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An atom with non-zero magnetic moment has an angular momentum of magnitude \(\sqrt{12}\,\hbar\). When a beam of such atoms is passed through a Stern-Gerlach apparatus, how many beams does it split into?

  • A.3
  • B.7
  • C.9
  • D.25

Answer: B

The beam splits into 7 beams.

The magnitude of an angular momentum with quantum number \(j\) is \(\sqrt{j(j+1)}\,\hbar\). Setting

\[ j(j + 1) = 12 \Rightarrow j = 3 \]

In a Stern-Gerlach apparatus the beam splits according to the projection \(m_{j}\), which takes the \(2j + 1 = 7\) values \(-3, -2, \ldots, 3\).

3 is the value of \(j\) itself, 9 is \((j)^{2}\) or the count for \(j = 4\), and 25 is \(2j + 1\) with 12 taken as \(j\); none gives the number of \(m_{j}\) states.

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