Unit 19
Practical Electricity
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contents
Electric Power and Energy
Dangers of Electricity
Safe Use of Electricity in the Home
Chapter Review
El e ct ct r i c p o w e r a n d e n e r g y H e at at i n g e l em e n t s
found in electric irons, irons, kettles kettles,, cookers and immersion heaters contains heating elements usually made of nichrome of of nichrome wire (which has a high resistance and high melting point)) coiled round an insulating fire-proof material point (e.g.fire-clay, silica or mica). Nichrome heats up when current flows through the wire.
e l ec ect r i c p o w e r a n d e n e r g y a . e l e c t r i c i rr o n
heat produced must be spread evenly over a large large metal base surface
thermostat control
contains a thermostat which switches the current off when the iron is is too too hot and on again when the iron cools below the temperature selected
chromiumplated metal base heating element
e l ec ect r i c p o w e r a n d e n e r g y b . e l ec ect r i c k e t t l e
earth pin
contact pins
insulated heating element enclosed
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
e l ec ect r i c p o w e r a n d e n e r g y c . eel c t r i c cco el e ct co o k e r s
two types of heating elements used on electric cookers: heating element set inside the body of the hotplate
flat hotplate
heating element wound inside tubular spirals
heating element wound inside tubular spirals
e l ec ect r i c p o w e r a n d e n e r g y o t h e r c o m m o n e l e ct c t r i cca al a p p l i a n ce s al
common electrical appliances with heating elements of different shapes: heating element
heating element
heating element
immersion heater soldering iron
heating element
toaster
hair dryer
Unit 19.1: Uses of Electricity d. Electrical lighting: Filament lamp
• The filament is made of tungsten of tungsten because of its high resistivity and melting point. point. • The filament is very thin, thin, giving it a higher resistance than the rest of the circuit (R=ρl ⁄ A) • When an electric current flows through the filament, the tungsten gets extremely hot (2500 oC) and generates light.
Figure 19.5 19.5 A filament lamp
Unit 19.1: Uses of Electricity Electrical lighting: Fluorescent lamp • The fluorescent lamp uses electrodes to produce light. • When electric charges are passed between the two electrodes, the mercury vapour in the glass tube emits ultraviolet light together with visible light.. light • The ultraviolet light is converted to visible light by the fluorescent powder coated on the inside of the glass tube.
Figure 19.6 19.6 A fluorescent lamp
Unit 19.1: Uses of Electricity Comparison of the filament lamp to the fluorescent lamp Type
Advantages
Disadvantages
Filament lamp
They are used in homes to Only a small percentage of the give a cosy and relaxed electrical energy supplied is atmosphere. converted to light. The majority of the energy is converted to thermal energy. This explains why a filament lamp feels warm when touched.
Fluorescent lamp
They are energy efficient. Thus, they are widely used in offices and schools for lighting. Tabl e 19.1 19.1
They cost more than filament lamps. As the mercury vapour in fluorescent lamps is toxic, the lamps must be handled carefully.
Unit 19.1: Uses of Electricity Electric motors
• They convert electrical energy into rotational kinetic energy.. energy • They are used in household appliances like the fan, food mixers, electric drills and even in hard disks and DVD drives. • They work on the principles of the magnetic effects of a current (Unit 21: Electromagnetism).
Unit 19.1: Uses of Electricity Key Ideas
1. Electric kettles, ovens and heaters make use of the heating effect of electricity to function. 2. An electric current is passed through the heating element (nichrome) in the appliance, causing thermal energy to be generated.
Unit 19.1: Uses of Electricity Test Yourself 19.1
1. Give two examples of household appliances that use the heating effect of a current. Answer:
Electric kettle and electric iron
Unit 19.1: Uses of Electricity Test Yourself 19.1 2.
What other types of lamps have you encountered other than filament and fluorescent lamps? Do you know which type is more efficient in converting electrical energy to light energy?
Answer: Light emitting diodes (LEDs) are efficient light sources.
