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Calculating the voltage and electric field strength between two parallel plates-again

  1. Dec 29, 2009 #1
    1. The problem statement, all variables and given/known data

    - How much is the electrical voltage U between two evenly electrified parallel flat plates, distanced d = 1 cm, if the strength of electric field between them is E = 1 V / m?
    - What is the strength of electric field to the left and to the right of each plate?
    - What is the force on the surface (area) unit between the two plates?
    - What are the voltage between the plates and the force on the surface unit as a result of doubling the distance between the two plates?
    - How is the strength of the electric field and force between the two plates altered (changed), if the voltage is maintained constant?
    Influencing constant is ε(0) = 8.85 × 10^-12 As / Vm.

    2. Relevant equations

    V= E*d
    F= Q*E
    V= Qd/ Aε(0)
    E= Q/ Aε(0)
    Q= CV

    3. The attempt at a solution

    First part: How much is the electrical voltage U between two evenly electrified parallel flat plates?

    V= 1 V/m* 0.01 m= 0.01 V

    Second part: What is the strength of electric field to the left and to the right of each plate?

    E= V/ d
    E= 0.01 v/ 0.01 m= 1 V/m

    Third part: What is the force on the surface unit between the two plates?

    E= ½ Q*V → Q= 2E/ V
    Q= 200 C

    F= Q*E= 200 N

    Fourth part: What are the voltage between the plates and the force on the surface unit as a result of doubling the distance between the two plates?

    d= 2 cm= 0.02 m

    V= d*E= 0.02 V
    Q= 2E/ V= 100 C
    F= Q*E= 100 N

    ARE MY CALCULATIONS CORRECT?

    Fifth part: How is the strength of the electric field and force between the two plates altered (changed), if the voltage is maintained constant?

    I need some help with this part. Any hints?

    Thank you for helping!
     
  2. jcsd
  3. Dec 29, 2009 #2
    Re: Calculating the voltage and electric field strength between two parallel plates-a

    The second part: The field between the plates is 1 V/m, but the field outside the plates isn't

    Third part: The E in [itex] E = \frac{1}{2}QV [/itex] is energy, the E in [itex] F = Q E [/itex] is the strength of the electric field, so this calculation is wrong. You need the force per square meter anyway. It's easiest to assume the area of the capacitor is 1 [itex] m^2 [/itex] and then to compute the capacity and the charge and then use [itex] F = Q E [/itex]

    Fourth part: wrong for the same reason as third part.

    Fifth part: if the voltage is the same, the electric field strength, wich is volt/meter must change
    if the distance changes. use V = E * d.
     
  4. Dec 29, 2009 #3
    Re: Calculating the voltage and electric field strength between two parallel plates-a

    Thank you for helping, I will just have to do additional :smile:calculating!
     
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