  If this system is very large, it is assumed that its temperature undergoes negligible changes from the absorption of small quantities of heat. It follows that if the physical volume is kept fixed, so that the only change in energy is through heating, then the heat and entropy are related by  ΔQ = T ΔSB.  Systems that satisfy these conditions may be considered to be good heat baths. A doubling in phase volume of the heat bath implies an increase in Boltzmann entropy of k ln 2, so requires heat absorption of kT ln 2.