*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)  

When an AC voltage source is connected across the primary winding of a transformer, a voltage appears across the secondary winding due to the principle of **mutual inductance**.

### Explanation:

1. **Correct Answer: Mutual Inductance**
   - **Mutual inductance** is the phenomenon where a changing magnetic field in one coil induces a voltage in another coil. In a transformer, the AC current in the primary winding creates a changing magnetic field, which induces a voltage in the secondary winding. This is the fundamental operating principle of transformers.

2. **Why Other Choices Are Incorrect:**
   - **Capacitive Coupling (Choice A):** This involves the transfer of energy between two circuits through a capacitance. It is not the primary mechanism in transformers, which rely on magnetic fields rather than electric fields.
   - **Displacement Current Coupling (Choice B):** Displacement current is a concept used in the context of changing electric fields in capacitors, not in the operation of transformers.
   - **Mutual Capacitance (Choice D):** This refers to the capacitance between two conductors. While there may be some small capacitive effects in a transformer, they are not responsible for the main operation of inducing voltage across the secondary winding.

### Useful Formula:

The voltage induced in the secondary winding can be described by the formula:

$$
V_s = V_p \times \frac{N_s}{N_p}
$$

where:
- $V_s$ = voltage across the secondary winding
- $V_p$ = voltage across the primary winding
- $N_s$ = number of turns in the secondary winding
- $N_p$ = number of turns in the primary winding

This formula shows how the voltage is related to the turns ratio of the transformer, which is a direct result of mutual inductance.