Q17 of 41 Page 221

An electric heater of resistance 8draws 15 A from the service mains for 2 hours. Calculate the rate at which heat is developed in the heater.

The rate at which heat is developed in the electric heater will be given by the power of the heater.


Thus, the rate at which heat is developed in the heater is 1800 joules per second.


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14

Compare the power used in the 2resistor in each of the following circuits:

(i) a 6 V battery in series with 1 and 2 a resistors.


(ii) a 4 V battery in parallel with 12 and 2 resistors.

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Two lamps, one rated 100 W at 220 V and the other 60 W at 220 V are connected in parallel to electron mains supply. What current is drawn from the line if the supply voltage is 220 V?

 16

Which uses more energy, a 250 W TV set in 1hr or a 1200 W toaster in 10 minutes ?

 18

Explain the following:

(a) Why is the tungsten used almost exclusively for the filament of electric lamps?


(b) Why are the conductors of electric heating devices such as bread toasters and electric irons made of an alloy rather than a pure metal ?


(c) Why is the series arrangement not used for domestic circuits?


(d) How does the resistance of a wire vary with its area of cross-section?


(e) Why are copper and aluminium wires usually employed for electricity transmission?