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

### Explanation:

1. **Why the correct answer is correct:**
   - The skin effect is a phenomenon where alternating current (AC) tends to flow near the surface of a conductor as the frequency increases. This is because the magnetic fields generated by the AC cause the current to be pushed towards the outer surface of the conductor.
   - As a result, the effective cross-sectional area through which the current flows is reduced, leading to an increase in the conductor's resistance at higher frequencies.

2. **Why the other choices are incorrect:**
   - **B: Resistance decreases as frequency increases because electron mobility increases**
     - This is incorrect because electron mobility does not increase with frequency. Instead, the skin effect causes the current to be confined to a smaller area, increasing resistance.
   - **C: Resistance increases as temperature increases because of the change in thermal coefficient**
     - While resistance does generally increase with temperature, this choice is unrelated to the skin effect, which is specifically about frequency dependence.
   - **D: Resistance decreases as temperature increases because of the change in thermal coefficient**
     - This is incorrect because resistance typically increases with temperature for most conductors, and this choice does not relate to the skin effect.

3. **Useful formulas or rules of thumb:**
   - The skin depth $\delta$ is given by:
     $$\delta = \sqrt{\frac{2\rho}{\omega \mu}}$$
     where:
     - $\rho$ = resistivity of the conductor
     - $\omega = 2\pi f$ = angular frequency
     - $\mu$ = permeability of the conductor
   - As frequency $f$ increases, the skin depth $\delta$ decreases, meaning the current flows in a thinner layer, increasing resistance.