Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Physics power question frustrating me

  1. Mar 27, 2010 #1
    1. The problem statement, all variables and given/known data

    Two bulbs P and Q labelled 12V , 24 W and 12 V and 36 W respectively are connected in series with a battery 24 V . Among the following statements , which one describes correctly about bulb P and Q

    (a) potential difference across P > pd across Q

    (b) resistance of Q > resistacne of P

    (c) current passing through Q > current in P

    (d) power dissipated from Q > power dissipated from P

    (e) potential difference and current in both bulbs are the same

    3. The attempt at a solution

    For (a) , i don think its right , since from P=VI , where I is constant here , when P increases ,V also increases .

    For (b) , i think its correct because from P=I^2 R , when P increases , R also increases .

    (c) its not correct since the current passing through both bulbs are the same

    (d) this is also correct , since given that Q is 36 W and P is 24 W

    (e) this is incorrect .

    Is my reasoning for each correct ?
     
  2. jcsd
  3. Mar 27, 2010 #2

    kuruman

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: power

    What does one mean when one says "a light bulb is labeled 12 V and 24 W"? What information can be extracted from this?
     
  4. Mar 27, 2010 #3
    Re: power

    the potential difference across it is simply 12 V and the power it requires to light is 24 W , is it that simple ? And also the current is 2 A .
     
  5. Mar 27, 2010 #4

    kuruman

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: power

    Not quite. A light bulb will give some light even if the applied voltage is less than the rated voltage. It means that when 12 V is applied to the bulb, the power dissipated in it will be 24 W. Can you use this information to find the resistance of the bulb?
     
  6. Mar 27, 2010 #5
    Re: power

    oh , yeah , P=V^2/R so when the power dissipated from the bulb is higher when the same amount of voltage is supplied to the bulb , the resistance will be higher .

    so (b),(e), (c) are out .

    is (d) correct ?
     
  7. Mar 27, 2010 #6

    kuruman

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: power

    Why don't you calculate the power dissipated in each resistor and answer your own question?
     
  8. Mar 27, 2010 #7
    Re: power

    ok , thank you for your help .
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook