Drag At Light Speed

A particle in B orbit does not accelerate indefinitely,

$\pi q c B=\pi q E=Ac^2$

where $c$ is light speed and $Ac^2$ is drag of free space at light speed.  So,

$A=\cfrac{\pi qB}{c}=\cfrac{\pi qE}{c^2}$

At last, a simple expression for the drag factor $A$.