*Question E5C01:
Which of the following represents pure capacitive reactance of 100 ohms in rectangular notation?*  

*Correct Answer is  A: 0 - j100*  

### AI Response: (may contain errors)  

To understand the representation of pure capacitive reactance in rectangular notation, we need to recall how reactance is expressed in complex numbers.

1. **Correct Answer Explanation:**
   - The correct answer is **A: 0 - j100**.
   - In rectangular notation, a complex number is expressed as $X + jY$, where $X$ is the real part and $Y$ is the imaginary part.
   - For capacitive reactance, the imaginary part is negative. Therefore, a capacitive reactance of $-j100$ ohms is represented as $0 - j100$.

2. **Why Other Choices are Incorrect:**
   - **B: 0 + j100**: This represents an inductive reactance of $+j100$ ohms, which is incorrect for a capacitor.
   - **C: 100 - j0**: This represents a purely resistive component of 100 ohms, which does not apply to capacitive reactance.
   - **D: 100 + j0**: This also represents a purely resistive component of 100 ohms, which is not relevant to capacitive reactance.

3. **Useful Formulas and Rules of Thumb:**
   - Capacitive reactance ($X_C$) is given by the formula:
     $$X_C = -\frac{1}{2\pi f C}$$
     where:
     - $f$ = frequency in hertz
     - $C$ = capacitance in farads
   - The negative sign indicates that capacitive reactance is represented in the negative imaginary part of the complex plane.

This understanding of reactance will help you correctly identify and represent capacitive and inductive components in future questions.