  Using data obtained for twelve galaxies as part of the {\it Spitzer} Infrared
Nearby Galaxies Survey (SINGS) and the Westerbork Synthesis Radio Telescope
(WSRT)-SINGS radio continuum survey, we study how star formation activity
affects the far-infrared (FIR)--radio correlation {\it within} galaxies by
testing a phenomenological model, which describes the radio image as a smeared
version of the FIR image. The physical basis of this description is that
cosmic-ray (CR) electrons will diffuse measurably farther than the mean free
path of dust-heating photons before decaying by synchrotron radiation. This
description works well in general. Galaxies with higher infrared surface
brightnesses have best-fit smoothing scale-lengths of a few hundred parsecs,
substantially shorter than those for lower surface brightness galaxies. We
interpret this result to suggest that galaxies with higher disk averaged star
formation rates have had a recent episode of enhanced star formation and are
characterized by a higher fraction of young CR electrons that have traveled
only a few hundred parsecs from their acceleration sites in supernova remnants
compared to galaxies with lower star formation activity.
