A small metal ball is suspended in a uniform electric field with the help of an insulated  thread. If high energy $X$ -ray beam falls on the ball, then the ball

An oil drop carries six electronic charges, has a mass of $1.6 \times 10^{-12} g$ and falls with a terminal velocity in air. The magnitude of vertical electrical electric field required to  make the drop move upward with the same speed as was formely moving is ........$kN/C$

Four charges $q, 2q, -4q$ and $2q$ are placed in order at the four corners of a square of side $b$. The net field at the centre of the square is

A hollow sphere of charge does not produce an electric field at any

Three charges are placed as shown in figure. The magnitude of $q_1$ is $2.00\, \mu C$, but its sign and the value of the charge $q_2$ are not known. Charge $q_3$ is $+4.00\, \mu C$, and the net force on $q_3$ is entirely in the negative $x-$ direction. The magnitude of $q_2$ is

A pendulum bob of mass $30.7 \times {10^{ - 6}}\,kg$ and carrying a charge $2 \times {10^{ - 8}}\,C$ is at rest in a horizontal uniform electric field of $20000\, V/m$. The tension in the thread of the pendulum is $(g = 9.8\,m/{s^2})$