 external fields. It must be remembered that the value <span class="math notranslate nohighlight">\(\gamma =\gamma (t).\)</span></p>
<div class="math notranslate nohighlight">
\[\frac {d} {dt} (\gamma m\dot r) -\gamma m r (\dot\theta) ^ 2 = q (E_r + r\dot\theta B_z).\]</div>
<p>We apply the Bush theorem and the independence of <span class="math notranslate nohighlight">\(B_z\)</span> from <span class="math notranslate nohighlight">\(r:\)</span></p>
<div class="math notranslate nohighlight">
\[-\dot\theta =\