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# Phyin c19 practical electricity

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• http://en.wikipedia.org/wiki/Battery_(electricity)
• ### Transcript

• 1.
• Describe the heating effect of electricity in appliances such as electric kettles, ovens and heaters
• Recall and apply P = V I and E = V I t to new situations or to solve related problems
• Calculate cost of using electrical appliances where the energy unit is kWh
• State hazards of using electricity
• (i) damaged insulation
• (ii) overheating of cables
• (iii) damp conditions
Learning outcomes
• 2.
• Explain the use of fuses and circuit breakers in electrical circuits and of fuse ratings
• Explain the need for earthing metal cases and for double insulation
• State meaning of ‘lives’, ‘neutral’, ‘earth’
• Describe the wiring in a mains plug
• Explain why switches, fuses and circuit breakers are wired into the live conductors
Learning outcomes
• 3. 19.1 Electric Power and Energy
• Heating Elements
• Electrical appliances use heating effect of electrical current
• Appliances have heating element – usually made of nichrome wire coiled round an insulating fire-proof materials
• Nichrome has high resistivity, high melting point, does not oxidise easily at high temperature
• 4. 19.1 Electric Power and Energy
• 5. 19.1 Electric Power and Energy
• Calculation of Electrical Energy
• When current flows through a conductor, energy released is E = V I t = I 2 Rt = V 2 t÷R
• Calculation of Electrical Power
• Electric power is the energy released per unit time
• P = V I = I 2 R = V 2 ÷R
• SI unit is watt ( W )
• 6. 19.1 Electric Power and Energy
• 7. 19.1 Electric Power and Energy
• 8. 19.1 Electric Power and Energy
• Measuring Electricity Consumption
• Kilowatt (kWh): unit of electrical energy
• 1 kWh is energy used by a 1 kW electrical appliance in 1 hour
• Energy released = power × time
• 1 kWh = 1 kW × 1 hour
• = 1000 W × 3600 s
• = 3600 000 J
• = 3.6 MJ
• Why is kilowatt-hour used
• instead of joule to measure
• electrical consumption?
• 9. 19.1 Electric Power and Energy
• 10. 19.1 Electric Power and Energy
• Measuring Electricity Consumption
• 1.5 kW air-con switched on for 8 hours uses 1.5 kW × 8 h = 12 kWh
• 5 kW electric oven switched on for 30 minutes uses 5 kW × 0.5 h = 2.5 kWh
• A 100 W light bulb switched on for 5 hours uses 0.1 kW × 5 h = 0.5 kWh
• 11. 19.1 Electric Power and Energy
• 12.
• When current flows through a conductor, energy released is
• E = V I t = I 2 Rt = V 2 t ÷ R
• Electric power is the energy released per unit time
• P = V I = I 2 R = V 2 ÷ R
• kWh is the unit of electrical energy used commercially
• 1 kWh is the energy used by a 1 kW electrical appliance in 1 hour
• 13.
• 14. 19.2 Dangers of Electricity
• Electric shock, fire
• 1. Damaged insulation
• exposed live wire can cause electric shock
• damaged insulation between live and neutral wire may result in a short circuit leading to fire
• 2. Overheating of cables
• thin wire  higher resistance  more heat produced damage wire insulation  cause fire
• overloaded power outlet  overheating  cause fire
• 15.
• 3. Damp conditions
• Dry skin has resistance of 100 000 ohms or more
• Wet skin reduces resistance drastically to few hundred ohms
• Keep switches, plugs, sockets, connecting wires dry
• Never operate electrical appliance with wet hands
19.2 Dangers of Electricity
• 16. 19.2 Dangers of Electricity Physiological effects of an electric current
• 17. 19.2 Dangers of Electricity
• 18.
• Two major dangers when using electricity are electric shock and fire
• Damaged insulation, overheating of cables and damp conditions are some causes of electrical accidents
• 19. 19.3 Safe Use of Electricity in the House
• Mains cable: live wire, neutral wire
• Mains cable connected to mains fuse , electricity meter and consumer unit
• Consumer unit has main switch or earth leakage circuit breaker
• Fuse or miniature circuit breaker (MCB) is used for each power circuit of the house
Consumer unit with circuit breakers for each circuit in the house
• 20. 19.3 Safe Use of Electricity in the House Household Electrical Wiring
• 21. 19.3 Safe Use of Electricity in the House
• 22. 19.3 Safe Use of Electricity in the House
• Fuse
• A safety device that protects the wiring and thus protect us
• Fuse melts and breaks when current exceeds its specific value
• Fuse ratings are 3 A, 5 A and 13 A
• Choose fuse which can take current slightly larger than maximum current allowable
• 23. 19.3 Safe Use of Electricity in the House
• 24. 19.3 Safe Use of Electricity in the House
• 25. 19.3 Safe Use of Electricity in the House
• Lighting Circuits
• Each circuit has its own fuse in the consumer unit
• 26.
• Switches
• Used to turn an electrical appliance on or off
• Used as a safety device to cut off flow of current in event someone accidentally touch a live wire and gets an electric shock
• Two way switches can be used to control a single lamp
19.3 Safe Use of Electricity in the House
• 27. 19.3 Safe Use of Electricity in the House
• The Earth Wire
• Serves as safety device
• Joined to metal case of appliance
• If live wire touches case, large current will flow to the Earth and blow fuse in live wire
• Avoids electric shock
• 28.
• Double Insulation
• Double insulation is the shielding and isolation of live electrical components
• Used in electrical appliances which are not earthed
• Appliances have 2-pin plug: live and neutral wire
• Examples: hair dryer, electric shavers, battery charger, television sets
• Casing made of plastic, wire cannot touch casing even if it become loose as there is double insulation
19.3 Safe Use of Electricity in the House
• 29. 19.3 Safe Use of Electricity in the House
• Three-Pin Plugs
• Live wire: brown
• Neutral wire: blue
• Earth wire: yellow and green stripes
• 30.
• Switches, fuses or circuit breakers must always be inserted along ‘live’ wires of circuits
• When switch is off or the fuse has ‘blown’, the ‘live’ wire is disconnected from the high voltage
19.3 Safe Use of Electricity in the House
• 31.
• Household wiring systems include lighting circuits and power circuits
• A power circuit consists of a live wire, a neutral wire and an earth wire
• Earthing metal cases and double insulation serve as safety devices
• Circuit switches, fuses and circuit breakers also serve as safety devices
• Circuit switches, fuses and circuit breakers must be connected on the ‘live’ side of a wiring system
• The wires in a three-core cable must be connected correctly to their corresponding pins
• 32.
• 33.
• What is the difference between power and energy?
• How do you calculate the cost of using electrical energy?
• What safety features are designed into electric circuits?
Questions
• 34.
• 35.
• 36.
• 37.
• 38.