Ch 19 using electricity 2-e

  • View
    1.522

  • Download
    0

Embed Size (px)

DESCRIPTION

 

Text of Ch 19 using electricity 2-e

  • 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