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Homework Help: Efficiency of a Lightbulb

  1. Jun 30, 2013 #1
    The problem statement, all variables and given/known data

    "A light bulb is 85% efficient. If it is supplied with 60J of electrical energy, how much of this energy is transferred to its surroundings as light energy?

    The attempt at a solution

    So I calculated that 85% of 60 is 51J, however I don't know if being "efficient" refers to energy that remains in the circuit, or energy that is released as light energy, seeing as this is the main purpose of a light bulb? I'm thinking it's the first, which would mean 9J is lost as light energy?
  2. jcsd
  3. Jun 30, 2013 #2
    I think it's safe to assume that the useful energy in this case is the light being emitted from the bulb. Which would mean that 51 J of light energy is emitted. The other energy doesn't "remain in the circuit" it is lost to heat.
  4. Jun 30, 2013 #3
    Ah ok! Thank you very much :)
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