A torch and the traffic lights use group of LEDs instead of filament bulbs.
e l ec ect r i c p o w e r a n d e n e r g y 2 .. H He eea c t o f a n e l e ct ct r i c c u r r e n t e a t i n g e f f e ct higher potential gain in potential energy
loss in potential energy
chemical energy heat energy
kinetic energy, etc
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
e l ec ect r i c p o w e r a n d e n e r g y 3 . Ca l cu c u l a t i o n o f e l e cctt r i ca ca l e n e r g y
a a volt volt is is defined defined as as the the potential potential difference difference across across a a component component such such that that one one joule joule of of work work is is done done in in taking taking one one coulomb coulomb of of charge charge from from one one point point to to the the other other one one joule joule of of energy energy is is released released is is one one coulomb coulomb of of electric electric charge charge flows flows through through a a potential potential difference difference of of one one volt volt
From previous topic:
V=
J C
E E= = VIt VIt
Q Q= = It It
E E= =
II22Rt Rt
E E= =
V2 R
tt
Unit 19.2: Measuring Electrical Energy Eg 1
A flashlight bulb is connected to a 3.0 V battery of negligible internal resistance. The ammeter shows a reading of 0.70 A. What is the electrical power used by the bulb?
Solution: Given
V =
3.0 V, I = 0.70 A.
Then the power P of the lamp is
P = VI
= 3.0 × 0.70 = 2.1 W Hence,
electrical power used is 2.1 W.
Eg. 2 Calculate the electrical energy consumed by a i. 2.5 V, 0.02 A bulb for 100s, ii. 60 W, 240 V electric iron for 20 minutes, iii. 4 kΩ immersion electric heater of current 2.3 A for 3 hours. Solution: i.
E E E
= = =
VIt 2.5 x 0.02 x 100 5J
Eg. 2 Calculate the electrical energy consumed by a i. 2.5 V, 0.02 A bulb for 100s, ii. 60 W, 240 V electric iron for 20 minutes, iii. 4 kΩ immersion electric heater of current 2.3 A for 3 hours. Solution: ii.
E E E E
= = = =
VIt Pt 60 x (20 x 60) 72 kJ.
seconds
Eg. 2 Calculate the electrical energy consumed by a i. 2.5 V, 0.02 A bulb for 100s, ii. 60 W, 240 V electric iron for 20 minutes, iii. 0.1 kΩ immersion electric heater of current 2.4 A for 3 hours.
seconds
Solution: iii.
E
=
I2Rt
E
=
(2.4)2 x 100 x (3 x 60 x 60)
E
=
6.22 MJ
e l ec ect r i c p o w e r a n d e n e r g y 4 . El El e c t r i c a l p o w e r
is is the the rate rate at at which which energy energy is released released P P= =
E t
P P= = VI VI
P P= = II22R R
P P= =
V2 R
-1) has a of a SI SI unit unit of watt of watt watt (W) (W) or joule per second (J s-1 )
other commonly used units include the milliwatt, kilowatt and megawatt all electrical appliances should indicate the correct correct working voltage and power consumption needed to use it. One unit unit of of electrical electrical energy energy
= =1 1 kWh kWh = =1 1 kW kW x x1 1 hour hour = = 1000 1000 x x 3600s 3600s = = 3,600,000 3,600,000 JJ
e l ec ect r i c p o w e r a n d e n e r g y m e a su s u r i n g e l e c t r i ci ci t y co n su m p t i o n
has has a a unit unit of of kilowatt-hour (kWh) 1 1 kWh kWh is is the the energy energy used used by by 1 1 kW kW electrical electrical appliance appliance in in an an hour hour Cost Cost of of electrical electrical energy energy = = units units of of electricity electricity used used x x cost cost per per unit
electric meter
Eg. 3 An electric fire is labelled 5 kW, 240 V. Calculate i. the current in the heater, ii. the resistance of the heater, iii. the cost of using the electric fire for 4 hours if the electricity costs 15 cents per unit. Solution:
i.
