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Homework Help: Electric field between 2 objects and electric force on object?

  1. Aug 2, 2013 #1
    1. The problem statement, all variables and given/known data

    Suppose two metal plates are separated by 1.00 cm. These plates are wide:
    at least 0.5 m square. We hook wires from a battery to each plate, so that there is a
    potential of 500 volts between them.

    2. Relevant equations

    (A) Find the electric field between the plates, in
    the middle where the field is quite uniform.

    (B) Suppose we introduce a small object
    which has +2.0 nC (nanocoulombs) of charge on it, placing it between the plates, near
    the middle where you just found the electric field. Determine the electric force on this

    3. The attempt at a solution


    I'm guessing I use the formula VAB = Ed.

    Step 1

    1.00 cm converted to meters = 0.01

    Step 2

    VAB = Ed.

    E = 5.0 x 104 V/m.

    Does the width of the plate have any influence though?


    I can't figure out how to do part (B)

    I'm thinking I use the formula F = qE.

    Step 1

    +2.0 nanoCoulomb's converted to Coulombs = +2.0 x 10-9 C

    Step 2

    F = qE.
    F =(2.0 x 10-9 C)(5.0 x 104 V/m)
    F = 1.0 x 10-4 N

    Again does width of the plate have any influence?
    Last edited: Aug 2, 2013
  2. jcsd
  3. Aug 3, 2013 #2


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    Homework Helper

    Welcome to PF!

    The electric field is uniform far from the edges of the plates, where the electric field lines are uneven and curved. In this problem, the plates are wide so the area near the edges is small compared to the total. The edge effect does not influence the surface charge density far from the edges, consequently, neither the electric field in the middle.


    Attached Files:

  4. Aug 3, 2013 #3

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Looks good to me - though the force and the electric field are both vectors and you have only given magnitudes. For completeness, you should say something about direction.

    The setup describes a parallel plate capacitor - the width of the plates does affect the physics but not the bit you need: that is already accounted for in the electric field. The only effect is that the field is not uniform near the edge.

    Note: +1nC charge in a 5V/m field experiences a 5nN force :)
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