In electromagnetics, we encounter serious problems when node-based elements are used to represent vector electric or magnetic fields. (See Electric Scalar Potential.) Node-based elements require special treatment for enforcing boundary conditions of electromagnetic field at material interfaces, conducting surfaces and geometric corners. Tangentially continuous vector elements or edge elements, whose degrees of freedom are associated with the edges of the finite element mesh, have been shown to be free of such shortcomings. ([414])
The tetrahedral element is the simplest tessellated shape and is able to model arbitrary 3-D geometric structures. It is also well suited for automatic mesh generation. The tetrahedral element, by far, is the most popular element shape for 3-D applications in FEA.
For the 1st-order tetrahedral element (KEYOPT(1) = 1), the degrees of freedom (DOF) are at the edges of element i.e., (DOFs = 6) (Figure 11.21: 1st-Order Tetrahedral Element). In terms of volume coordinates, the vector basis functions are defined as:
 | (11–278) |
 | (11–279) |
 | (11–280) |
 | (11–281) |
 | (11–282) |
 | (11–283) |
where:
| hIJ = edge length between node I and J |
| λI, λJ, λK, λL = volume coordinates (λK = 1 - λI - λJ - λL) |
 λI,
 λJ,
 λK,
 λL = the gradient of volume coordinates |
The tangential component of the approximated field is constant along the edge. The normal component of field varies linearly.
Tangential vector bases for hexahedral elements can be derived by carrying out the transformation mapping a hexahedral element in the global xyz coordinate to a brick element in local str coordinates.
For the 1st-order brick element (KEYOPT(1) = 1), the degrees of freedom (DOF) are at the edges of element (DOFs = 12) (Figure 11.22: 1st-Order Brick Element). The vector basis functions are cast in the local coordinate
 | (11–284) |
 | (11–285) |
 | (11–286) |
where:
| hs, ht, hr = length of element edge |
 s,
 t,
 r = gradient of local
coordinates |
