Page 52 - CHAPTER-1 (Electricity)

CHAPTER 1

ELECTRICITY

9. Which of the following uses more energy, a 250 W TV set in 1 h,

or a 1200 W toaster in 10 minutes?

Ans. Energy used by a 250 W TV set in 1 h = 250 W × 1h = 250 Wh

Energy used by 1200 W toaster in 10 min. (i.e., 1/6 h) = 1200 W × (1/6)

h = 200 Wh

Thus, a 250 W TV set uses more power in 1 h than a 1200 W toaster

in 10 minutes.

10. An electric heater of resistance 8 is drawing 15 A from the service

mains from 2 hours. Compute the rate at which heat is developed

during this time in the heater.

Ans. Here, I = 15A, R = 8 , t = 2h

2
2
Rate at which heat is developed, i.e, electric power, P = I R = (15)
× 8 = 1800 W = 1800 J/s.

Note: The time given in the question is called superfluous

(irrelevant to the question) data.

11. Explain the following :

(a) Why is tungsten the most exclusive filament chosen for

incandescent lamps?

(b) Why is it that the elements of electric heating devices, such as

toasters and electric irons, made up of an alloy rather than a pure

metal?

(c) Why does the series arrangement not find itself being used for

domestic circuits?

(d) How does the variation in the resistance of a wire occur with

its cross-sectional area?

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

electricity transmission.

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