P = 5000 = I =
VI 240 x I 20.8 A
Eg. 3 An electric fire is labelled 5 kW, 240 V. Calculate i. the current in the heater, ii. the resistance of the heater, iii. the cost of using the electric fire for 4 hours if the electricity costs 15 cents per unit. Solution:
ii.
V = 240 = R =
RI R x 20.8 11.5 Ω
Eg. 3 An electric fire is labelled 5 kW, 240 V. Calculate i. the current in the heater, ii. the resistance of the heater, iii. the cost of using the electric fire for 4 hours if the electricity costs 15 cents per unit. Solution:
iii. Cost of Energy
= = =
VIt x cost per unit 5 x 4 x 0.15 $3.00
Eg. 4 A filament lamp, rated as 60 W, 240 V, is connected to a 240 V power supply. Find (a) the current flowing through the lamp, (b) the resistance of the filament, and (c) the energy produced by the lamp when it is switched on for 8 hours.
Eg. 4 Solution: Given P = 60 W and V = 240 V, (a) The current I is I = P/V = 60/240 = 0.25 A (b) The resistance R of the filament is P = V2/R i.e. R = V2/P = 2402/60 = 960
Ω
Or R can be calculated from R = V/I = 240/0.25 = 960 (c) Converting time t of 8 hours into seconds gives 8
×
60 × 60 = 2.88 × 104 s
Hence the electrical energy E used up when the lamp is switched on for 8 hours is E = P × t = 60 × 2.88 × 104 = 1.73 × 106 J = 1.73 MJ
Ω
Calculating the cost of electricity consumption
• The cost of electricity consumed is calculated based on the number of kilowatt-hours (kWh) of electrical energy used. • One kilowatt-hour (kWh) is the amount of electrical energy used by 1 kW device in one hour.
Eg.5 A 1.5 kW electrical heated is used to heat a large container of water for 2 hours. Calculate the amount of electrical energy used by the heater in (a) kWh, (b) J. Solution: Given P = 1.5 kW, t = 2 hrs (a) Energy used E (in kWh) = P
×
t
= 1.5 × 2 = 3.0 kWh (b) E (in J) = P × t
= 1500 × (2 × 60 × 60) = 1.08 × 107 J
Electrical bill…
Eg. 6 Figure
shows a utilities bill of a household for one month. How is the cost of electricity shown in the bill derived?
Solution:
From Figure 19.12, under the heading ‘Electricity Services’, the number of units of electricity consumed is 112 kWh at a rate of $0.1957 (or 19.57 cents per kWh). Therefore, the cost of electricity consumption is 112 × $0.1957 = $21.92 which is correctly indicated in the last column of the bill.
Test Yourself 19.2
1.
(a) A 240 V mains power supply delivers a current of 9.0 A through an air-conditioner. Find the power supplied in watts. (b) An air-conditioner is used for 1.5 hours each day. Using the answer in (a), given that the electricity tariff is $0.18 per kWh, calculate the cost of using the air-conditioner in a month (30 days).
