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Homework Help: Voltage across a bulb affects power output

  1. Jul 11, 2012 #1
    1. The problem statement, all variables and given/known data
    I'm studying for my GCSE exam and I need to know how changing the voltage across a bulb affect it's power output for my secondary evidence, i have searched the internet for the answer but can't find anything. Anyone know how it affects it OR any useful websites to use? thankyou.

    2. Relevant equations

    3. The attempt at a solution
    I have wrote that the bigger the voltage going into the bulb, the more the power output will be because P=VxI but I need to write more? thankyou.
  2. jcsd
  3. Jul 11, 2012 #2
    Hi gracep. You can write 3 "different" recipes for the power output:

    [itex]P=V\cdot I[/itex]
    [itex]P=I^2\cdot R[/itex]

    These 3 things are all related through Ohm's Law.

    What you need is something that relates voltage and power and where everything else in the equation is a constant.

    So, it depends on the problem you have. If you have a constant intensity you would use the first relation. If you have a constant resistance (wich is probably the more common situation), then you want to use the third relation.

    From that you can easily see how the power behaves when changing voltage.
  4. Jul 11, 2012 #3


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    Gold Member

    What he said..

    Rather than mentioning P=IV, I would point out that the power is proportional to V2.

    That's because increasing V also increases I. It would be so better to rearrange the equation given using Ohms Law...

    Ohms Law..

    You were given P=IV so substitute for I giving


    Hence power is proportional to V2.

    I supose you might get an extra mark for pointing out that R might not be constant. It might vary with temperature.
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