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Watt is going on?

  1. Apr 26, 2012 #1
    Ok, I'm preparing to get shot down here because maybe this is a really dumb question but here we go...

    How does an electric current know to supply more energy (joules per second) to higher wattage appliances?

    For example:

    At home I can plug a 10W, 50W, 60W or 100W light bulb into a socket and they all work and emit different brightnesses. Why?

    What is it about the 100W light bulb that makes it draw more power? It is the resistance? If so does it have more or less than a 60W? Iif it has less R then surely it wouldn't get so hot and therefore so bright and if it had more R then, well, I can't understand why that would make it bright either.

    Have I just asked the most stupid question ever on Physics Forum? If anyone can help me clear this up that would be great.

  2. jcsd
  3. Apr 26, 2012 #2
    V = IR and Power = VI . Does it answer your question ?

    P.S And this is the worst explanation ever :P
    Last edited: Apr 26, 2012
  4. Apr 26, 2012 #3


    User Avatar

    Staff: Mentor

    You are right in saying that the lower resistance device draws more current. With the house voltage being the same in each case, then the lowest resistance bulb is delivered the most power, i.e., the most heat energy, so it's able to produce the most light energy.
    If you lower the resistance then it draws more current. Power = voltage x current

    The reason different metal resistances glow differently is almost entirely due to the temperature of their surface, the hotter each tiny glowing area on the surface, the whiter (or bluer) it glows. To cause a small flashlight bulb's filament to glow about as white as does the filament in your large 100W bulb, despite the greatly reduced power in the smaller bulb, the filament is wound smaller and is much thinner and shorter in order to concentrate the heat energy in a small volume of filament metal and raise the local surface temperature to nearly that of the local surface temperature of the larger bulb's much larger filament. The rarified gases in the bulb are designed to reduce the rate at which heat escapes from the bulb, meaning less electrical energy is needed to maintain the high temperature of the glowing filament.

    Compare the incandescent light bulb with the heating element on your electric stove. Your stove element is a much lower resistance so draws much greater current and generates a lot more heat energy. But the stove element has much greater volume (it's very thick and also much longer) than the filament in any light globe so the heat energy distributed over the element's much greater surface area fails to raise the local surface temperature much above a dull red glow. The heat is not concentrated in a small space and so fails to raise the temperature nearly as much as happens in a light bulb, before the element's heat escapes to warm the metal of the saucepan resting on it.

    I hope that helps you to make the resistance power heat temperature connection. i0om5.gif

  5. Apr 27, 2012 #4
    It is better to remain silent and be thought a fool than to speak out and remove all doubt. Jokes apart, stupid is one who had a question but made no effort to get an answer. Feel free to ask more questions.
  6. Apr 27, 2012 #5
    remember: "the only stupid question is the one you never asked"
    Last edited: Apr 27, 2012
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