A charged particle of charge $Q$ is held fixed and another charged particle of mass $m$ and charge $q$ (of the same sign) is released from a distance $r$. The impulse of the force exerted by the external agent on the fixed charge by the time the distance between $Q$ and $q$ becomes $2r$ is

In the figure shown the electric potential energy of the system is ($q$ is at the centre of the conducting neutral spherical shell of inner radius $a$ and outer radius $b$)

A point charge $Q$ is placed at the centre of a spherical cavity of radius $b$ carved inside a solid conducting sphere of radius $a$. Then the total energy of the system is   $\left[k=\frac{1}{4\pi {\epsilon }_0}\right]$

Four charges are fixed along the $y$-axis as shown. A positive charge approaches the system along the $x$-axis with initial speed just enough to cross the origin. Then its total energy at the origin is

Three charges $+q,-q$ and $+2q$ are placed at the vertices of a right-angled triangle (isosceles triangle) as shown. The net electrostatic energy of the configuration is