Question

two lamps one rated 100w 220v and the other 60w 220v are connected in parallel to electric main supply find the current drawn by two bulbs from the line of the supply voltage is 220v

Answer 1

Both the bulbs are connected in parallel. Therefore, potential difference across each of them will be same that is 220 V, Current drawn by the bulb of rating 100 W is given by

I=P/V

 =100/220=0.4545...A

Similarly, current drawn by the bulb of rating 60 W is given by

I=P/V

 =60/220=0.2727...A

∴ the total current drawn from the line =0.4545+0.2727=0.727A

Answer 2

Power = P = 100 W

Voltage = V = 220v

Resistance = R

P = V²/R

100 = 220 ×220/R

R = 220 × 220/100

= 484 Ω

Power = P = 60 W

Voltage = V = 220v

Resistance = R

P = V²/R

60 = 220 × 220/R

R = 220× 220/60

= 806.7 Ω

As the resistors are connected in parallel ,

total resistance = 1/R

1/R = 1/484 + 1/806.7

= 806.7 + 484/484×806.7

= 1290.7/390442.8

R = 390442.8/ 1290.7 = 302.5 ohms

Total resistance = 302.5 Ω

I = current

V = IR

220 = I × 302.5

I = 220/302.5

= 0.73 A