The three pages all discuss the Lorentz force density and its application to a source-free, static volume in electromagnetism. The first defines the Lorentz force density as the force per unit volume exerted on matter, mathematically expressed as $\mathbf{f}{\text{matter}} = \mathbf{\rho}\mathbf{E} + \mathbf{J} \times \mathbf{B}$, which comprises an electric force component ($\mathbf{\rho}\mathbf{E}$) and a magnetic force component ($\mathbf{J} \times \mathbf{B}$) . The other two then explain that the total electromagnetic force on the field inside a volume is zero if the volume is source-free ($\mathbf{\rho}=0 and \mathbf{J}=0$) and static ($\partial/\partial t = 0$), because the source-free condition immediately causes the Lorentz force density ($\mathbf{f}{\text{matter}}$) to vanish throughout the volume, meaning there is nothing for the field to push on, and thus the total integrated force is zero.
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