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Battery Power problem.

  1. Nov 15, 2012 #1
    1. The problem statement, all variables and given/known data
    A light bulb is rated at 3 watts when connected to a 6 volt battery. How much power will it dissipate if it is connected to a 12 volt battery?

    2. Relevant equations

    P = IV

    3. The attempt at a solution

    3w = I(6v)
    I = 1/2

    P = (1/2)(12v)
    P = 6w.

    Okay, so I heard the answer was 12w from a friend, but I got 6w. I'm not really sure about my answer.. because I guess the current really wouldn't stay the same right?
  2. jcsd
  3. Nov 16, 2012 #2

    Simon Bridge

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    You assumed the current would stay the same.
    If you doubled the voltage across the bulb, what do you expect to happen to the current? (V=IR)

    Since P=IV - what would you expect to happen to the power?
  4. Nov 16, 2012 #3


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    Welcome to PF, Workout

    I've got to level with you, with a real light bulb nothing stays the same when it is heated up to a higher temperature. Your choice of current staying the same isn't bad. But it is very likely the person who made up the question thought the resistance would stay the same. Try it that way, and you should get the 12 Watts.

    A little off topic, if you ever need to completely drain a battery (NiCd batteries improve in performance if this is done once in a while), connect it to a light bulb of about the same rated voltage. As the battery loses its charge, its voltage will drop and deliver less power to the bulb. The bulb cools, causing its resistance to decrease, which helps maintain the current. All this works out nicely to discharge the battery completely in a reasonable time.

    The reason why old light bulbs often fail when they are turned on is that their resistance is really low when cold, an unusually large current flows and it burns out the weak filament like a fuse.
  5. Nov 16, 2012 #4


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    I agree with Delphi51 and Simon..

    I hate this sort of question because if you know a bit more than the examiner expects you can get the answer wrong. For this problem you have to assume the bulbs have a constant resistance, because the construction of the fillament isn't being changed, it's still the same material, thickness, length etc

    If you want to show off add a statement that you have assumed the bulbs have a constant resistance but in practice this may not be the case. It might be proportional to temperature.
  6. Nov 16, 2012 #5


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    This can be risky, and for nimh and li-po type batteries, permanent degradation occurs if they are discharged below a certain voltage. In the case of a nicd battery pack with the cells in series, cell reversal can occur due to imbalance if the voltage per cell is discharged to below 1.1 volt per cell.
  7. Nov 16, 2012 #6
    Ah okay thanks for the help. I will assume the resistance is constant, that seems like the more reasonable assumption at least!
  8. Nov 16, 2012 #7


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    If you overdischarge a li-po it can become unstable and catch fire when recharged. It should not be recharged without extreme caution. In fact many chargers will refuse to charge one that's been discharged too much due to the fire risk. See vids on youtube of lipo batteries that have been abused on purpose.

    Lead acid cells can also be damaged by deep discharge - even those types designed to tollerate deep discharge don't really seem to like it and will live longer if you can avoid it.
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