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HW: What is energy used? What is wattage?

  1. Feb 6, 2009 #1
    Cutnell Physics 7th Ed pg 600 # 39

    1. The problem statement, all variables and given/known data

    A camera flash has a capacitor.
    Capacitor has a rating of: 850uF
    The potential difference between the places is: 280V

    a) Determine Energy used to produce flash.
    b)If flash lasts 3.9E-9 S, what is the power or wattage of the flash?

    2. Relevant equations

    V=C/F
    W=VA
    A=C/S

    3. The attempt at a solution

    a) (280V)(850uF)= 0.238C

    b) (0.238 C)/(3.9E-3 S)= 61.03A
    W=???

    The back of the book has a) =33J b) 8500W
    Where did I go wrong?

    Thanks in advance.
     
  2. jcsd
  3. Feb 6, 2009 #2

    rl.bhat

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    Energy = 1/2*C*V^2
    Power = E/t
     
  4. Feb 7, 2009 #3

    LowlyPion

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