Resistors in series and parallel; power

Explanation of question 14

A resistor is connected across a 12V supply. A 6V supply is now substituted for this supply. The new power will be
  1. four times the original power
  2. twice the original power
  3. one-half the original power
  4. one-quarter the original power

    The answer is :- 4

    Assume the resistor as a value of 24 Ohms. I=V/R

    At 12v I would be I = 12/24 = 0.5Amps. W = V x I or W= 12 x 0.5 = 6w

    At 6v I would be I=6/24 = 0.25Amps. W = V x I or W = 6 x 0.25 =1.5w

    1.5w is one-quarter of the original power of 6w 

    OR:-

    P = V2 / R.

    R is a constant, therefore, P V2. <

    New power will be (½)2 = ¼ of the original power.

        R is a constant, therefore, P V2. <

        New power will be (½)2 = ¼ of the original power.

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