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Instantaneous Power calculation

  1. Apr 7, 2016 #1
    1. The problem statement, all variables and given/known data
    The energy of a system increases at a rate of 3.5t + 6.2t^2, in joules.
    What is the instantaneous power at t=3.1 s?

    2. Relevant equations

    3. The attempt at a solution
    dW=3.5 + 12.4t
    P=(3.5 + 12.4t joule) / (3.1s)=1.12 + 4t, in watts

    Yes/ no???
  2. jcsd
  3. Apr 7, 2016 #2

    Ray Vickson

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  4. Apr 7, 2016 #3


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    Staff: Mentor

    No, it's dW/dt = 3.5 + 12.4t

    Why did you divide by 3.1? What units would your answer have?
  5. Apr 7, 2016 #4


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    no, they are asking for the instantaneous power at t=3.1

    why would the power at 3.1 secs be dependent on time?

    1 watt does equal 1 joule per second.

    I'll give you a few hints.
    The average power dissipated during a time period is energy/ the time period. What happens when you decrease that time period to a very small value?

    another hint. To find energy simply sum (integrate) the power. how do you go the other way?
  6. Apr 7, 2016 #5
    So i just need to derive the equation and plug in 3.1s?
    I divided by 3.1 because instantaneous power is derivative of work w/ respect to time.
  7. Apr 7, 2016 #6
    which i now realize i was thinking of W/Δt.. oops
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