# the local Lorentz force density is zero if there are no charges or currents present to act upon The Null Force Condition: The simulation visually confirms that the electromagnetic force is exclusively mediated through charges and currents . Even though the first two panels show non-zero Electric and Magnetic fields permeating the volume, the total force density in the third panel is identically zero, precisely because the source conditions are set to $$\rho=0$$ and J=0. This proves that fields only exert force where matter (charges or currents) exists to receive that force. {% embed url="" %} {% embed url="" %}