1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Inf. Parallel Plate Capacitors and Potential Energy

  1. Mar 13, 2015 #1
    Hopefully no one minds that I post a couple questions in a row as long as I'm following the rules:

    1. The problem statement, all variables and given/known data

    Two infinite parallel plates labelled W and X are separated by 5.2 cm. The electric potential between the plates is 150 V. An electron starts from rest at time tW and reaches plate X at time tX. The electron continues through the opening and reaches point P at time tP

    c.) Find the electron’s kinetic energy and potential energy at point *P.
    d.) Find the minimum speed of the electron at time tw needed for it to escape to infinity.

    2. Relevant equations
    U = qV , energy conserved

    3. The attempt at a solution
    We can calc. the KE X easily: Ux = 0, Ex = Uw, Uw = 150V * -1.60*10^-19 C = 1/2 m(e) v^2 so we have v and there is no electric field once it is outside of the plates. A previous given answer indicates that the speed is constant once we hit X (I don't understand why other than that no outside forces are acting on the electron if we exclude gravity), so how can we calculate U and K at *P?
  2. jcsd
  3. Mar 14, 2015 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Do we know the initial position of the electron?
    If the electron is accelerated through a potential difference of V then it's kinetic energy is V electron-volts.

    Per your question: do you know how to calculate the potential due to a single plate?
    Do you know the superposition principle?
  4. Mar 14, 2015 #3
    Yes it is at rest at W.

    I know the EF is σ/2e0 everywhere due to each plate and you can find V by integrating E through a distance. And I know you can also find V by adding the potentials from each plate. But we don't now the Surface density of the plates.
  5. Mar 14, 2015 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    You also know the potential difference between the plates is 150V... this gives you the kinetic energy at X.
    Per the charge density - just put it equal to ##+\sigma## for the +ve plate and ##-\sigma## for the -ve plate.
  6. Mar 14, 2015 #5
    Right, outside of the PPC the EF is 0 everywhere, so the ΔV would have to be 0 right? That means the potential doesn't change so the KE can't change as well (at point X and beyond).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted