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Homework Help: Two electricity problems

  1. Mar 12, 2006 #1
    Hi, I don't get how to do this at all because I missed the lesson so could someone please help me? I appreciate it a lot!

    First problem:

    http://img55.imageshack.us/img55/9403/physics8tl.jpg [Broken]

    a) if the switch closes, what is i1? i1 is current at that position

    b) graph v2 versus t graph

    c) after a long time, if the switch opens, what is i2?

    d) calculate Q (charge) of the capacitor

    e) calculate U (potential energy) of the capacitor

    Second problem:

    There is a capacitor with two plates with very large surface area. They are set d distance apart and have equal but opposite charge densities.

    a) diagram the current through the plates (no idea how to do this).

    b) If a dielectric is added, does the electric potential increase, decrease, or stay the same? What mechanism causes this effect? (I think it goes down because of the q=cv formula but i dunno what mechanism).

    Thanks a lot!
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Mar 12, 2006 #2


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    Homework Helper

    I honestly don't know how much you know and how much you don't know due to this 'missed lesson'. If you missed a lesson, you should get some sort of help or read up on your own. You would just be copying blindly otherwise.
  4. Mar 12, 2006 #3


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    Indicated current (I) flows from + side of battery, along the wires, to the - side of the battery. At a capacitor, the + charges accumulate on the + plate, and the E-field from them chases (the same number of) charges off the - plate.

    As the current flows through a resistor, charges lose PE (friction-dominated), according to dV = I R .

    Now, at time t=0, what is the potential difference across the capacitor?

    question 2 part a : there is an E-field between the plates, but NO current.
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