Answer: (a) P = VI
= 240×9.0 = 2.16 kW (b) Total cost = 2.16 × 1.5 × 0.18 × 30 = $17.50
5 . D an a n g e r s o f e l e c t r i ci ci t y Two major dangers when using electricity
risk risk of of an an electric electric shock shock
risk risk of of fire fire
Dangers are caused by
the the use use of of electrical wires with damaged insulation
the the overheating overheating of of cables cables
touching touching electrical electrical appliances appliances with with wet wet hands hands
d a n g e r s o f e l e c t r i ci ci t y a. dam aged in sulat ion The The insulation insulation of of a a wire wire protects protects us us because: because:
an exposed live wire can can electrocute someone who who accidentally touches it it live and neutral wires wires may may come come into contact in a short circuit, circuit circuit,, circuit, causing a large current to to flow flow and ignite a fire
exposed electrical wire
d a n g e r s o f e l e c t r i ci ci t y b . o v e r h e at a t i n g o f cca a b l e ss
as as resistance is inversely proportional proportional to to the the cross-sectional cross-sectional area, area, a a thin thin wire wire possesses possesses high high resistance resistance thin thin wires wires produce produce more more heat heat which which heat heat up up the the wires wires faster faster thin thin wires wires damage damage the the wire wire insulations insulations and and may may cause cause a a fire fire thin thin wires wires are are used used for for electrical electrical appliances appliances which which need need low low power power e.g. e.g. lamps lamps and and radios radios thick thick wires wires are are used used for for appliances which which require require high high power power e.g. e.g. electric electric irons irons and and kettles kettles
d a n g e r s o f e l e c t r i ci ci t y c . O v e r h e a t i n g o ff c aa bles ab
if if too many electrical electrical appliances were used at the same same time, time, the the total total power power drawn by by them them through through the the electric cable from the mains supply may be very large cable cable becomes becomes overloaded and overheated, which may result in a fire
d a n g e r s o f e l e c t r i ci ci t y d. D Daam m p co n d i t i o n s
dry skin has a resistance of 100 000 Ω Ω or or more 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 especially with with sweat, sweat, contains salts of good conductibility may cause resistance to fall to a few hundred ohms
•
For example, if a hair dryer with the main power switched on happened to fall into a bathtub while a person is bathing, the person may get electrocuted.
•
The water provides a conducting path for a large current to flow.
Unit 19.3: Dangers of Electricity Test Yourself 19.3
1. State the danger caused by the following conditions: (a) Damaged insulation (b) Overheating of cables (c) Using electrical devices in damp conditions. Answer:
(a) Damaged insulation may cause the live wire to be exposed and when touched, can cause severe electric shock to the user and may lead to severe injuries or even death. (b) Overheating of cables may damage the insulation and may cause a fire. (c) Water provides a conducting path for current to flow and thus a person can get electrocuted.
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e 6a. E Ell e ct c t r i ci ci t y i n t h e h o m e Electricity is supplied to the house through the the mains cable which consists of a live wire and and a neutral neutral wire
the mains cable is connected to a mains fuse, fuse, an electricity electricity meter meter and and consumer consumer unit a live wire allows current to flow flow through; through; has has aa certain potential
consumer unit
main switch
a neutral neutral wire wire is is at at zero zero potential
electric meter mains fuse
circuit showing how electricity is supplied to the home
live live neutra neutrall
Unit 19.4: Safe Use of Electricity at Home
Figure 19.18
Typical home circuitry
pp 383
Safety features installed at home for safe use of electricity: a. Circuit breakers b. Fuses c. Correct placement of switch in the circuit d. Three-pin plug e. Earth wire f. Double insulation of certain appliances
Unit 19.4: Safe Use of Electricity at Home a. Circuit breakers • These are safety devices that can switch off the electrical supply when there is excessive current in the circuit.
Figure 19.19 19.19 Circuit breaker
Unit 19.4: Safe Use of Electricity at Home a. Circuit breakers
•
• •
Two circuit breakers that can be found near the front door of a house: (a)The (a) The Miniature Circuit Breaker (MCB) (b)The (b) The Earth Leakage Circuit Breaker (ELCB) or the Residual Current Detector (RCD) The MCB prevents excessive current flow through the circuit by tripping or breaking it. The ELCB monitors the amount of current flowing from the live wire. The ELCB detects small current leakages from the live wire to the earth wire. When this happens, the current in the live wire will be greater than the neutral wire, causing the ELCB to ‘trip’.
b. Fuses A fuse is a short, thin piece piece of wire that heats up and melts when when EXCESSIVE EXCESSIV E current flowing through it, i.e. higher than its rating. Fuses protect the house circuits (electrical appliances) from damage due to too large a current or overloading. Fuses come in 1A, 2A, 5A , 10A and 13A.
Figure 19.20 19.20 A glass cartridge fuse contains a thin metal wire which melts when excessive current flows through it.
