Three equal charges are placed at the corners of an equilateral triangle. Which of the graphs below correctly depicts the equally-spaced equipotential surfaces in the plane of the triangle? (All graphs have the same scale.)

   

A non-conducting ring of radius $0.5 \mathrm{m}$ carries total charge of $1.11\times {10}^{-10} \mathrm{C}$ distributed non-uniformly on its circumference producing an electric field everywhere in space.
The value of the line integral $\underset{\mathrm{l}=\mathrm{\infty }}{\overset{\mathrm{l}=0}{\int }}-\overrightarrow{E}·\mathrm{d}\overrightarrow{l}$

( $l=0$ being centre of the ring) in volt is

A solid spherical conducting shell has inner radius $a$ and outer radius $2a$. At the center of the shell is located a point charge $+Q$. What must the excess charge of the shell be in order for the charge density on the inner and outer surfaces of the shell to be exactly equal?