"math notranslate nohighlight">\(\vec {dl} = (dr, dz)\)</span>, the rate of change of the flow covered by this circle can be found from the second Maxwell equation <span class="math notranslate nohighlight">\(\nabla\vec {B} = 0.\)</span> Thus,</p>
<div class="math notranslate nohighlight">
\[\dot\psi = 2\pi r (-B_r\dot z + B_z\dot r).\]</div>
<p>After integration over time, from the above equations we obtain the following expression:</p>
<div class="math notranslate nohighlight">
\[\dot\theta = (-\frac {q} 