Patch test for thermal-electrical elements

$T=C_0+C_{1}x+C_{2}y+C_{3}z$, where T is the temperature, $C_0$ through $C_3$ are arbitrary constants, and x, y, z denote spatial location. $\varphi =D_0+D_{1}x+D_{2}y+D_{3}z$, where $\varphi$ is the electrical potential, $D_0$ through $D_3$ are arbitrary constants, and x, y, z denote spatial location. Temperature and electrical potentials are prescribed at every node along the boundary of the mesh.