highlight">\(F_r\)</span> from the Lorentz force <span class="math notranslate nohighlight">\(\vec {F} = e\vec {E} +\displaystyle\frac {e} {c}\vec {\upsilon}\times\vec {B}\)</span>:</p>
<div class="math notranslate nohighlight">
\[F_r = eE_r -\frac {e\upsilon_z B_\alpha} {c} = eE_r (1-\displaystyle\frac {\upsilon ^ 2} {c ^ 2}) =\displaystyle\frac {eE_r} {\gamma ^ 2}.\]</div>
<p>It is useful to express the field through the current <span class="math notranslate nohighlight">\(I =\rho_0\upsilon\pi a ^ 2\)</sp