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# Phyin c19 practical electricity

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• http://en.wikipedia.org/wiki/Battery_(electricity)
• ### Phyin c19 practical electricity

1. 1. <ul><li>Describe the heating effect of electricity in appliances such as electric kettles, ovens and heaters </li></ul><ul><li>Recall and apply P = V I and E = V I t to new situations or to solve related problems </li></ul><ul><li>Calculate cost of using electrical appliances where the energy unit is kWh </li></ul><ul><li>State hazards of using electricity </li></ul><ul><li>(i) damaged insulation </li></ul><ul><li>(ii) overheating of cables </li></ul><ul><li>(iii) damp conditions </li></ul>Learning outcomes
2. 2. <ul><li>Explain the use of fuses and circuit breakers in electrical circuits and of fuse ratings </li></ul><ul><li>Explain the need for earthing metal cases and for double insulation </li></ul><ul><li>State meaning of ‘lives’, ‘neutral’, ‘earth’ </li></ul><ul><li>Describe the wiring in a mains plug </li></ul><ul><li>Explain why switches, fuses and circuit breakers are wired into the live conductors </li></ul>Learning outcomes
3. 3. 19.1 Electric Power and Energy <ul><li>Heating Elements </li></ul><ul><li>Electrical appliances use heating effect of electrical current </li></ul><ul><li>Appliances have heating element – usually made of nichrome wire coiled round an insulating fire-proof materials </li></ul><ul><li>Nichrome has high resistivity, high melting point, does not oxidise easily at high temperature </li></ul>
4. 4. 19.1 Electric Power and Energy
5. 5. 19.1 Electric Power and Energy <ul><li>Calculation of Electrical Energy </li></ul><ul><li>When current flows through a conductor, energy released is E = V I t = I 2 Rt = V 2 t÷R </li></ul><ul><li>Calculation of Electrical Power </li></ul><ul><li>Electric power is the energy released per unit time </li></ul><ul><li>P = V I = I 2 R = V 2 ÷R </li></ul><ul><li>SI unit is watt ( W ) </li></ul>
6. 6. 19.1 Electric Power and Energy
7. 7. 19.1 Electric Power and Energy
8. 8. 19.1 Electric Power and Energy <ul><li>Measuring Electricity Consumption </li></ul><ul><li>Kilowatt (kWh): unit of electrical energy </li></ul><ul><li>1 kWh is energy used by a 1 kW electrical appliance in 1 hour </li></ul><ul><li>Energy released = power × time </li></ul><ul><li>1 kWh = 1 kW × 1 hour </li></ul><ul><li> = 1000 W × 3600 s </li></ul><ul><li> = 3600 000 J </li></ul><ul><li> = 3.6 MJ </li></ul><ul><li>Why is kilowatt-hour used </li></ul><ul><li>instead of joule to measure </li></ul><ul><li>electrical consumption? </li></ul>
9. 9. 19.1 Electric Power and Energy
10. 10. 19.1 Electric Power and Energy <ul><li>Measuring Electricity Consumption </li></ul><ul><li>1.5 kW air-con switched on for 8 hours uses 1.5 kW × 8 h = 12 kWh </li></ul><ul><li>5 kW electric oven switched on for 30 minutes uses 5 kW × 0.5 h = 2.5 kWh </li></ul><ul><li>A 100 W light bulb switched on for 5 hours uses 0.1 kW × 5 h = 0.5 kWh </li></ul>
11. 11. 19.1 Electric Power and Energy
12. 12. <ul><li>When current flows through a conductor, energy released is </li></ul><ul><li>E = V I t = I 2 Rt = V 2 t ÷ R </li></ul><ul><li>Electric power is the energy released per unit time </li></ul><ul><li>P = V I = I 2 R = V 2 ÷ R </li></ul><ul><li>kWh is the unit of electrical energy used commercially </li></ul><ul><li>1 kWh is the energy used by a 1 kW electrical appliance in 1 hour </li></ul>
13. 14. 19.2 Dangers of Electricity <ul><li>Electric shock, fire </li></ul><ul><li>1. Damaged insulation </li></ul><ul><ul><li>exposed live wire can cause electric shock </li></ul></ul><ul><ul><li>damaged insulation between live and neutral wire may result in a short circuit leading to fire </li></ul></ul><ul><li>2. Overheating of cables </li></ul><ul><ul><li>thin wire  higher resistance  more heat produced damage wire insulation  cause fire </li></ul></ul><ul><ul><li>overloaded power outlet  overheating  cause fire </li></ul></ul>
14. 15. <ul><li>3. Damp conditions </li></ul><ul><ul><li>Dry skin has resistance of 100 000 ohms or more </li></ul></ul><ul><ul><li>Wet skin reduces resistance drastically to few hundred ohms </li></ul></ul><ul><ul><li>Keep switches, plugs, sockets, connecting wires dry </li></ul></ul><ul><ul><li>Never operate electrical appliance with wet hands </li></ul></ul>19.2 Dangers of Electricity
