  The flexoelectric conversion of mechanical to electrical energy in nematic
liquid crystals is investigated using continuum theory. Since the electrical
energy produced cannot exceed the mechanical energy supplied, and vice-versa,
upper bounds are imposed on the magnitudes of the flexoelectric coefficients in
terms of the elastic and dielectric coefficients. For conventional values of
the elastic and dielectric coefficients, it is shown that the flexoelectric
coefficients may not be larger than a few tens of pC/m. This has important
consequences for the future use of such flexoelectric materials in devices and
the related energetics of distorted equilibrium structures.
