Question

Two bulbs rated 100W at 220V and another 60W at 220V are connected in parallel to the electric mains supply. what current is drawn 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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