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Math Help - Calculating percent efficiency...

  1. #1
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    Calculating percent efficiency...

    Hi all, I had this question in my homework today...

    "An electric tea kettle consumes 576.2 kJ of electrical energy while heating a pot of water. By the time the water boils it has gained 381.4 kJ of heat.
    a) What is the percent efficiency of the conversion of electrical energy to heat done by the kettle?





    b) Why was there less energy absorbed by the water than was supplied by the kettle?"

    --

    For part A, I used the percent efficiency formula, and calculated the answer at 151%. I'm not sure if this is right...if it isn't, can someone please explain how to get the correct answer?
    I'm also unsure as to the answer for part B...

    Thanks in advance for any help, much appreciated.
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  2. #2
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    Re: Calculating percent efficiency...

    a. 100% efficiency is perfect. You can never be better than perfect. Maybe you have your formula backwards....

    b. 381 kJs of the 576 kJs went into the water. Where did the other 195 kJs go?
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  3. #3
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    Re: Calculating percent efficiency...

    I tried again with the values switched, and I got 66.2%, which seems to make a lot more sense!

    And I'm supposing it's because all the energy doesn't go into heating up the water, some would also go into the air or into other nearby objects.


    Thanks a lot!
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  4. #4
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    Re: Calculating percent efficiency...

    Good job on both a and b!
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