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Can someone show mw the working out here

  1. Oct 3, 2014 #1
    Q = C (90pF) x V (25 volts)
    90 x 10e-12
    x 25 = 2.25 x 10e-9 Coulombs (2.25nC)
    I get this equation for charge but my maths is letting me down.
    How do I get from 90pF (90 to power of -12) x 25 = 2.25 x 10 to power of -9
    I don't know the working out with negative exponents. I may have to duplicate in a test so I need a simple way.
    This subject is doing my head in - the chemistry, physics and maths does not come naturally...grrr
     
  2. jcsd
  3. Oct 4, 2014 #2

    Mentallic

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    90pF is not 90 to the power of -12 but rather 90 times 10 to the power of -12.

    So you're trying to figure out why

    [tex]90*10^{-12}*25=2.25*10^{-9}[/tex]

    90*25 = 2250 so

    [tex]90*25*10^{-12}=2250*10^{-12}[/tex]
    [tex]=225*10^{-11}[/tex]
    [tex]=22.5*10^{-10}[/tex]
    [tex]=2.25*10^{-9}[/tex]
     
  4. Oct 4, 2014 #3
    Thanks - especially for pointing out my error. My notes have the 10 and that's what I've been working with, but you've shown how easily I can leave something vital out. This really isn't my area and I'll be glad when this subject is done - at least the physics, maths and chemistry part. The subject is high risk hazards in the workplace.
    Your workings are exactly what I needed, just a simple way to get to the end!
    Thanks so much.
    Back to dealing with the physics of deceleration in free fall and soft landings. Yay!
     
  5. Oct 5, 2014 #4

    Mentallic

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    I can understand how you could be eager to leave those subjects behind you when every piece of maths that you have to calculate gives you trouble and possibly intimidates you as well. But at least try not to let the subjects leave a sour taste in your mouth as you move on from them. Maths really is elegant in its logic, even if the logic can sometimes be hard to come by.

    Even simpler (although I don't recommend it because you should learn to become comfortable with using the index notations as they pop up everywhere) would be to avoid converting to indices altogether.
    90pF * 25 = 2250pF = 2.25nF
    Since 1000pF = 1nF.

    Sounds fun! ;)
     
  6. Oct 5, 2014 #5

    SteamKing

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    Working with negative exponents is no different than working with positive exponents. If you can add and subtract negative numbers, you're all set to work with negative exponents. The laws of exponents work the same:

    2-2 * 2-4 = 2-2-4 = 2-6 = 1 / 26 = 1/64

    24 * 2-2 = 24-2 = 22 = 4

    If you continue to study physics and chemistry, you are also going to need to be proficient at working in scientific notation.
     
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