Physics help please Exam soon?

Question

Two circular disks spaced 0:50 mm apart form a parallel-plate capacitor. Transferring 3.0 x10^9 electrons from one disk to the other causes the electric eld strength to be 2.0x10^5 N=C. What are the diameters of the disks?

Answer 1

Distance between parallel plates,d = 0.5 mm

                                                        = 0.5*10^-3 m

Electric field strength,F=2*10^5 N

Number of electrons = 3*10^9 

Charge,q=6.2*10^-19*3*10^9 C

               =18.6*10^-10 C

Area,A=(pi/4)D^2

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Electric field strength

F=Eq

E=F/q

   =2*10^5/18.6*10^-10

   =0.1075*10^15 N/C

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Using capacitance formulae

q/Ed = e_0A/d

_A=q/Ee₀

_Where

e_{0 =Permittivity of empty space =8.854*10^-12 C^2/N-m^2}

(pi/4)D^2=(18.6*10^-10 C)/(0.1075*10^15 N/C)(8.854*10^-12 C^2/N-m^2)

(pi/4)D^2=19.54*10^-7

D^2=2.4876*10^-6

D=0.001577 m

   =1.577 mm

Diameter of the disk ,D=1.577 mm