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Homework Help: Why does a light bulb burn out?

  1. Mar 25, 2007 #1
    1. The problem statement, all variables and given/known data

    The problem is that I cannot figure out why a filament will burn out in a particular spot. Higher resistance = lower current why does the filament heat up more?

    2. Relevant equations

    R2= R1[1+a(T2-T1)]
    Resistance = (resistivity * L)/ A
    R= V/I

    3. The attempt at a solution

    I know that tungsten oxide evaporates form the filament when a critical temperature is reached. This causes the cross sectional area of the filament to decrease and increases resistance. Increased resistance results in increased temperature. The filament gives off more heat as the cross sectional area decreases. This could be because the current is reduced so this excess energy has to go somewhere so it is converted to heat energy and radiated from the filament. I just don't understand how a resistor dissipates heat and why the light bulb 'choses' to burn out in a certain spot.
  2. jcsd
  3. Mar 25, 2007 #2


    User Avatar
    Gold Member

    I'm not very knowledgable about such things but it seems to me you have answered your own question.

    "...tungsten oxide evaporates form the filament when a critical temperature is reached..."
    "...Increased resistance results in increased temperature..."
    and finally:
    "...tungsten oxide evaporates form the filament when a critical temperature is reached..."

    i.e. it's a runaway process.

    Whatever spot starts thinning first is the spot that will burn out first. A theoretically perfectly manufactured filament will evaporate evenly until the whole filament burns out in a puff.
  4. Mar 25, 2007 #3
    so how exactly does a resistor decrease current and convert energy to heat energy? what is happening on a molecular level?
  5. Mar 25, 2007 #4
    Thermal energy is generated via electronic collisions with the scattering center (fixed lattice atoms). At one position in the lattice you may have an energetic electron collide with the fixed lattice, this can change the structure of the lattice at that point which will interrupt the flow of electrons in that region. With this "defect" now in place, other electrons will collide with the defect further altering the current path, etc.
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