Each of 2$N$ electrons (mass $m )$ is free to move along the $x$ -axis. The potential-energy function for each electron is $U(x)=\frac{1}{2} k^{\prime} x^{2},$ where $k^{\prime}$ is a positive constant. The electric and magnetic interactions between electrons can be ignored. Use the exclusion principle to show that the minimum energy of the system of 2$N$ electrons is $\hbar N^{2} \sqrt{k^{\prime} / m} .$ Hint: See Section 40.5 and the hint given in Problem $41.47 . )$