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  1. #1
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    Physics capacitor question...

    Hi guys i'm pretty new here and i'm just looking for abit of help really. I'm attempting my physics assignment and i can't for the world of me figure out how to work this one out and searching online for equations is just making it more confusing for me! The question is below.

    A 4700 uF capacitor is charged to a p.d. of 10v. When the capacitor is discharged through a small electric motor, the motor lifts a weight of 0.1N through a height of 0.12m.
    I need to work out the energy stored in the charged capacitor - I've done this one I think! E = 1/2 CV^2 which is 1/2 * 4700 * 100
    The gravitational potential energy gained by the weight
    The efficiency of the energy transfer - This one i think i can do!

    Any help at all would be greatly appreciated so thanks in advance!!
    Last edited by Xiah; 6th May 2011 at 14:31.

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  2. #2
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    Re: Physics capacitor question...

    You have overlooked "uF". Stored energy in the cap= 0.5*4700e-6*100 =235 mJ (millijoule)
    0.1N does 0.1*0.12=12mJ much work.
    e=12/235=0.05 (!)


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  3. #3
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    Re: Physics capacitor question...

    Wtransfer is the energy transferred = Work physical
    Wloss = energy lost in the transfer
    Total energy = Wloss + Wtransfer = Work Capacitor
    Efficiency = Wtransfer/total energy

    Work (physical) = .1N*.12M =.012Nm
    Work Capacitor = 0.5*4700E-6*100 = 0.235 Joules = .235 Nm
    Wloss = Wcapacitor=Wphysical = .235-.012 = .223
    Efficiency = .012/.235 = .051 = 5%

    Work (physical) = F*d
    http://en.wikipedia.org/wiki/Work_(physics)
    Work Capacitor = 0.5CV^2
    Capacitance - Wikipedia, the free encyclopedia
    I Joule = 1N*1m
    Joule - Wikipedia, the free encyclopedia


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  4. #4
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    Re: Physics capacitor question...

    I thought the gravitational potential energy gained by the weight would be = mgh 0.1 * 9.8 * 0.12 = 0.1176?

    Thank you both so much for your help by the way!



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    Re: Physics capacitor question...

    It´s a weight of 0.1N not a mass of 0.1Kg
    Already was multiplied by 9.8 ( weight = mg )



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  6. #6
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    Re: Physics capacitor question...

    yes _Eduardo_ you are correct



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    Re: Physics capacitor question...

    Much appreciated thankyou :)



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