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    anishsaro's Avatar
    anishsaro Posts: 3, Reputation: 1
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    #1

    Feb 3, 2014, 11:09 AM
    Capacitance question
    A photographic flash unit consists of a capacitor previously charged by a 100V source. The average power delivered to the flash tube is 2000W and the flash lasts 0.040s. The capacitance of the capacitor is:
    A 40 x 10^-6 F
    B 80 x 10^-6 F
    C 160 x 10^-6 F
    D 80 x 10^-3 F
    E 160 x 10^-3 F
    Cat1864's Avatar
    Cat1864 Posts: 8,007, Reputation: 3687
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    #2

    Feb 3, 2014, 11:16 AM
    We won't do your homework or answer a test question for you. If you tell us what you think the answer is and why, we can help you.
    anishsaro's Avatar
    anishsaro Posts: 3, Reputation: 1
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    #3

    Feb 3, 2014, 11:20 AM
    A photographic flash unit consists of a capacitor previously discharged by a 100V source. The average power delivered to the flash tube is 2000W and the flash lasts 0.040s. The capacitance of the capacitor is?
    Cat1864's Avatar
    Cat1864 Posts: 8,007, Reputation: 3687
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    #4

    Feb 3, 2014, 11:24 AM
    Quote Originally Posted by anishsaro View Post
    A photographic flash unit consists of a capacitor previously discharged by a 100V source. The average power delivered to the flash tube is 2000W and the flash lasts 0.040s. The capacitance of the capacitor is?
    So...what's your answer?
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #5

    Feb 3, 2014, 11:27 AM
    I'm sure your text has an equation for energy stored in a capacitor as a function of its capacitance C and voltage V. Hint: Remember that power times time is equal to energy.
    anishsaro's Avatar
    anishsaro Posts: 3, Reputation: 1
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    #6

    Feb 3, 2014, 02:31 PM
    NOTE: it is 1000V.. not 100V

    Actually, I found the way out! I only didn't think of the equation for energy... Energy = 0.5*C*(V)^2

    i.e; using W = Pt to find work done in joules
    then equating it to Energy = 0.5*C*(V)^2 as work done = energy
    therefore, C = Pt/(0.5*(V)^2)

    C = 160 x 10^-6 F
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #7

    Feb 3, 2014, 02:54 PM
    Quote Originally Posted by anishsaro View Post
    NOTE: it is 1000V.. not 100V

    ...

    C = 160 x 10^-6 F
    Good! I was a bit worried because none of the answers worked for the data you originally provided.
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