*Question T5A09:
Which of the following describes alternating current?*  

*Correct Answer is  C: Current that alternates between positive and negative directions*  

### AI Response: (may contain errors)  

To understand alternating current (AC), let's break down the correct answer and the other choices:

### Correct Answer: 
**C: Current that alternates between positive and negative directions**

- **Explanation**: 
  - Alternating current is characterized by the flow of electric charge that periodically reverses direction. In a typical AC waveform, such as a sine wave, the current flows in the positive direction for half of the cycle and in the negative direction for the other half. This is what defines AC.

### Why the Other Choices are Incorrect:
- **A: Current that alternates between a positive direction and zero**  
  - This choice is misleading because it suggests that the current only goes from positive to zero, without ever going negative. AC does not just stop at zero; it also flows in the negative direction.

- **B: Current that alternates between a negative direction and zero**  
  - Similar to choice A, this option implies that the current only flows from negative to zero, omitting the positive direction. AC includes both positive and negative flows.

- **D: All these answers are correct**  
  - Since choices A and B are incorrect, this option cannot be true. Not all the provided descriptions accurately represent AC.

### Useful Concepts:
- **Waveform**: The most common waveform for AC is the sine wave, which can be described mathematically as:
  $$ I(t) = I_{max} \sin(2 \pi f t + \phi) $$
  where:
  - $I(t)$ = instantaneous current at time $t$
  - $I_{max}$ = peak current
  - $f$ = frequency in hertz
  - $\phi$ = phase angle in radians

- **RMS Value**: The root mean square (RMS) value of AC is often used to represent its effective value:
  $$ I_{RMS} = \frac{I_{max}}{\sqrt{2}} $$

This understanding of AC will help you answer related questions on the exam effectively.