The Weak Form PDE node is the default node on the top geometric entity level in The Weak Form PDE Interfaces and may also be added to The Coefficient Form PDE Interfaces and The General Form PDE Interfaces. It contains one weak form expression for each dependent variable in the interface (see Equation 16-9).

Enter the weak expressions that COMSOL Multiphysics (together with any other weak expressions on the same domain) sets equal to 0 in the weak field. For example, in a 2D Component model with one dependent variable, the default expression on the domain level is -test(ux)*ux-test(uy)*uy+1[m^-2]*test(u). This is the weak formulation of Poisson’s equation with the right-hand side f = 1. On other geometric entity levels, the default weak expression is 0.