Kinetic energy of a relativistic massive point charge in a constant and uniform electric field
$$\frac{mv^2}{2}=\frac{mc^2}{2}(1-e^{-2\frac{\Delta Uq}{mc^2}})$$
The kinetic energy is derived under the assumption that Coulomb's force is proportional to $$\ 1-v^2/c^2$$
For simplicity, I took the case when there is no initial velocity.
Accelerated particle is emitted from the origin
Derivation:
From Newton's Second Law: $$m\frac{dv}{dt}=qE(1-v^2/c^2)$$
Here $$E$$ - the electric field intensity
We integrate:
$$\frac{dv}{(1-v^2/c^2)}=\frac{qE}{mc}dt$$
$$\frac{v}{c}=th(\frac{qE}{mc}t)$$ [1]
Once again, we integrate
$$\frac{dx}{dt}=c\ th(\frac{qE}{mc}t)$$
$$x=\frac{mc^2}{qE}\ ln\{ch(\frac{qE}{mc}t)\}$$
Next we do recalculate the electric field intensity into a potential difference
$$\Delta U=xE$$
$$\Delta Uq=mc^2\ ln\{ch(\frac{qE}{mc}t)\}$$
$$e^{\frac{\Delta Uq}{mc^2}}=ch(\frac{qE}{mc}t)$$
We use the equation [1], to eliminate the time
$$\frac{mv^2}{2}=\frac{mc^2}{2}th^2(\frac{qE}{mc}t)=\frac{mc^2}{2}\frac{e^{2\frac{\Delta Uq}{mc^2}}-1}{e^{2\frac{\Delta Uq}{mc^2}}}=\frac{mc^2}{2}(1-e^{-2\frac{\Delta Uq}{mc^2}})$$
$$\frac{mv^2}{2}=\frac{mc^2}{2}(1-e^{-2\frac{\Delta Uq}{mc^2}})$$
The kinetic energy is derived under the assumption that Coulomb's force is proportional to $$\ 1-v^2/c^2$$
For simplicity, I took the case when there is no initial velocity.
Accelerated particle is emitted from the origin
Derivation:
From Newton's Second Law: $$m\frac{dv}{dt}=qE(1-v^2/c^2)$$
Here $$E$$ - the electric field intensity
We integrate:
$$\frac{dv}{(1-v^2/c^2)}=\frac{qE}{mc}dt$$
$$\frac{v}{c}=th(\frac{qE}{mc}t)$$ [1]
Once again, we integrate
$$\frac{dx}{dt}=c\ th(\frac{qE}{mc}t)$$
$$x=\frac{mc^2}{qE}\ ln\{ch(\frac{qE}{mc}t)\}$$
Next we do recalculate the electric field intensity into a potential difference
$$\Delta U=xE$$
$$\Delta Uq=mc^2\ ln\{ch(\frac{qE}{mc}t)\}$$
$$e^{\frac{\Delta Uq}{mc^2}}=ch(\frac{qE}{mc}t)$$
We use the equation [1], to eliminate the time
$$\frac{mv^2}{2}=\frac{mc^2}{2}th^2(\frac{qE}{mc}t)=\frac{mc^2}{2}\frac{e^{2\frac{\Delta Uq}{mc^2}}-1}{e^{2\frac{\Delta Uq}{mc^2}}}=\frac{mc^2}{2}(1-e^{-2\frac{\Delta Uq}{mc^2}})$$