Question

A bulb is rated at 220V- 100W. What is its resistance? Five such bulbs burn for 4 hrs. What is the energy consumed? Calculate the cost if the rate is 50paise per unit?

Answer 1

Using Ohms law:
Current (I) =Voltage(V)/Resistance(R)
therefore R=V/I

while power(P)= current(I)x Voltage(V)    that means I=P/V
       Hence R = V2/P
                       =220 squared/100  =48400/100
                           =484 ohms
Energy consumed by the five bulbs in 4hrs.
Energy=power x time  (while power for each bulb is 100W or 100 joules/second)
                energy consumed by 1 bulb:
                   If 1 second= 100 joules
                   then 40 x 60 x60= (100 x 40 x60 x60)/1 joules
                                                =14 400 000 joules
                                                 =14 400 Kj
  then the five bulbs = 5 x 14 400
                                   =72,000 Kj (this the amount of energy consumed)
Cost:
1 bulb = 100W
Then 5 = 500W
1 unit of electricity = 1KW
If 1KW (1000w)= 50 paise
then 500 w = (500 x 50)/ 1000
                     =25000/100
                     =25 paise