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Homework Help: Electric bulb power

  1. Sep 16, 2013 #1
    Suppose I have 10.2 W power source... Now how many 1 W bulb can I use here?

    Obviously I can use 10 bulbs at their best here... But can I use an eleventh one? Usually we do it like this 10.2/1=10.2~10 but my ques is there is some power left here... So why can't we use one more?

    This will result in that bulb glowing dim? I am thinking maybe all of the 11 bulb will then glow dimmer... Maybe that's why we always take the floor of the number we get...

    Am I right?
  2. jcsd
  3. Sep 16, 2013 #2


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    Staff: Mentor

    You are correct -- adding the last bulb will pull down the voltage of the power source, which will dim all of the other bulbs a little bit.
  4. Sep 16, 2013 #3
    Thank you...
  5. Sep 17, 2013 #4


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    Science Advisor
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    Gold Member

    Hold on a moment. It's not really that simple. Berkeman gave you the short answer.

    The 11th bulb might cause the output voltage to fall but not always. You can't really answer the question without knowing why the designer of the power supply specified a 10W limit.

    It could be he specified a 10W limit so that the power supply wouldn't overheat when used in a hot climate. If you are in a nice air conditioned office the power supply might well deliver the full voltage at 11, 12, 13W or more without any problem.

    On the other hand it might only appear to be happy delivering >10W at the full voltage. In reality some components inside might be operating slightly hotter then intended. That might reduce the life time of the product. The effect can be quite dramatic. The life expectancy of some capacitors can halve for every 10C rise in temperature over 20C. He might have limited it to 10W to ensure the company had an acceptable failure rate within the warranty period.

    It could be berkeman is right and that the designer limited it to 10W to ensure the output voltage stayed within the design tolerance. However that doesn't mean the power supply voltage definitely will drop when slightly overloaded.
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