Current density and carrier drift
Current measures signed charge flow across a surface.
Key knowledge
Vector current density J gives current I=∫J·n dA across an oriented surface. For one carrier species of signed charge q and number density n_c, its drift contribution is J=n_cqv_d; contributions from species add. Random microscopic velocity is not the same as net drift, and negative carriers drifting one way give conventional current in the opposite direction.
Values and equations
I=∫_S J·n dA; J=n_cqv_d
- I
- Signed transported electric current through S A
- J
- Vector transported-charge current density A m⁻²
- n
- Oriented surface unit normal 1
- A
- Surface area m²
- n_c
- Carrier number density m⁻³
- q
- Signed carrier charge C
- v_d
- Mean carrier drift velocity m s⁻¹
- S
- Chosen crossing surface
Conditions and scope
- State the oriented crossing surface and the macroscopic averaging/carrier model.
- The drift formula describes transported carrier charge; polarization/displacement-current terms in Maxwell’s equations are distinct. Signal propagation speed is not the carrier drift speed.
Sources · 1
References
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Evidence and review
Read §9.2 drift-current relation9.2 and vector Current Density/surface-integral9.4–9.5 body. Preserve signed carrier charge rather than mixing electron charge magnitude with its drift direction.
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Entry details
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- foundation-physics-current-density-drift
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- foundations-refresh.1
- Structured reference record
- Foundational reference additions