4 . Two circular disks spaced at 500 mm apart form a parallel plate capacitor. W

Question

4. Two circular disks spaced at 500 mm apart form a parallel plate capacitor. When 3.00 x 109 electrons

are transferred from the left to the right plate,

(initially both neutral) causes the electric field

strength to be 2.00 x 105 N/C.

a) Calculate the potential difference between

the plates.

Which plate is at the higher potential?

b) Calculate the radius of each disk.

c) How much potential energy is stored in the

capacitor?

d) The space between the disks is next filled up with

an oil of dielectric constant K=4. What are the new

values of the potential difference between the plates

and the energy stored by the capacitor?

Explanation / Answer

Charge in left plate = -1* 3*10^9 * -1.602 x 10-19 C = + 4.86*10^-10 C

Charge in right plate = - 4.86*10^-10 C

a) Potential difference = V = E*d = 2*10^5 * 0.5m =10^5 Nm/C

The +ve plate is at higher potential.

b) Q/V = epsion * A / d

A = d * Q/ ( epsilon * V) epsilon = 8.85 × 10-12

Substituting the values we get, A = 2.74*10^-4 m2.

C) Potential Energy U = QV/2

U = 2.43*10^-5 J = 24.3 microJ

D) Vnew = V/k = 2.5*10^4 Nm/C
   Unew = U/k = 6.075 microJ

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