Assertion: Half of the ring is uniformly positively charged and other half uniformly negatively charged. Then, electric field and electric potential both are zero at centre.

Reason: At the centre of uniformly charged ring, electric field is zero.

Assertion: Spherical shell $A$ in the shown figure has a charge $-q$ and shell $B$ has charge $+q.$ When these two are connected by a thin conducting wire, the whole of the electrostatic potential difference between the shell is lost.

Reason: Whole of the inner charge will transfer to outer shell $B$.

Assertion: A charged sphere has a radius $R$. If at a distance $\frac{R}{2}$ from the surface towards centre, the potential is $V_1$ and a distance $\frac{R}{2}$ from the surface towards infinity, the potential is $V_2$ then $V_1>V_2$.

Reason: Inside a solid sphere, $V-r$ variation is parabolic. Where $r$ is the distance from centre?

Assertion: For a non-uniformly charged thin circular ring with net charge zero, the electric field at any point on axis of the ring is zero.

Reason: For a non-uniformly charged thin circular ring with net charge zero, the electric potential at each point on axis of the ring is zero.

Assertion: In a region where a uniform electric field exists, the net charge within a volume of any size is zero.

Reason: The electric flux within any closed surface in the region of a uniform electric field is zero.

Assertion: We have a solid charged sphere. There are two points $P$ and $Q\text{.} P$ is a point lying between centre and surface of the sphere. $Q$ is at finite and non-zero distance from the surface outside the sphere. There will be no situation where electric potential is equal at $P$ and $Q$.

Reason: Rate of change of electric field with distance from centre merely represents, magnitude of electric potential at that point.

Assertion: Cyclotron is used to accelerate heavy charged particles, such as a proton. It is not suitable for accelerating light charged particles such as electrons.

Reason: Specific charge of a proton is more than compared to the electron.

Reason: Net electrostatic potential at the centre is zero. 

Assertion: Inside a solid charged sphere $E-r$ variation is linear.

Reason: Inside that solid charged sphere $V-r$ variation is parabolic.