Electrostatics Question 736

Question: A system consists of a uniform charged sphere of radius R and a surrounding medium filled by a charge with the volume density ρ=αr, where α a positive constant is and r is the distance from the center of the charge. The charge of the sphere for which the electric field intensity E outside the sphere is independent of r is-

Options:

A) πR2α

B) 4πR2α

C) 2πR2α

D) 3πR2α/4

Show Answer

Answer:

Correct Answer: C

Solution:

[c] EpdS=Q+Qε0

Where Q’ is the charge outside the sphere

Q1RrdV=Rrαr×4πr2dr

=4πα(r22)r=4πα(r22R22)=2πα(r2R2)

Ep4πr2=Q+2πα(r2R2)ε0

Ep=Q4πr2ε0+α2ε0αR22r2ε0

E is independent of r if

Q4πr2ε0αR22r2ε0=0

Q=2πR2α



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