*Question E5D01:
What is the result of conductor skin effect?*  

*Correct Answer is  A: Resistance increases as frequency increases because RF current flows closer to the surface*  

### AI Response: (may contain errors)  

The correct answer to the question about the conductor skin effect is:

**A: Resistance increases as frequency increases because RF current flows closer to the surface.**

### Explanation of the Correct Answer:
- The skin effect refers to the phenomenon where alternating current (AC) tends to flow near the surface of a conductor rather than uniformly throughout its cross-section. 
- As the frequency of the AC increases, the depth at which the current penetrates the conductor decreases. This results in a higher effective resistance because the current is concentrated in a smaller area, leading to increased resistance.
- The skin depth ($\delta$) can be calculated using the formula:

$$
\delta = \sqrt{\frac{2}{\omega \mu \sigma}}
$$

where:
- $\omega = 2\pi f$ (angular frequency)
- $\mu$ = permeability of the material
- $\sigma$ = conductivity of the material

### Why the Other Choices are Incorrect:
- **B: Resistance decreases as frequency increases because electron mobility increases.**
  - This statement is incorrect because, while electron mobility can affect resistance, the skin effect specifically causes resistance to increase at higher frequencies due to the concentration of current near the surface.

- **C: Resistance increases as temperature increases because of the change in thermal coefficient.**
  - This choice is misleading. While it is true that resistance generally increases with temperature due to the thermal agitation of atoms, it does not relate to the skin effect or frequency.

- **D: Resistance decreases as temperature increases because of the change in thermal coefficient.**
  - This statement is incorrect. For most conductive materials, resistance increases with temperature, not decreases. 

### Summary:
- The skin effect is crucial for understanding how RF signals behave in conductors, especially at higher frequencies. 
- Remember that as frequency increases, the effective resistance increases due to the current flowing closer to the surface of the conductor. This is a key concept for the Extra Class exam.