Electric and magnetic pressure cancellation in a matched TEM line ($f_E=\epsilon a{V}_{\mathrm{rms}}^2/(2d^2)={\mu }_0{I}_{\mathrm{rms}}^2/(2a)=f_B$)

In a matched lossless parallel-plate transmission line, a single travelling wave obeys $V/I=Z_0=(d/a)\sqrt{{\mu }_0/\epsilon }$. The electric attraction per unit length is $f_E=\epsilon a{V}_{\mathrm{rms}}^2/(2d^2)$; each plate feels the field of the other plate, excluding its self-field. Opposite sheet currents give magnetic repulsion $f_B={\mu }_0{I}_{\mathrm{rms}}^2/(2a)$. Substituting the travelling-wave impedance proves their equality, so the ideal net transverse electromagnetic force is zero at every phase. A standing wave or independent voltage and current need not have this cancellation.