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Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
Ch 19   using electricity 2-e
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Ch 19 using electricity 2-e

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  1. Chapter 19 Using Electricity (Part 1)
  2. In this chapter, you will learn to:1. state some electrical hazards and precautionary measures to ensure the safe use of electricity at home2. explain what is meant by power and state its units3. solve simple problems on the cost of using electrical appliances using kilowatt-hour as a unit of electrical energy consumption4. show an appreciation that Singapore has no natural resources of her own, and thus has to conserve energy5. discuss the importance of reducing electrical energy wastage
  3. Dangers of Electricity
  4. Dangers of ElectricityElectrocution 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 well2. 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.
  5. Dangers of ElectricityElectrical hazards caused by improper use of electricity are: Burns Electrocutions Electrical fires Explosions
  6. Dangers of Electricitya) Damaged insulationThe 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
  7. Dangers of Electricityb) 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
  8. Dangers of Electricityc) 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
  9. Dangers of Electricityd) 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.
  10. Dangers of Electricityd) 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.
  11. Safe Use of Electricity at home
  12. 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)
  13. safe use of electricity in the home 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
  14. safe use of electricity in the home 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
  15. safe use of electricity in the home 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
  16. safe use of electricity in the home 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
  17. safe use of electricity in the home Three-pin plugs wiring a 3-pin plug check there are no loose strands anywhere in the plug before replacing the cover
  18. safe use of electricity in the home Three-pin plugs wiring a 3-pin plug check there are no loose strands anywhere in the plug before replacing the cover
  19. safe use of electricity in the home 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
  20. 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 madewith 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.
  21. safe use of electricity in the home 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
  22. 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.
  23. safe use of electricity in the home Earth wire fault causes live wire to be in electrical contact with the metal case fuse heating element
  24. safe use of electricity in the home 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.
  25. safe use of electricity in the home Consumer unit consumer unit with circuit breakers for each circuit in the house
  26. safe use of electricity in the home 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
  27. Chapter 19 Using Electricity (Part 2)
  28. In this chapter, you will learn to:1. state some electrical hazards and precautionary measures to ensure the safe use of electricity at home2. explain what is meant by power and state its units3. solve simple problems on the cost of using electrical appliances using kilowatt-hour as a unit of electrical energy consumption4. show an appreciation that Singapore has no natural resources of her own, and thus has to conserve energy5. discuss the importance of reducing electrical energy wastage
  29. 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
  30. electric power and energy 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
  31. electric power and energy 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
  32. electric power and energy electric kettle  when a current flows through the heating element, water around the element is heated first by conductionearth pin  heat is spread through the water by contact insulated convection pins heating element enclosed
  33. electric power and energyelectric 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
  34. electric power and energy other common electrical appliances  common electrical appliances with heating elements of different shapes: heating heating heating element element elementimmersion heater soldering iron hair dryer heating element toaster
  35. 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.
  36. 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).
  37. 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
  38. Energy usage in a homeExample 1An oven rated 1.2kW is switched on for 2hours. Calculate the electrical energy usedby it.SolutionElectrical energy used = Power  Time = 1.2 kW  2 h = 2.4 kWh
  39. Example 2 :If 500W are consumed by an electrical appliancein 5 hours, what is the number of kilowatt-hourconsumed ?Solution : E (kWh) = P (kW) x t (h) = (500 / 1000) x 5 = 2.5 kWh1 kW = 1000 W1 W = 0.001 or 1/1000 kW
  40. Example 3If 1 kWh of energy costs 15 cents, how muchwould it cost to operate a 750 W electric iron for 4hours? Solution : Energy = Power x time = 0.75 kW x 4 h = 3 kWh Cost = 3 kWh x 15 cents = 45 cents
  41. Example 4How much would you have to pay the Public UtilitiesBoard if you used two 40W lamps and a 120 Wtelevision 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
  42. Example 5If you watched a 120 W television for 2 hours andused a 20 W table lamp for 4 hours every day for30 days, how much would you have to pay at theend of the 30 days, assuming that electrical energycosts 15 cents per kWh?Answer: $1.44 or 144 cents
  43. 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.
  44. Reducing wastage of electricity• 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.
  45. Reducing wastage of electricity• 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.

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