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Form 4 Physics Mains Electricity Questions and Answers
State why alternating current (a.c) is used for transmitting electricity over long distances.
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1.
Figure 17 shows the electric wiring of an electric heater. A, B, C are the mains wires. Identify A, B, and C.
2.
An electric heater is made of a wire of resistance 100O and connected to a 240V supply. Determine the: (i) Power rating of the heater. (ii) Current flowing in the circuit. (iii) Time taken for the heater to raise the temperature of 200g of water from #23^0C# to #95^0C# (specific heat capacity of water is #4200J Kg^-1K^-1#). (iv) Cost of using the heater for two hours a day for 30 days.
3.
The following table shows electrical appliances to be used in a house. The electrical rating for each appliance is shown. The following fuses are available, 5A, 15A, 30A and 45A. Determine which one of the fuses is suitable for the house.
4.
State the reason why electrical power is transmitted over long distances at very high voltage.
5.
State why alternating current (a.c) is used for transmitting electricity over long distances.
6.
A house has one 100W bulb, two 60W bulbs and one 30W bulb. Determine the cost of having all the bulbs switched on for 70 hours, given that the cost of electricity is 40 cents per kilowatt hour.
7.
In domestic wiring systems lamps in the circuit are required to be parallel and not in series. State two reasons for this requirement.
8.
When a transformer is connected to an ac source, the output voltage is found to be 24V. if the power input is 200W, determine the output current. (Assume the transformer is 100% efficient).
9.
(a) State two factors that determine the resistance of metallic conductor. (b) Explain how a fuse safeguards electrical appliances against excessive currents. (c) A hair dryer is rated 2.5kW, 240V. (i) Determine whether a 10A fuse may be suitable for the hair dryer. (ii) Determine the cost of using the dryer for 3 hours if the cost of electricity is Kshs. 0.80 per kilowatt hour.
10.
(a) Electrical energy is transmitted at very high voltages and low current. (i) Describe how the high voltages are attained. (ii) State two reasons why thick aluminium wires are preferred to copper wires for transmission over long distances. (b) Figure 4 shows an electric cooker connected to the mains supply. X, Y and Z are the mains wires. State with a reason the name of wire Y. (c) A student has
11.
Figure 5 shows part of the lighting circuit in a house. State two errors in the wiring circuit.
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