Theory of friction: Coulomb drag between two closely spaced solids

We calculate the friction force between two metallic bodies with flat surfaces separated by a vacuum slab of thickness $d,$ and moving with a relative velocity $v.\mathrm{}$ The separation $d$ is assumed to be so large that the only interaction between the bodies is via the Coulomb field. The friction force depends linearly on the velocity $v$ (for small $v$) and decays rapidly with increasing $d.\mathrm{}$ In most practical cases, the Coulomb drag makes a negligible contribution to the friction force, but it may make an important contribution in some special cases.