*Question G5C01:
What causes a voltage to appear across the secondary winding of a transformer when an AC voltage source is connected across its primary winding?*  

*Correct Answer is  C: Mutual inductance*  

### AI Response: (may contain errors)  

To understand why the correct answer is "Mutual inductance," let's break down the concepts involved:

### Correct Answer: Mutual Inductance
- **Explanation**: When an AC voltage is applied to the primary winding of a transformer, it creates an alternating magnetic field around the winding. This changing magnetic field induces a voltage in the secondary winding through the principle of mutual inductance. The relationship can be described by the formula:

$$
V_s = -M \frac{dI_p}{dt}
$$

where:
- $V_s$ = induced voltage in the secondary winding
- $M$ = mutual inductance between the primary and secondary windings
- $I_p$ = current in the primary winding

This shows that the induced voltage in the secondary is directly related to the rate of change of current in the primary winding.

### Why the Other Choices are Incorrect
- **A: Capacitive coupling**: 
  - Capacitive coupling occurs when an AC signal is transferred between two conductors through a capacitor. In transformers, the primary and secondary windings are magnetically coupled, not capacitively coupled. Therefore, this option does not explain the voltage induction in transformers.

- **B: Displacement current coupling**: 
  - Displacement current is a concept from Maxwell's equations that describes a changing electric field in a capacitor. It does not apply to transformers, which rely on magnetic fields for operation. Thus, this choice is not relevant to the operation of a transformer.

- **D: Mutual capacitance**: 
  - Mutual capacitance refers to the ability of two conductors to store charge in relation to each other. Like capacitive coupling, this concept does not apply to transformers, which operate based on magnetic fields rather than electric fields. Therefore, this option is also incorrect.

### Summary
- The correct answer is **Mutual Inductance** because it describes the mechanism by which a changing magnetic field in the primary winding induces a voltage in the secondary winding.
- The other options (capacitive coupling, displacement current coupling, and mutual capacitance) do not accurately describe the operation of a transformer. 

Understanding these principles will help you grasp how transformers function and prepare you for related questions on the Extra Class exam.