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Homework Help: Calculation of power produced by an electric fish

  1. May 29, 2016 #1
    1. The problem statement, all variables and given/known data
    An electric fish is capable of generating a voltage of 220V. This drives a pulse of 15A lasting for 2ms. The fish produces 200 pulses in one second. What is the average power output?

    2. Relevant equations

    3. The attempt at a solution
    At first glance I thought we just have to multiply 220 with 15 using the equation P=VI. But that doesn't give the answer. So at another glance I was convinced that 200 is also supposed to be multiplied because in one second 200 such "power" is produced. But that still doesn't gives the answer. Can anyone explain what am I missing?
    Last edited: May 29, 2016
  2. jcsd
  3. May 29, 2016 #2
    Try Q=It for charge, then use E=QV and then do P=E/t?
  4. May 29, 2016 #3
    What am I supposed to do with 2ms?
  5. May 29, 2016 #4
    That is the time t. 2 milliseconds.
  6. May 29, 2016 #5
    So why cant I just apply P = VI?
  7. May 29, 2016 #6
    I'm not sure. Maybe because it says "average" power. When I get questions like this that give me loads of information, I deduce that I probably need to use all of it and P=IV doesn't use all of it. I don't know why P=IV doesn't work, maybe someone else can enlighten you, but if you want to get the right answer in exams/homeowork/coursework maybe try to use all the information given (unless there are many parts to the question).
  8. May 29, 2016 #7
    That wasn't really an explanation...
  9. May 29, 2016 #8
    No, it wasn't an explanation because I don't know why you can't use P=IV. I was just trying to help you to get similar questions correct in the future.
  10. May 29, 2016 #9
    I also used the same trick and multiplied everything and got the answer correct. I want to know the logic behind it
  11. May 29, 2016 #10
    Ok maybe it should be visualized that the fish produces pulses all over his body, at the same time....

    So the pulses are "averaged out" during one second, such that 200 pulses are during each second.

    Nowhere does it say, that the pulses go in a straight line...

    pulse 1, pulse 2 etc...

    pulses 1-5 could begin at the same time during 0,02 seconds (20ms)
  12. May 29, 2016 #11
    Okay then what does 2ms has to do with anything?
  13. May 29, 2016 #12
    the fish actually produces 4 seconds worth of electrical current I believe, in each second.

    Some of the fishes pulses are happening simulatenously ( yet those pulses actually happen within that one second of measured time)

    The time affects the electric energy or electric work.

    I suspect you could then calculate power after wards

    E = I * t*U
    current x time x voltage = work

    watts should be work done / time in seconds

    try if that give correct result
  14. May 29, 2016 #13


    User Avatar

    Staff: Mentor

    Consider the total energy produced over a full second. Each individual pulse lasts only 2 ms, and 200 of them don't total a full second so there is some "dead time" in there. Hence the need to look at the average over time.
  15. May 30, 2016 #14
    Ok the moderator removed my post...

    I think that true time duration for one pulse is only 2 milliseconds like it says in the problem statement.

    I misread originally that it was 20 ms

    If there are 200 pulses in each second.

    Then it is known that one pulse has duration of 0.002 seconds.

    Try to work from there according to darkmatter5's instruction
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