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A bulb is rated 40W, 220V. Find the current drawn by it, when it is connected to a 220V supply. Also find its resistance. If the given bulb is replaced by a bulb of rating 25W, 220V,will be there be any change in the value of current and resistance? Justify your answer and determine the change.

 

Answer:

Case 1:

Given,

  • Power, P = 40 W
  • Potential difference, V = 220 V

Let I be the current through the bulb and R be the resistance.

We know that,

P = V * I 

I = P / V

  = 40 / 220

   = 0.18 A

We know that resistance is given as

R = V 2 / P

    = (220) 2 / 40

    =  1210 ohm.

Therefore,

  • The current drawn by the bulb = 0.18 A
  • The resistance of the bulb = 1210 ohm

Case 2:

Given,

  • Power, P = 25 W
  • Potential difference, V = 220 V

 

Let I be the current through the bulb and R be the resistance.

We know that,

P = V * I 

I = P / V

  = 25 / 220

   = 0.113 A

We know that resistance is given as

R = V 2 / P

    = (220) 2 / 25

    =  1936 ohm.

Therefore,

  • The current drawn by the bulb = 0.113 A
  • The resistance of the bulb = 1936 ohm

 

Thus, there is a change in the value of current and resistance in the two cases. 

The current in case 1 is more since its resistance is lesser than the resistance of case 2.

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CA Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Economics, Accounting and English at Teachoo