  Willis dynamic homogenization theory revealed that the effective linear
momentum of elastic composites is coupled to their effective strain. % This
result, which is partially due to asymmetry at the subwavelength scale, implies
that spatially-uniform but time-varying stress can induce temporal variations
in the macroscopically observable momentum. Recent generalization of Willis'
dynamic homogenization theory to the case of piezoelectric composites further
revealed that their effective linear momentum is also coupled to the effective
electric field. Here, we introduce the simplest possible model -- a
one-dimensional discrete model -- that exhibits this so-called electromomentum
coupling in subwavelength composites. We utilize our model to elucidate the
physical origins of this phenomenon, illustrate its mechanism, and identify
local resonances which lead to elevated Willis- and electromomentum coupling in
narrow frequency bands. The results provide intuitive guidelines for the design
of this coupling in piezoelectric metamaterials.
