Show that the fermion kinetic contribution \(\mathcal{L}_{\text {kin-ferm }}\) in Eq. (9.4.20) can be rewritten in terms of the physical vector boson fields as Eq. (9.4.22) with the electroweak currents \(J_{W}^{\mu \pm}, J_{Z}^{\mu}\) and \(J_{\mathrm{em}}^{\mu}\) given in Eq. (9.4.23).