For a photon gas, this quantity is proportional to volume times temperature to the third powertimes 
For a photon gas, this quantity is proportional to volume times temperature to the third powertimes the zeta function of 3, and unlike for an ideal gas this quantity is not conserved. Fock statesare defined by a constant value for this quantity, and for a given state the operator for thisquantity is equal to the product of the creation and annihilation operator for that state. Thisdimensionless quantity is conjugate to the (*) chemical potential, and the chemical potential isequal to the derivative of free energy with respect to this parameter; in other words, the chemicalpotential is the change in energy when this quantity is increased by one. For 10 points, name thisquantity, which for one mole of substance is equal to Avogadro's number.
For a photon gas, this quantity is proportional to volume times temperature to the third powertimes the zeta function of 3, and unlike for an ideal gas this quantity is not conserved.
particle number [accept any answer along the lines of "number of particles" or "number of atoms" or "number of molecules"]