Question

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

Answer 1

HI !

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

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