Solution
= Solution
With $u^\mu=\gamma(c,\mathbf v)$ and $d/d\tau=\gamma d/dt$, the spatial components of the <relativistic Lorentz force> give
$$
\frac{d(\gamma m\mathbf v)}{dt}=q(\mathbf E+\mathbf v\times\mathbf B).
$$
The time component gives $mc^2\,d\gamma/dt=q\mathbf E\cdot\mathbf v$. It says <magnetic fields> do no work and electric power changes relativistic energy.