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Homework Help: Temperature of Bulb Filament

  1. Feb 1, 2009 #1
    1. The problem statement, all variables and given/known data

    A common flashlight bulb is rated at 0.3 A and 3.3 V (the values of current and voltage under operating conditions). If the resistance of the bulb's tungsten filament at room temperature (20.0 degrees Celsius) is 1.10 Ohms , estimate the temperature of the tungsten filament when the bulb is turned on.

    Take the temperature coefficient of resitivity for tungsten to be 4.5e10-3

    2. Relevant equations

    R = V/I

    3. The attempt at a solution
    R = 3.3V / 0.3A = 11 Ohms when turned on

    Little lost on how to find the temperature when turned on.
  2. jcsd
  3. Feb 1, 2009 #2


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    What are the units on that coefficient of resistivity? Perhaps we can make up a formula relating it to temperature and resistance.
  4. Feb 1, 2009 #3
    I'm curious about a custom I see universally in this forum. It's been quite a few years since I was in school and I wonder if this custom indicates a change in the way EE is being taught nowadays. Traditionally E instead of V has been used to represent electromotive force just as I represents current instead of A. In the past a distinction has been made between the unit of electromotive force and its designator.

    Are instructors and textbooks now using V for electromotive force?
  5. Feb 1, 2009 #4


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    How much does the resistance of the filament change?

    With that Δ in resistance perhaps you can figure the Δ in temp needed to develop that much resistance?
  6. Feb 1, 2009 #5


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    I remember that, Skeptic! I've been in high school for 30 years and all our books and government exams just used V for electric potential, which is a bit confusing leading inevitably to saying "Voltage" instead of "potential". But P and E are busy already so V is a relief.
  7. Feb 1, 2009 #6
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