a) A solid sphere, made of an insulating material, has a volume charge density of  ? = a/r,  where r is the radius from the center of the sphere, a is constant, and  a > 0.  What is the electric field within the sphere as a function of the radius r? Note: The volume element dV for a spherical shell of radius r and thickness dr is equal to 4?r2dr. (Use the following as necessary: a, r, and ?0.)     b.) What If? What if the charge density as a function of r within the charged solid sphere is given by  ? = a/r^2?  Find the new magnitude and direction of the electric field within the sphere at radius r. (Use the following as necessary: a, r, and ?0.)

Physics for Scientists and Engineers: Foundations and Connections
1st Edition
ISBN:9781133939146
Author:Katz, Debora M.
Publisher:Katz, Debora M.
Chapter25: Gauss’s Law
Section: Chapter Questions
Problem 43PQ: The nonuniform charge density of a solid insulating sphere of radius R is given by = cr2 (r R),...
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a)
A solid sphere, made of an insulating material, has a volume charge density of 
? = a/r,
 where r is the radius from the center of the sphere, a is constant, and 
a > 0.
 What is the electric field within the sphere as a function of the radius rNote: The volume element dV for a spherical shell of radius r and thickness dr is equal to 4?r2dr. (Use the following as necessary: ar, and ?0.)
 
 
b.)
What If? What if the charge density as a function of r within the charged solid sphere is given by 
? = a/r^2?
 Find the new magnitude and direction of the electric field within the sphere at radius r. (Use the following as necessary: ar, and ?0.)
 
Expert Solution
Step 1

a)

ρ = volume charge density = a/r

small charge enclosed is given as

dq = ρ dV

dq = (a/r ) ( 4πr2dr )

dq = 4π a r dr

total charge is given as

Q = \int_{0}^{r}4π a r dr

Q = 4π a r2/2

using gauss's law

E A = Q/\epsilon _{o}

E (4π r2) = 4π a r2/(2\epsilon _{o})

E = a/(2\epsilon _{o})

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