Three charges $Q,\, + q$ and $ + q$ are placed at the vertices of a right-angled isosceles triangle as shown. The net electrostatic energy of the configuration is zero if $Q$ is equal to

Two equal charges $q$ are placed at a distance of $2a$ and a third charge $ - 2q$ is placed at the midpoint. The potential energy of the system is

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 distance between $Q$ and $q$ becomes $2r$ is

A particle of mass $m$ and charge $q$ is placed at rest in a uniform electric field $E$ and then released. The kinetic energy attained by the particle after moving a distance $y$ is

How much kinetic energy will be gained by an $\alpha - $particle in going from a point at $70\,V$ to another point at $50\,V$

Two identical particles of mass $m$ and charge $q$ are shot at each other from a very great distance with an initial speed $v$. The distance of closest approach of these charges is