Let \(F^{\mu u}\) be the electromagnetic tensor and consider the symmetric stress-energy tensor defined in Eq. (3.3.9), \(\bar{T}^{\mu}{ }_{u}=-F^{\mu \tau} F_{u \tau}+\frac{1}{4} \delta^{\mu}{ }_{u} F^{\sigma \tau} F_{\sigma \tau}\). In the presence of an external conserved fourvector current density \(j^{\mu}\) show that \(\partial_{\mu} \bar{T}^{\mu u}=0\) becomes \(\partial_{\mu} \bar{T}^{\mu u}=(1 /

c) j_{\mu} F^{\mu u}\).