CHAPTER 3 DOMESTIC ELECTRICITY 1 Electrical Power How to calculate power drawn from a source? Three equivalent expressions of power [HKCEE] [Power] Calculate the power dissipated in the 10Ω resistor. Page 1
[HKCEE] [Power] A resistor and a bulb are connected in parallel to a 9 V battery as shown above. The reading of the ammeter is5 A. If the power dissipated by the resistor is 18 W, find the power dissipated by the bulb. [HKCEE] [Power] Three identical light bulbs are connected to a cell as shown in the diagram. After some time, the filament ofbulb Z breaks. What will happen to the brightness of the remaining light bulbs? High voltage transmission of electricity Why electricity generated must be transmitted using very high voltage? Page 2
[HKCEE] [Power transmission] Which of the following statements about long distance power transmission at high alternating voltages arecorrect? (1) Alternating voltages can be stepped up or down efficiently by transformers. (2) For a given transmission power, the current will be reduced if a high voltage is adopted. (3) The power loss in the transmission cables will be reduced if a high voltage is adopted. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) [HKCEE] [Power transmission] 100 kw of electrical power is transmitted at 20 kv through cables of total resistance 4 Ω. Find the potentialdrop and power loss in the cables. 2 Power Rating Rated Value: The meaning of 220V, 60W Page 3
[HKCEE][Rated value] Three light bulbs P, Q and R of ratings 24 V, 80 W, 12 V, 80 W and 12 V, 40 W respectively are connectedin parallel to a 12 V battery. Which of the bulbs will be the brightness, and which will be dimmest? [HKCEE][Rated value] Two light bulbs A and B of ratings 10 W, 6 V and 5 W, 6 V respectively are connected in series to a 6Vbattery. Which of the following statements is/are correct? (1) The resistance of A is smaller than that of B. (2) The current through A is the same as that through B. (3) A is brighter than B. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only E. (1), (2) and (3) Page 4
[HKCEE][Rated value] The above diagram shows the label attached to a rice cooker. Which of the following statements is/are truewhen the cooker is working at its rated value? (1) The resistance of the cooker is 96.8 Ω (2) The cooker draws a current of 4.4 A from the mains supply (3) The cooker consumes 1 kw h of the electrical energy in 2 hours. A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only E. (1), (2) and (3) [HKCEE][Electricity Cost] The following table shows three electrical appliances which Clara used in a certain month: Calculate the cost of electricity used. Note: 1 kw h of electricity costs $ 0.86. Page 5
[Further question: Why common induction cookers usually have maximum output power 2800W?] 3 Household wiring Alternating Current vs Direct Current Page 6
Three wires and plug Why the earth pin is slightly longer than the other two pins? Fuse Why the fuse and switch must be installed in the live wire but not the neutral wire? Page 7
[Plug][HKCEE] The diagram above shows a three-pin plug and the wires connecting to it. To which of the pins should each ofthe wires X, Y and Z be connected? [Plug][HKCEE] The photograph shows a three-pin plug of an appliance. Which of the following is a function of pin P? A. It prevents the appliance from being short-circuited. B. It protects the user from getting an electric shock. C. It provides a return path for the current. D. It can break the circuit when the current flowing through the appliance is too large. [Wiring][HKCEE] Which of the following diagrams correctly shows the connection of the wires of an iron to the pins of a plug?(l : Live, N : Neutral, E : Earth) Page 8
[Wiring][HKCEE] The switch S and the fuse of an iron are incorrectly fitted in the neutral wire as shown above. Which of thefollowing statements is correct? A. The iron will not operate even when S is on. B. The iron will still operate even when S is off. C. The iron will still operate, but if there is high current the fuse will not blow. D. The iron will still operate, but if the live wire accidentally touches the body of the iron the current cannotbe conducted to the earth. E. The iron will still operate, but the heating element will remain at a high potential even when S is off. [Fuse][HKCEE] If a 15 A fuse is installed in the plug of an electric kettle of rating 220 V, 900 W, which of the followingdescriptions is correct? A. The kettle will not operate. B. The kettle will be short-circuited. C. The output power of the kettle will be increased. D. The chance of the kettle being damaged by an excessive current will be increased. Household wiring plan Page 9
Lighting circuit two way switch Ring main [Circuit Breaker][HKCEE] A household circuit breaker is marked 220 V, 15 A. Now an iron rated 220 V, 1100 W and a cooker rated 220 V, 550 W are connected in parallel to the mains socket. How many light bulbs rated 220 V, 100 W atmost can be connected in parallel to the mains without triggering the circuit breaker? A. 12 B. 16 C. 17 D. 20 FINAL REMARKS The understanding of electrical power and household wiring is of great importance in our daily life. For academic purpose, students should know the calculation of power and understand the design of household circuit. For practical purpose, students should know how household electricity is being supplied, through all the circuit components, and be able to utilize household electricity. Page 10