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e fuse
protects protects electrical electrical appliances appliances from from damage damage when when excessive excessive current current flows flows through through fuse fuse ratings: ratings: choose choose a a fuse fuse which which can can take take a a current current slightly slightly larger larger than than the the maximum maximum current current that that can can pass pass through through the the electrical electrical appliance appliance before before it it overheats overheats ..
ceramic insulator
plastic base
5A fuse wire
metal contacts
ceramic tube fuse wire soldered to ends
fuse wire in a fuse
cartridge fuse
For example a 5 A fuse is used for a lighting circuit that uses 4 A.
Unit 19.4: Safe Use of Electricity at Home b. Fuses
•
A fuse should be connected to the live wire so that the appliance will not become charged after the fuse has melted due to an overflow of current.
•
Before you change a fuse, always switch off the mains power supply.
Unit 19.4: Safe Use of Electricity at Home Eg. 7 A hot water heated is rated 2880 W, 240 V. Calculate the operating current and suggest a suitable rating for a fuse to protect the heater from overheatin overheating. g. Solution: Given: Power of heater P = 2880 W, voltage V = 240 V Let the operating current be I. P = VI, then I = P/V = 2880/240 = 12 A
The current I in the water heater is 12 A. A suitable fuse is one with a fuse rating that is slightly higher than the current flowing through the device. Thus a 13 A fuse would be suitable for the water heater.
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e c. sw it ch es
are used to turn electrical electrical appliances appliances on appliances on or or off live wire
wire to electrical appliance earth wire
electric contact knob cam springy metal
wall switch
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e sw it ch es
two way switches can be used to to control control a a single single lamp lamp lamp neutral 240 V supply live
fuse electric contact two-way switches
springy metal
Unit 19.4: Safe Use of Electricity at Home c. Switches
• •
Switches break or complete and electrical circuit. If the switch is fitted onto the neutral wire, the appliance will be ‘live’ even if the switch if ‘off’.
Figure 19.23 19.23 Incorrect: The switch is fitted on the neutral wire
Unit 19.4: Safe Use of Electricity at Home c. Switches
•
Switches must be fitted onto the live wire so that switching off disconnects the high voltage from an appliance.. appliance
Figure 19.24 19.24 Correct: The switch is fitted on the live wire
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e c . Sw i t c h e s a n d f u s e s i n a c i r c u i t Switches Switches and and fuses fuses must must always always be be wired wired into the the ‘live’ ‘live’ wire wire of of the the household household circuit. circuit. When When the the switch switch is is off off or or fuse has ‘blown’, ‘blown’, the exposed ‘live’ wire of the socket socket can can be be safely touched because the the flow flow of current current has has been cut off.
switch lamp live fuse
neutral
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e s w i t c h e s a n d f u s es es i n a c i r c u i t
neutral
live (off)
live (on)
correct
neutral
incorrect
d. 3- pin plugs
Underline: Live
Brown
Neutral
Blue
Earth
Yellow with green stripes
Unit 19.4: Safe Use of Electricity at Home d.
Plugs and sockets
•
A fused plug is used to connect appliances to the mains supply via the power socket.
Figure 19.26 19.26 A fused (safety) plug
•
Inside the plug, there is a cartridge fuse that blows and breaks the circuit when excessive current flows in the appliance.
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e d . T h r e ee- p i n p l u g s The The power power circuits circuits join join all all the the sockets sockets around around the the walls walls of of a a house house to to the the mains mains supply supply through through a a wiring system system called called a a ring ring circuit. circuit. In In this this circuit, circuit, there are are two two possible possible paths paths through through which which the the current current can can flow. flow. earth neutral live
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e w i r i n gg a 3 - p i n p l u g
live wire: brown brown
neutral wire: blue blue
neutral
earth wire: wire: yellow and and green (or just green)) just green coloured wires should be connected correctly to their respective pins ensure that a fuse of the correct value is fitted
earth
live
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e e . Ea r t h i n g m e t a l c a se s eess
an an earth earth wire wire serves serves as as a a safety safety device device incorporated incorporated in the power power circuit circuit to to prevent prevent electric electric shocks shocks joined joined to the earthing case case (metal (metal case case or or chassis chassis of of an an appliance) appliance)
if someone happens to touch the kettle, current can flow through his body to the earth, giving him an electric shock
through an error, the metal case of the kettle is not earthed
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e e a r tt h i n g m e t a l cca se se s
fault causes live wire to be in electrical contact with the metal case
fuse
heating element
Unit 19.4: Safe Use of Electricity at Home e. Earthing • The earth wire (green and yellow) is a low-resistance wire and is usually connected to the metal casing of the appliance.
