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Seniority of Wattage vs. Amperage

  1. Jun 14, 2012 #1
    Was wondering if I kept the wattage for say a 100 watt bulb constant but varied the voltage would the light be able to continue functioning normally?

    Also, assuming this is true does this hold for more complicated circuitry?

  2. jcsd
  3. Jun 14, 2012 #2
    You don't have that freedom. The bulb looks at the voltage and decides what current it will draw. The power is then VxI.
  4. Jun 14, 2012 #3
    The bulb sees 120 volts. We know it would draw about .83 amps, right? 120x.83 = 100

    The bulb sees 240 volts. Would it then draw half as many amps?

    The bulb sees 60 volts. Would it then draw twice as many amps?

    More generally, does varying the voltage affect the wattage of a light bulb?
    Last edited: Jun 14, 2012
  5. Jun 14, 2012 #4


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    Staff: Mentor

    Are you familiar with Ohm's Law? If you increase the voltage across a resistor, what does that do to the current through the resistor?

    I = V/R

    P = V^2/R = I^2 * R
  6. Jun 14, 2012 #5


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    Staff: Mentor

    Never anthropomorphise lightbulbs. They hate it when you do that. :tongue2:
  7. Jun 14, 2012 #6
    So if I wanted to keep 100 watts going to a bulb with 144 ohms of resistance and I doubled the voltage to 240 I'd need to add 432 ohms (576-144) worth of resistance to the circuit?

    So in theory I could run a 120v 100 watt light bulb on a 240v line if this were added: http://goo.gl/vy2Ig [Broken]

    Thanks for fielding my questions guys.
    Last edited by a moderator: May 6, 2017
  8. Jun 15, 2012 #7
    Don't, Don't and Don't again.

    Apart form the fact that what you propose will not work, This is a serious safety issue. 240 volt mains is seriously more dangerous than 120 volt mains.

    If you must run a 120 volt bulb from 240 then run two in series. This will work safely.
    Do not connect them in parallel, all you will achieve is two blown bulbs.

    Do you understand what series means?
    Last edited by a moderator: May 6, 2017
  9. Jun 15, 2012 #8
    Well is my math at least right?

    If so, why would it be dangerous aside from the extra voltage?

    Could you explain simply the diff between series and parallel?
  10. Jun 15, 2012 #9
    Instead of trying to tell experts (and there are quite a few more expert than I am) here how to do something, how about just explaining your goal ie what you want to achieve and asking for help.
  11. Jun 15, 2012 #10


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    Staff: Mentor

    You don't understand the difference between series and parallel circuits, and you are wanting to start off working with AC Mains circuits? That's not a good thing to do. Please learn the basics of electricity and electronics first, and then find a good local mentor who can help you safely learn about working with AC Mains circuits. The shock and fire hazards are very real when working with those kinds of voltages and that much available power.

    Here is your starter on series and parallel circuits:


  12. Jun 16, 2012 #11
    Berkeman you seem to have a short circuit between 'theoretical' and 'actual'. I can ask questions all day about 240v circuits without being silly enough to try to power a 100w light bulb off of it.
  13. Jun 16, 2012 #12


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    Staff: Mentor

    Two problems...

    First, you have not been clear that you do not intend to try any of this. We are genuinely concerned for your safety and those around you.

    Second, when you post stuff like that in the forums, other newbies can see it and think that they can do it safely. Not a good idea.

    We take safety seriously here at the PF. This thread is closed.
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