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

Choices:  
A: 0 - j100  
B: 0 + j100  
C: 100 - j0  
D: 100 + j0    

Correct Answer is  A: 0 - j100  

To represent impedance in rectangular form, use:
$$Z = R + jX$$
- $R$ is resistance (real part).
- $X$ is reactance (imaginary part).
- Inductive reactance is positive ($+jX$).
- Capacitive reactance is negative ($-jX$).

Why A is correct (0 - j100):
- “Pure capacitive reactance of 100 Ω” means no resistance ($R = 0$) and capacitive reactance of magnitude 100 Ω.
- Capacitive reactance is negative in rectangular form, so $Z = 0 - j100$.

Why the others are incorrect:
- B: 0 + j100 → Pure inductive reactance (not capacitive).
- C: 100 - j0 → Pure resistance of 100 Ω (no reactance).
- D: 100 + j0 → Pure resistance of 100 Ω (no reactance).

Useful formulas and rules:
- $X_L = 2\pi f L$ → inductive, appears as $+jX_L$.
- $X_C = -\dfrac{1}{2\pi f C}$ → capacitive, appears as $-j|X_C|$.
- Rule of thumb: Inductors are $+jX$; capacitors are $-jX$.