Solution
= Solution
The relative <orbital speed> is $Ze^2/(4\pi\epsilon_0\hbar n)$. Divide the circumference by this <speed> to find
$$
\boxed{T_n=\frac{4\epsilon_0^2h^3n^3}{Z^2\mu e^4}.}
$$
Positronium has twice the <hydrogen> period at the same <principal quantum number>. The symbol $n$ here labels principal levels with $n=1$ the <ground state>; if “nth <excited state>” is read literally as excitation count, replace $n$ by $n+1$.