Active Reading Solution: Small System, Big Reservoir

  1. If System S has energy ε, that leaves ε of energy for the reservoir. The reservoir must therefore be in a state with 999 0-energy atoms and 1 ε-energy atoms. That one excited atom could be any of the 1000 atoms, so that's 1000 microstates.

  2. If System S has energy 2ε, that leaves no energy for the reservoir. The reservoir must therefore be in a state with all of its atoms in their ground states, which is only one possible microstate.