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Some physics qustions

  • 12-05-2008 06:35PM
    #1
    sei046
    Registered Users, Registered Users 2 Posts: 2,110 ✭✭✭


    Explain briefly why and electric charge builds up on the inner plates of a paralell plate capacitor.


Comments

  • #2
    ApeXaviour
    Registered Users, Registered Users 2 Posts: 5,523 ✭✭✭ApeXaviour


    Hey sei046.
    The system is electrically biased.

    In a very basic woolly sense, you have one side that is connected to the positive terminal of a battery, and the other connected to the negative terminal. The battery pulls electrons from the plate connected to the positive terminal, leaving it positively charged. On the negative side there are an abundance of electrons, which are repelled from the negative terminal of the battery, but attracted to the positively charged plate. But, they cannot cross the bridge (dielectric) between the plates, so they collect on the plate connected on the negative plate.

    Sort of like this bad bad attempt at ascii diagram:
    |+   -|
               |+   -|
+ _____________|+   -|_____________ -
               |+   -|
               |+   -|
               |+   -|

    That an okay explanation? Were there any other questions?


  • #3
    sei046
    Registered Users, Registered Users 2 Posts: 2,110 ✭✭✭sei046


    Dead on man cheers, I had a fairly vague understanding but I did not know how to put it into words.

    lets say a dust particle has a charge of -2nC and a mass of 4x10^-9 kg and gets trapped in between the potential of two point charge, equal and opposite.

    lets say the dust had an inital velocity of 10^6 m/s


    SO


    (+5nC)
    ( 2nC )>>(in. vel)>>
    (-5nC)

    <
    1cm
    >
    <
    5cm
    >


    Describe the 1d motion of the dust particle ( ie uniform motion, uniformly accelerated motion) The succesive phases?


  • #4
    ApeXaviour
    Registered Users, Registered Users 2 Posts: 5,523 ✭✭✭ApeXaviour


    Interesting question! The acceleration is not constant, which rules out the use of plain aul vf = vi + a*t.

    The acceleration is position/time dependent. You can derive it's dependence from the coulomb force:
    a = f/m = E*q/m etc. right?

    After that calculus is required (or at least preferred):
    avari.gif
    From: http://hyperphysics.phy-astr.gsu.edu/hbase/avari.html

    Good luck, :) it looks like it'll get a bit messy tbh. If you really can't get through it for some reason, say why and if I've time, I'll give it a shot tomorrow.


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