*Question G5A09:
What unit is used to measure reactance?*  

*Correct Answer is  B: Ohm*  

### AI Response: (may contain errors)  

The correct answer to the question about the unit used to measure reactance is **Ohm**.

1. **Why the correct answer is correct:**
   - Reactance is a measure of the opposition that a circuit presents to the flow of alternating current (AC) due to capacitance or inductance. It is measured in Ohms (Ω), the same unit used for resistance. The formula for reactance is:
     - For capacitive reactance ($X_C$):
       $$X_C = \frac{1}{2 \pi f C}$$
     - For inductive reactance ($X_L$):
       $$X_L = 2 \pi f L$$
   - Here, $f$ is the frequency in hertz, $C$ is the capacitance in farads, and $L$ is the inductance in henries. Both types of reactance are expressed in Ohms.

2. **Why the other choices are incorrect:**
   - **A: Farad**: This is the unit of capacitance, not reactance. It measures how much charge a capacitor can store per volt.
   - **C: Ampere**: This is the unit of electric current. It measures the flow of electric charge, not the opposition to it.
   - **D: Siemens**: This is the unit of conductance, which is the reciprocal of resistance. It measures how easily current can flow through a component, not the opposition to current flow.

3. **Useful formulas or rules of thumb:**
   - Remember that reactance is frequency-dependent:
     - Capacitive reactance decreases with increasing frequency, while inductive reactance increases with increasing frequency.
   - The total impedance ($Z$) in an AC circuit can be calculated using:
     $$Z = \sqrt{R^2 + X^2}$$
   - Where $R$ is resistance and $X$ is reactance (which can be either $X_C$ or $X_L$).

Understanding these concepts will help you grasp the role of reactance in AC circuits and prepare you for the Extra Class exam.