# Struggling With Part C of Electric Field Calculation

• johnio09
In summary, the conversation is about the struggle with Part C of a problem involving 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. The answer of 8.329 V was determined to be wrong without an explanation as to why. Advice was given to make a sketch and to read the guidelines for further assistance.
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.

johnio09 said:
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 ,

johnio09 said:
I've figured out parts A and B
Perhaps you can enlighten us ?

johnio09 said:
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 !

##\ ##

## What is the common mistake when calculating the electric field in Part C?

A common mistake is neglecting vector components and directions. It's crucial to break down the electric field into its x, y, and z components and ensure that you are summing these vectorially, not just algebraically.

## How do I handle multiple charges in Part C of the electric field calculation?

When dealing with multiple charges, calculate the electric field due to each charge separately, considering their positions and magnitudes. Then, use vector addition to find the net electric field at the point of interest.

## Why is my calculated electric field magnitude incorrect?

Ensure that you are using the correct formula, $$E = k \frac{q}{r^2}$$, where $$E$$ is the electric field, $$k$$ is Coulomb's constant, $$q$$ is the charge, and $$r$$ is the distance from the charge to the point of interest. Double-check your distance calculations and unit conversions.

## How do I account for the direction of the electric field in Part C?

The direction of the electric field is along the line connecting the charge and the point of interest, pointing away from positive charges and toward negative charges. Use unit vectors to properly represent the direction in your calculations.

## What tools or techniques can help simplify Part C of the electric field calculation?

Using symmetry can greatly simplify calculations. For example, in cases with symmetrical charge distributions, certain components of the electric field may cancel out. Additionally, vector calculus tools and software like MATLAB or Python can help manage complex calculations.

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