Drift velocity is the average velocity at which free charge carriers (like electrons) move in a conductor under the influence of an electric field. It’s a slow, net motion distinct from the random thermal motion of particles.
The expression for drift velocity (vd v_d vd) is:
vd=InqA v_d = \frac{I}{n q A} vd=nqAI
Where:
- I I I: Current in the conductor (amperes)
- n n n: Number density of charge carriers (per cubic meter)
- q q q: Charge of each carrier (e.g., 1.602×10−19 1.602 \times 10^{-19} 1.602×10−19 C for an electron)
- A A A: Cross-sectional area of the conductor (square meters)
Alternatively, it can be expressed in terms of the electric field:
vd=μE v_d = \mu E vd=μE
Where:
- μ \mu μ: Mobility of the charge carriers (m²/(V·s))
- E E E: Electric field strength (V/m)
This shows drift velocity depends on the applied field and material properties. Typically, drift velocities are very small, on the order of mm/s, despite high random speeds of electrons.