*Question G5A08:
What is impedance?*  

*Correct Answer is  C: The ratio of voltage to current*  

### AI Response: (may contain errors)  

Impedance is a fundamental concept in AC circuits, representing the opposition that a circuit presents to the flow of alternating current. It is a complex quantity that combines both resistance and reactance.

1. **Correct Answer: C - The ratio of voltage to current**

   - Impedance ($Z$) is defined as the ratio of voltage ($V$) to current ($I$) in a circuit. Mathematically, it is expressed as:
     $$Z = \frac{V}{I}$$
   - Impedance is measured in ohms ($\Omega$), similar to resistance, but it also accounts for the phase difference between voltage and current due to reactance.

2. **Why the other choices are incorrect:**

   - **Choice A: The ratio of current to voltage**
     - This describes conductance, not impedance. Conductance is the reciprocal of resistance and is given by $G = \frac{1}{R}$, where $R$ is resistance.
   
   - **Choice B: The product of current and voltage**
     - This describes power, specifically apparent power in an AC circuit, which is given by $S = VI$. It is measured in volt-amperes (VA).
   
   - **Choice D: The product of current and reactance**
     - This does not correctly describe impedance. Reactance is a component of impedance, but multiplying it by current does not yield impedance.

3. **Useful Formulas:**

   - Impedance in a purely resistive circuit: $Z = R$
   - Impedance in a purely inductive circuit: $Z = jX_L$, where $X_L = 2\pi fL$ (inductive reactance)
   - Impedance in a purely capacitive circuit: $Z = -jX_C$, where $X_C = \frac{1}{2\pi fC}$ (capacitive reactance)
   - In general, impedance is a complex number: $Z = R + jX$, where $R$ is resistance and $X$ is reactance.

Understanding impedance is crucial for analyzing and designing AC circuits, as it affects how circuits respond to different frequencies and how power is distributed within the circuit.