Four charges each equal to $(-Q)$ are placed at the four corners of a square and a charge $q$ is placed at its centre. If the system of charges is in equilibrium, the value of $q$ is

Figure shows three arrangements of electric field lines. In each arrangement, a proton is released from rest at point A and then accelerated through point B by the electric field. Points A and B have equal separations in the three arrangements. If $p_1,p_2$ and $p_3$ are linear momentum of the proton at point B in the three arrangement respectively, then

An electron is placed on $X$-axis where the electric potential depends on $x$ as shown in figures (the potential does not depend on $y$ and $z$ ). What is the electric force on the electron?