Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2022/ii/paper-3/35a/b/i/solution

For an ultrarelativistic particle, . Including the two spin states of an electron, counting wavevectors in a sphere gives the density of states
The grand canonical partition function of the ideal Fermi gas is therefore
Since the grand potential satisfies , integration by parts gives
Thus
This is the equation of state of an ideal ultrarelativistic gas.

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