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A typical light bulb is 2% efficient. It means that only 2% of electric energy is converted into visible electromagnetic radiation and 98% is converted into ultraviolet radiation known as heat. Suppose a 100W bulb is turned on for 10h.

a. how much energy is used by the lamp? express in kWh.

b. how much energy is wasted due to heat?

c. suppose that the light bulb is turned on for a longer period of time. would you expect its efficiency to increase or decrease?

Please help me...i was absent from class the day we learned this...

  • physics....again -

    I should tell you that the opening statement is wrong. The other 98% percent is infrared (heat) radiation. There is almost no ultraviolet produced. The so called "typical" incandescent light bulbs are no longer in the majority, either.

    As far as the questions go, for #1, multiply watts by the number of seconds to get energy produced (in Joules). For #2, (heat only) take 98% of 100 W.

    c. Use common sense for #3. Or reread the first sentence of the question.

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