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sonutulsiani
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Homework Statement
A sphere of radius R has volume charge density = B/r for r < R, where B is a constant and = 0 for r > R. (a) Find the total charge on the sphere. (b) Find the expressions for the electric field inside and outside the charge distribution. (c) Sketch the magnitude of the electric field as a function of the distance r from the sphere’s center
Homework Equations
The Attempt at a Solution
I got the charge by integrating (4 pi B r dr) over 0 to R as (2 pi B [R^2]). For the part b, that's where I am confused.The solution that I have and the answer that I got are different as the approach is different.
What I have done is:
Q/(Q inside) = (4/3 pi R^3)/(4/3 pi r^3). Substituting Q in this equation from above and using the Gauss' law, I got E = (B r)/(2 R ε0)
But in the solution:
For r<R, they have just simply substituted Qinside = (2 pi B [r^2]) and got E = B/(2 ε0)
For r>R, they have just simply substituted Qinside = (2 pi B [R^2]) and got E = [B(R^2)]/[2 (r^2) ε0]
What here I am confused is 2 things.
1. How does the total charge become the inside charge as the solution says.
2. Notice the Q inside for r<R, it has small r but r>R has big R.