Question

Is drift velocity positive?

Answer 1

Yes, **drift velocity** is typically considered **positive**, but it depends on how it is defined and the reference direction. Let me explain in detail:

### **What is Drift Velocity?**
Drift velocity refers to the average velocity at which charged particles (like electrons or ions) move in response to an electric field within a conductor, such as a wire. It is given by the formula:

\[
v_d = \frac{I}{nqA}
\]

Where:
- \( v_d \): Drift velocity
- \( I \): Current in the conductor
- \( n \): Number of charge carriers per unit volume
- \( q \): Charge of each carrier
- \( A \): Cross-sectional area of the conductor

### **Direction of Drift Velocity**
Drift velocity is a **vector quantity**, meaning it has both magnitude and direction. Its sign depends on the type of charge carrier and the direction of current:

1. **Positive Charge Carriers (e.g., Holes in Semiconductors):**
   - If the charge carriers are positive, the drift velocity is in the **same direction as the electric field** and the current. In this case, \( v_d \) is positive.

2. **Negative Charge Carriers (e.g., Electrons):**
   - For negative charges (like electrons in a conductor), they move in the **direction opposite to the electric field**, as they are repelled by the negative terminal and attracted to the positive terminal.
   - However, by convention, current is defined as the flow of positive charge. Thus, the current direction and the drift velocity direction of electrons are opposite. If you assign the drift velocity a sign relative to current direction, it could be negative.

### **Conventional Definition**
In most cases, **drift velocity is considered positive**, as it describes the magnitude of the motion of charge carriers. The sign convention is typically applied to the electric current or charge carrier's flow direction when analyzing circuits.

### **Practical Example**
If you are studying a copper wire carrying electrons, the **electrons' drift velocity** may technically point in the opposite direction of the conventional current. Still, its absolute magnitude (positive drift velocity) is what is usually calculated or discussed in practice.

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In summary:
- **Drift velocity is generally positive when considering its magnitude.**
- The **sign** depends on the context, reference direction, and type of charge carrier, but these factors are typically clarified in physics problems or engineering analyses.