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Question

The intensity of an electric field depends only on the coordinates x and y as follows:
E=a(x^i+y^j)x2+y2
where, a is a constant and ^i and ^j are the unit vectors of the x and y axes. Find the charge within a sphere of radius R with the centre at the origin.

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Solution

We know that equation of sphere with origin as centre is x2+y2+z2=R2

At any point P(x,y,z) on the sphere a unit vector perpendicular to the sphere radially outwards is
^n=xx2+y2+z2^i+yx2+y2+z2^j+zx2+y2+z2^k=xR^i+yR^j+zR^k

Let us find the electric flux passing through a small area dS at point P on the sphere,
dϕ=E.^ndS={ax2R(x2+y2)+ay2R(x2+y2)}=(aR)dS

Here, we note that dϕ is independent of the coordinates x,y and z. Therefore, total flux passing through the sphere is
ϕ=dϕ=aRdS=(aR)(4πR2)
ϕ=4πaR
From Gauss's law,
ϕ=qinϵ0
(4πaR)=qinϵ0
qin=4πϵ0aR

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