  Several physical models have recently been proposed to obtain unidirectional
motion of an overdamped Brownian particle in a periodic potential system. The
asymmetric ratchetlike form of the periodic potential and the presence of
correlated nonequilibrium fluctuating forces are considered essential to obtain
such a macroscopic motion in homogeneous systems. In the present work, instead,
inhomogeneous systems are considered, wherein the friction coefficient and/or
temperature could vary in space. We show that unidirectional motion can be
obtained even in a symmetric nonratchetlike periodic potential system in the
presence of white noise fluctuations. We consider four different cases of
system inhomogeneity We argue that all these different models work under the
same basic principle of alteration of relative stability of otherwise locally
stable states in the presence of temperature inhomogeneity.