15. 16. 19.2 Dangers of Electricity Physiological effects of an electric current
16. 17. 19.2 Dangers of Electricity
17. 18. <ul><li>Two major dangers when using electricity are electric shock and fire </li></ul><ul><li>Damaged insulation, overheating of cables and damp conditions are some causes of electrical accidents </li></ul>
18. 19. 19.3 Safe Use of Electricity in the House <ul><li>Mains cable: live wire, neutral wire </li></ul><ul><li>Mains cable connected to mains fuse , electricity meter and consumer unit </li></ul><ul><li>Consumer unit has main switch or earth leakage circuit breaker </li></ul><ul><li>Fuse or miniature circuit breaker (MCB) is used for each power circuit of the house </li></ul>Consumer unit with circuit breakers for each circuit in the house
19. 20. 19.3 Safe Use of Electricity in the House Household Electrical Wiring
20. 21. 19.3 Safe Use of Electricity in the House
21. 22. 19.3 Safe Use of Electricity in the House <ul><li>Fuse </li></ul><ul><li>A safety device that protects the wiring and thus protect us </li></ul><ul><li>Fuse melts and breaks when current exceeds its specific value </li></ul><ul><li>Fuse ratings are 3 A, 5 A and 13 A </li></ul><ul><li>Choose fuse which can take current slightly larger than maximum current allowable </li></ul>
22. 23. 19.3 Safe Use of Electricity in the House
23. 24. 19.3 Safe Use of Electricity in the House
24. 25. 19.3 Safe Use of Electricity in the House <ul><li>Lighting Circuits </li></ul><ul><li>Each circuit has its own fuse in the consumer unit </li></ul>
25. 26. <ul><li>Switches </li></ul><ul><li>Used to turn an electrical appliance on or off </li></ul><ul><li>Used as a safety device to cut off flow of current in event someone accidentally touch a live wire and gets an electric shock </li></ul><ul><li>Two way switches can be used to control a single lamp </li></ul>19.3 Safe Use of Electricity in the House
26. 27. 19.3 Safe Use of Electricity in the House <ul><li>The Earth Wire </li></ul><ul><li>Serves as safety device </li></ul><ul><li>Joined to metal case of appliance </li></ul><ul><li>If live wire touches case, large current will flow to the Earth and blow fuse in live wire </li></ul><ul><li>Avoids electric shock </li></ul>
27. 28. <ul><li>Double Insulation </li></ul><ul><li>Double insulation is the shielding and isolation of live electrical components </li></ul><ul><li>Used in electrical appliances which are not earthed </li></ul><ul><li>Appliances have 2-pin plug: live and neutral wire </li></ul><ul><li>Examples: hair dryer, electric shavers, battery charger, television sets </li></ul><ul><li>Casing made of plastic, wire cannot touch casing even if it become loose as there is double insulation </li></ul>19.3 Safe Use of Electricity in the House
28. 29. 19.3 Safe Use of Electricity in the House <ul><li>Three-Pin Plugs </li></ul><ul><li>Live wire: brown </li></ul><ul><li>Neutral wire: blue </li></ul><ul><li>Earth wire: yellow and green stripes </li></ul>
29. 30. <ul><li>Switches, fuses or circuit breakers must always be inserted along ‘live’ wires of circuits </li></ul><ul><li>When switch is off or the fuse has ‘blown’, the ‘live’ wire is disconnected from the high voltage </li></ul>19.3 Safe Use of Electricity in the House
30. 31. <ul><li>Household wiring systems include lighting circuits and power circuits </li></ul><ul><li>A power circuit consists of a live wire, a neutral wire and an earth wire </li></ul><ul><li>Earthing metal cases and double insulation serve as safety devices </li></ul><ul><li>Circuit switches, fuses and circuit breakers also serve as safety devices </li></ul><ul><li>Circuit switches, fuses and circuit breakers must be connected on the ‘live’ side of a wiring system </li></ul><ul><li>The wires in a three-core cable must be connected correctly to their corresponding pins </li></ul>
31. 33. <ul><li>What is the difference between power and energy? </li></ul><ul><li>How do you calculate the cost of using electrical energy? </li></ul><ul><li>What safety features are designed into electric circuits? </li></ul>Questions