Potential Difference problem with sphere and capacitor

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

A thin spherical shell made of plastic carries a uniformly distributed negative charge -6e-10 coulombs (indicated as -Q1 in the diagram). Two large thin disks made of glass carry uniformly distributed positive and negative charges 1.5e-05 coulombs and -1.5e-05 coulombs (indicated as +Q2 and -Q2 in the figure). The radius R1 of the plastic spherical shell is 6 mm, and the radius R2 of the glass disks is 4 meters. The distance d from the center of the spherical shell to the positive disk is 18 mm.

(a) Find the potential difference V1 – V2. Point 1 is at the center of the plastic sphere, and point 2 is just outside the sphere.

(b) Find the potential difference V2 – V3. Point 2 is just below the sphere, and point 3 is right beside the positive glass disk.

Remember that the sign of the potential difference is important.

2. Relevant equations
Esphere = (1/4ε0)*(q/r2)
Ecapacitor = Q/A/ε0
ΔV = – (E*ΔL)

3. The attempt at a solution

Ecapacitor = Q/A/ε0
= (1.5e-5)/(4^2∏)/(8.85e-12) = 3.4e4 N/C

ΔV = – (E*ΔL)
= – (3.4e4 * .006) = -202.32 V/m


I don’t understand how to do part b. I really could use some help. Part A is correct.


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