# Struggling With Part C of Electric Field Calculation

• johnio09

#### johnio09

Homework Statement
Consider a solid conducting sphere with a radius 1.5 cm and charge -4.4pC on it. There is a conducting spherical shell concentric to the sphere. The shell has an inner radius 3.7 cm and outer radius 5.1 cm and a net charge 27.4 pC on the shell. A) denote the charge on the inner surface of the shell by Q'2 and that on the outer surface of the shell by Q ''2 . Find the charge Q''2. Answer in units of pC. B) Find the magnitude of the electric field at point P, midway between the outer surface of the solid conducting sphere and the inner surface of the conducting spherical shell. Answer in units N/C. C) Find the potential V at point P. Assume the potential at r = infinity. Answer in units of volt.
Relevant Equations
E =kQ/r^2
V = kQ/r
I've figured out parts A and B but I'm struggling with Part C. I used the equation V = kQ1/r1 + kQ2/r2 where Q1 = -4.4e-12C ; k = 8.98755e9 r1 = 0.026 m Q2 = 27.4e-12 and r2 = .051-.026 My answer (8.329 V) is wrong but I have no idea why. Please help if you can.

My answer (8.329 V) is wrong but I have no idea why.
I have no idea either why your answer 8.329 V is wrong because my mind-reading abilities are not what they used to be. I can't help you find what's wrong unless you post what you did and how you got that answer.

• berkeman
Hello @johnio09 , ##\qquad## !​

I've figured out parts A and B
Perhaps you can enlighten us ?

wrong but I have no idea why
How do you know it's wrong ? Because the book answer is different ?

Perhaps you used the wrong equation ? What's the idea behind it ?

Several ways out are feasible. My advice: make a sketch of ##|E| ## vs ##r##.

Oh, and do read the guidelines . Follow them as best you can and we'll get along just fine !

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