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I Heating effects of different lightbulbs

  1. Jun 17, 2017 #1
    Suppose I have two light bulbs, an LED and a halogen, but both have the same power P. Their radiation spectra are different, the LED radiating much less IR. Putting them in identical closets from which no heat may escape, will the walls and air in each come to some temperature T which, although increasing, will be the same for both?
  2. jcsd
  3. Jun 17, 2017 #2


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    What do you think?

    First can I check what you mean by "power P" ?

    Is it..

    a) The same power going into each light?
    b) The same amount of visible light emitted by each light?
  4. Jun 17, 2017 #3
    both consume and radiate the same power
    what do I think? I think that there would be a time when both were at the same temperature ("equilibrium" between walls and air) and that temperature would be increasing. I think it possible that one would reach that equilibrium more quickly than the other.
  5. Jun 17, 2017 #4

    Vanadium 50

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    That can't be true (in the visible, the question you were asked) given your previous statement.

  6. Jun 17, 2017 #5
    I do not understand that. A bulb which is rated at 13 W, e.g. means that perhaps 8 W is radiated as visible light, 5 W as heat (IR). Whatever, whatever the bulb consumes must equal the energy radiated or else energy will be being stored somehow in the bulb.
  7. Jun 17, 2017 #6


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    F Todd: You are correct. Conservation of energy means all bulbs consume as much as they emit overall. That's why in b) I asked if you meant equal _visible_ output. (A 500 lumen halogen consumes more power than a 500 Lumen LED).

    If the input power is the same then I would expect both boxes to behave more or less identically. However there might be minor differences. For example a halogen will be hotter than an LED so more of the power will leave the bulb by conduction/convection rather than radiation - so the temperature gradient between bulb and outside will be different (I think).
    Last edited: Jun 17, 2017
  8. Jun 18, 2017 #7
    Sadly, the experiment will not run for very long as the LED lamp will overheat and fail long before the halogen lamp maxes out...
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