A graph of the $x$ component of the electric field as a function of $x$ in a region of space is shown. The $y$ and $z$ components of the electric field are zero in this region. If the electric potential is $10 \mathrm{V}$ at the origin, then the potential at $x=2.0 \mathrm{m}$ is 

 

An electron travelling in a uniform electric field passes from a region of potential $V_1$ to a region of higher potential $V_2$. Then

Three concentric conducting spherical shells carry charges as follows: $+4Q$ on the inner shell, $-2Q$ on the middle shell and $-5Q$ on the outer shell. The charge on the inner surface of the outer shell is