Question

Two lamps, one rated 100W; 220V, and the other 60W; 220V, are connected in parallel to electric mains supply. find the current drown by two bulbs from the line, if the supply voltage is 220 V.

Answer 1

Power = P = 100 W

Voltage = V = 220v

Resistance = R

P = V²/R

100 = 220 ×220/R

R  = 220 × 220/100

     = 484 Ω

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Power = P = 60 W

Voltage = V = 220v

Resistance = R

P = V²/R

60 = 220 × 220/R

R = 220× 220/60

    = 806.7 Ω

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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

The current drawn is 0.73 A

Answer 2

For fst lamp

P=100W

V=220V

P=VI

100=220.I

I=5/11

For scnd lamp

P=60W

V=220V

P=VI

60=220.I

I=3/11

So total current drawn by two lamps 5/11+3/11

8/11A

0.72A