  A simplified model has previously described the inductive charging of
colliding identical grains in the presence of an external electric field. Here
we extend that model by including heterogeneous surface charge distributions,
grain rotations and electrostatic interactions between grains. We find from
this more realistic model that strong heterogeneities in charging can occur in
agitated granular beds, and we predict that shielding due to these
heterogeneities can dramatically alter the charging rate in such beds.
