Page 51 - CHAPTER-1 (Electricity)

CHAPTER 1

ELECTRICITY

7. Compare the power used in the 2 resistor in each of the

following circuits :

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

(ii) a 8 V battery in parallel with 24 and 2 resistors.

Ans. (i) Since 6 V battery is in series with 1 and 2 resistors,

the current in the circuit,

I = 6V = 6V =2A
1Ω+2Ω 3Ω

2
2
Power used in 2 resistor, P = I R = (2A) × 2 = 8W
1
(ii) Since 8 V battery is in parallel with 24 and 2 resistors,

potential drop across 2 resistor, V = 8V.

Power used in 4 resistor, P = V /R = 64/2 = 32
2
2
Clearly, P1/P2 = 8/32 = 1:4

8. Two lamps, one rated 220 W at 220 V, and the other 110 W at 220

V, are connected in parallel combination to the electric mains

supply. Compute the current drawn from the line if the supply

voltage is 220 V?

Ans. Resistance of first lamp, r = (220) /220 = 220
2
1
resistance of the second lamp, r = (220) /110 = 440
2
2
Since the two lamps are connected in parallel, the equivalent

resistance is given by

1
1
1
= + = r +r 1
2
R p r 1 r 2 r r
1 2
or R = 440*220/(440+220) = 440/3 = 146.67
p
Current drawn from the line, i.e., I = V/ R = 220/(440/3) = 3/2 = 1.5A
p

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