# Predefined Variables

A new model contains a list of predefined variables by default.

A predefined variable may be deleted or modified and only have an effect if you explicitly refer to that variable.

 c0 The speed of light in free space in m/sec. eps0 The permittivity of free space in F/m. mu0 The permeability of free space in H/m. pi The mathematical constant $\pi$ (Ludolph’s number). zf0 The characteristic impedance of free space in Ohm.