Mon)

• Attempt independently first before consulting answer guide

• Will be marked

TYS Topic 19Do on foolscap paper. Do not submit

• Class to do all questions

• Attempt independently first before consulting answer guide

• Teacher to go through selected questions during March holiday lesson

Documents

Practical Electricity (Amos) (1)