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Energy transfer in a bulb

  1. Apr 5, 2008 #1
    1. The problem statement, all variables and given/known data

    An electric bulb is marked 120 V, 40 W.
    (a) Calculate the current in this bulb when operating on a 120 V supply.
    (b) Describe 3 different ways in which energy can be transferred from the filament when it is operating normally.
    (c) If the light bulb is now connected to a 240 V supply it will work for perhaps an hour as a 160 W bulb before its filament breaks.
    (i) How will the relative importance of the answers you have given to (b) change when the bulb is run on this increased voltage?
    (ii) Suggest a reason why this bulb can operate for so long a time before its filament breaks.

    2. Relevant equations

    3. The attempt at a solution

    (a) i calculated the current using P = IV

    (b) the different ways are : conduction, convection and radiation.

    (c)(i) I can't understand what the question asks

    (ii) I am thinking that maybe it's because the increase in current is small. the current flowing increases from 0.33A to 0.66A
  2. jcsd
  3. Apr 5, 2008 #2

    Gib Z

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    Homework Helper

    c) is asking how the importance of each conduction, convection and radiation, change when the bulbs voltage increases. For example, perhaps it will begin to emit more energy as heat rather than light?
  4. Apr 5, 2008 #3
    ah well, i suppose the heat transferred by all of these ways will increase because the current increases?! is it right?
  5. Apr 5, 2008 #4
    Have you learned anything about the black body emission, and how it depends on temperature?
  6. Apr 6, 2008 #5
    errmm no. i'm going to google that. thanks
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