Discussion
Under parity and time reversal transformations, which of the following statements is(are) TRUE about the electric dipole moment p and the magnetic dipole moment μ?
Answer: A
The true statement is p is odd under parity and μ is odd under time reversal; it is the only one the key accepts in this multiple-select question.
The electric dipole moment \(\mathbf{p} = \sum q\mathbf{r}\) is built from position vectors. Parity sends \(\mathbf{r} \rightarrow -\mathbf{r}\), so \(\mathbf{p}\) is odd under parity; time reversal leaves positions unchanged, so \(\mathbf{p}\) is even under T.
The magnetic dipole moment comes from circulating currents or spin, \(\boldsymbol{\mu} \propto \mathbf{r} \times \mathbf{v}\) or \(\propto \mathbf{S}\), an axial vector like angular momentum. It is even under parity (both factors change sign) but odd under time reversal, since reversing time reverses every velocity and spin.
So 'μ is even under time reversal' and 'p is even under parity' are both false, which rules out the other three statements.
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