  When the magnetic field has a parallel component to the current density ${\bf
J}$ there appear force-free effects due to flux cutting and crossing. This
results in an anisotropic ${\bf E}({\bf J})$ relation, being ${\bf E}$ the
electric field. Understanding force-free effects is interesting not only for
the design of superconducting power and magnet applications but also for
material characterization. This work develops and applies a fast and accurate
computer modeling method based on a variational approach that can handle
force-free anisotropic ${\bf E}({\bf J})$ relations and perform fully three
dimensional (3D) calculations. We present a systematic study of force-free
effects in rectangular thin films and prisms with several finite thicknesses
under applied magnetic fields with arbitrary angle $\theta$ with the surface.
The results are compared with the same situation with isotropic ${\bf E}({\bf
J})$ relation. The thin film situation shows gradual critical current density
penetration and a general increase of the magnitude of the magnetization with
the angle $\theta$ but a minimum at the remnant state of the magnetization
loop. The prism model presents current paths with 3D bending for all angles
$\theta$. The average current density over the thickness agrees very well with
the thin film model except for the highest angles. The prism hysteresis loops
reveal a peak after the remnant state, which is due to the parallel component
of the self-magnetic-field and is implicitly neglected for thin films. The
presented numerical method shows the capability to take force-free situations
into account for general 3D situations with a high number of degrees of
freedom. The results reveal new features of force-free effects in thin films
and prisms.
