1. The question "why it -j?" likely refers to the use of the imaginary unit in electrical engineering or complex numbers. 2. In mathematics and engineering, the imaginary unit is denoted as $j$ or $i$, where $j = \sqrt{-1}$. 3. Electrical engineers prefer to use $j$ instead of $i$ to avoid confusion with current, which is commonly denoted by $i$. 4. The negative sign in front of $j$ (i.e., $-j$) often appears in formulas involving phase shifts or complex conjugates. 5. For example, in AC circuit analysis, $-j$ represents a phase shift of $-90^\circ$ or a rotation in the complex plane. 6. So, "why it -j?" means the negative imaginary unit is used to indicate direction or phase in complex numbers, especially in engineering contexts.