In a region of space an electric field line is in the shape of a semicircle of radius R. Magnitude of the field at all point is E. A particle of mass m having charge q is constrained to move along this field line. The particle is released from rest at A. Find its kinetic energy when it reaches point B.

$$As the electric field is tangential to each point on the field line, the electrostatic force acting on the charge will also be tangential to the field line at each point.

So for a small element on the semicircular field, if work energy theorem is applied, then

$dW=\overrightarrow{F}.\overrightarrow{dS}=q\overrightarrow{E}.\overrightarrow{dS}=qE.dS (as \theta =0^0)$

So integrating over the entire path

$W=K_B-K_A=K_B-0={\int }_0^{\pi R}qE.dS=qE.\pi R$