*Question E5B06:
What is susceptance?*  

*Correct Answer is  C: The imaginary part of admittance*  

### AI Response: (may contain errors)  

Susceptance is a concept used in AC circuit analysis, particularly when dealing with complex impedance and admittance.

### Explanation:

- **Correct Answer: C: The imaginary part of admittance**

  Admittance ($Y$) is the measure of how easily a circuit allows the flow of electric current. It is the reciprocal of impedance ($Z$). Admittance is a complex number and can be expressed as:

  $$ Y = G + jB $$

  where:
  - $G$ is the conductance (the real part),
  - $j$ is the imaginary unit,
  - $B$ is the susceptance (the imaginary part).

  Susceptance ($B$) represents the reactive component of admittance, which is associated with the storage and release of energy in the circuit's inductors and capacitors.

### Why the Other Choices are Incorrect:

- **A: The magnetic impedance of a circuit**

  This choice is incorrect because susceptance is not related to magnetic impedance. Impedance is a broader term that includes both resistance and reactance, not specifically magnetic properties.

- **B: The ratio of magnetic field to electric field**

  This choice is incorrect because the ratio of magnetic field to electric field is related to the characteristic impedance of a medium, not susceptance.

- **D: A measure of the efficiency of a transformer**

  This choice is incorrect because susceptance does not measure efficiency. Transformer efficiency is typically related to power losses and is not directly connected to susceptance.

### Useful Formula:

- **Admittance and Impedance Relationship:**

  $$ Y = \frac{1}{Z} $$

  where $Z$ is the complex impedance, expressed as $Z = R + jX$, with $R$ being resistance and $X$ being reactance.

Understanding susceptance as the imaginary part of admittance helps in analyzing how reactive components (inductors and capacitors) affect the overall behavior of AC circuits.