•
Earthing prevents users from getting an electric shock if the live wire is not properly connected and touches the metal casing of the appliance.
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e e .. W h y i s e a r t h i n g n o t r e q u i r e d ? Inside Inside a a house, house, there there are are a a few lighting lighting circuits circuits and and power power circuits. circuits. Each Each circuit circuit has has its its own own fuse fuse in in the consumer consumer unit. unit. Lighting circuits circuits
have have their their own own fuse fuse in in the the consumer unit seldom seldom carry carry more more than than 1A 1A of of current; current; little heat produced, produced, therefore therefore thin thin cables cables are are used. There There is is no no metal parts exposed. (Insulation?)
s a f e u s e o f e l e ct c t r i ci ci t y i n t h e h o m e f. Double insulat ion Some Some electrical electrical appliances appliances (e.g.hair (e.g.hair dryers, dryers, electric electric shavers shavers and and television television sets) sets) connected connected to to power power circuits circuits are are not not earthed earthed by by having having only only a a 2-pin 2-pin plug plug (live (live and and neutral neutral wires). wires).
double insulation is used live electrical electrical components are carefully shielded and isolated from the user accessible metal parts cannot become live unless two independent independent layers layers of of insulation fail symbol for double insulation
Unit 19.4: Safe Use of Electricity at Home f. Double insulation
•
This is a safety feature in an electrical appliance that can substitute the earth wire.
•
Devices that have double insulation normally use a two-pin plug as only the live and neutral wires are required.
•
The electric cable is insulated from the internal components of the appliance.
•
The internal components are also insulated from the external casing.
Figure 19.29 19.29 This fan has double insulation. Notice the symbol for double insulation on the specification sheet.
Key Idea
1.
Safety features of home electrical circuit: (a) Circuit breakers. They serve two functions: (i) To prevent excessive current flow through the cables, and (ii) To detect small leakage current to earth.
(b) Fuses prevent excessive current flow through the appliance. The fuse is
always connected to the live wire. (c) Switches are always fitted on the live wire. (d) Three-pin plugs are used to connect appliances to the mains socket. The plug contains a fuse within the case. It also connects the earth wire to the appliance. (e) The earth wire is connected to the metal casing of appliances. It provides a low resistance conducting path for the current to flow to earth should the metal case suddenly become ‘live’.
Test Yourself 19.4
1. What is the function of a circuit breaker? What is the difference between the Miniature Circuit Breaker and the Earth Leakage Circuit Breaker? Answer:
A circuit breaker switches off the electrical supply in a circuit when there is an overflow of current or leakage of current. The MCB breaks the circuit when the current flowing through it is excessive but the ELCB detects small current leakages to earth.
Test Yourself 19.4
2. Why do some appliances use a three-pin plug while others use a two-pin plug? Answer:
Appliances with double insulation may use a two-pin plug. Appliances that requires earthing will need to use a threepin plug.
Test Yourself 19.4
3. What is the function of a fuse? Where must it be connected in a circuit? Answer:
A fuse prevents excessive current flow through the appliance. It is connected to the live wire.
Test Yourself 19.4
4. What is the purpose of the earth wire? Where must it be connected? Answer:
The earth wire provides a conducting path for current to flow to earth and prevents the metal casing from becoming ‘live’. It is connected to the metal casing of appliances.
Practical electricity
has
Heating effect is based on
Power P = VI which gives
Energy E = VIt expressed in
Energy unit kWh
may have
Hazards caused by
a) damaged insulation b) overheating of cables c) damp conditions
requires
Safety precautions using
• fuses • circuit breakers • earthing metal cases • double insulation
flows in
Wiring consists of
• live wire • neutral wire • earth wire