The average thermal energy for a monoatomic gas is (where kB is Boltzmann constant and T is absolute temperature):
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The average thermal energy for a monoatomic gas is (where kB is Boltzmann constant and T is absolute temperature):

G1 Physics · Accepted Answer

Step 1: A monoatomic ideal-gas molecule has three translational degrees of freedom. Step 2: Equipartition theorem gives average energy (1/2)kBT per quadratic degree of freedom. Step 3: Therefore ⟨E⟩ = 3 × (1/2)kBT = (3/2)kBT. Final answer: (A).