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Q. An imaginary, closed spherical surface $S$ of radius $R$ is centered on the origin. A positive charge $+q$ is originally at the origin and electric flux through the surface is $\phi_{E}$. Three additional charges are now added along the x axis: $−3q$ at $x=-\frac{R}{2}, +5q$ at $x=\frac{R}{2}$ and $4q$ at $x=\frac{3R}{2}$, The flux through S is now

UPSEEUPSEE 2016

Solution:

According to Gauss law, charge $=+q$
Electric flux, $\phi_{E}=\frac{q_{\text {inside }}}{\epsilon_{0}}=\frac{q_{0}}{\epsilon_{0}}$
Total charge inside the spherical Gaussian surface of radius $R$ is
$q_{\text {inside }} =(-3 q)+5 q +q$
$=-3 q+6 q$
$=3 q$
$\therefore $ Flux through $S$
$\phi=\frac{3 q}{\epsilon_{0}}=3 \phi_{E}$