  We present the energetic aspect of open systems which may exchange particles
with their environments. Our attention shall be paid to the scale that the
motion of the particles is described by the classical Langevin dynamics. Along
a particular realization of the stochastic process, we study the energy
transfer into the open system from the environments. We are able to clarify how
much energy each particle carries when it enters or leaves the system. On the
other hand, the chemical potential should be considered as the concept in macro
scale, which is relevant to the free energy potential with respect to the
number of particles. Keywords: open systems, stochastic energetics, chemical
potential
