Gaussian Surface and Charge Density

mardi 30 juillet 2013

1. A solid dielectric sphere of radius 10 cm has an electric charge uniformly distributed throughout its volume. The electric field at 5 cm from the center of the sphere is 8.6 x 10^4 N/C, pointing radially outward. Calculate the magnitude and direction of the electric field at a point 15 cm from the center of the sphere.



2. Gaussian Law





3. I solved it like this:



E(4∏r2) = Q(r3/a3)/ ε0

E = Qr/4∏ε0a3

Q = 1.91 * 10^-7C

E = kQ/r2

= 76.4kN/C radically outward.



My question is what is the difference between doing it the above way versus with charge density:

Finding r < a (inside the sphere)

Equation: E(4∏r2) = 4/3(∏r3)ρ/ ε0

ρ = 4.5666 *10^-5



Find r > a (outside the sphere)

E(4∏r2) = 4/3(∏a3)ρ/ ε0

E = 76.4 kN/C radically outward






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