Magnetic forces do no work
= Magnetic forces do no work
{title2=$(q\mathbf v\times\mathbf B)\cdot\mathbf v=0$}
The magnetic part of the <Lorentz force> is perpendicular to the particle <velocity>. Its power is therefore zero, so with no other forces the <kinetic energy> remains constant. This remains true relativistically because $dE/dt=\mathbf F\cdot\mathbf v$; a magnetic force can change momentum direction without changing total energy.