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Electricity · Physics

Current is charge flow. Drift is the microscopic story.

Electric current measures how much charge passes a cross-section per unit time. At the microscopic level, charges drift slowly while signals propagate through the circuit much faster.

This topic

Electric Current

Define current, interpret drift velocity, and connect microscopic motion to current density.

Definition
What current means
Electric current is the rate at which charge passes through a chosen cross-section. It is a scalar quantity defined relative to a direction convention.
  • Definition: I = ΔQ/Δt
  • Conventional current direction vs electron motion
  • Steady vs time-varying current (conceptual)
  • Units: ampere (C/s)
Start nano-lesson Start by choosing a direction.
Microscopic
Drift velocity
In a conductor, charges undergo random thermal motion but also develop a small average drift velocity when an electric field is present.
  • Drift is an average, not the full motion
  • Drift speed is typically very small
  • Why current can be steady even with random motion
  • How drift direction depends on charge sign
Start nano-lesson Random + drift = real charge motion.
Model
Microscopic view of current
Current arises from many charges moving through a cross-section. The microscopic picture connects charge density, cross-sectional area, and drift speed.
  • Carrier density and how it matters
  • Role of cross-sectional area
  • Why thicker wires carry more current (for the same drift)
  • Limits: collisions and scattering (preview)
Start nano-lesson Many slow carriers can make a large current.
Field quantity
Current density
Current density describes how current is distributed in space. It is a vector pointing in the direction of conventional current flow.
  • Definition: J = I/A for uniform flow
  • Vector direction: along conventional current
  • Non-uniform cases: J varies across the cross-section
  • Connects naturally to Ohm’s law in materials (next topic)
Start nano-lesson J tells you “how concentrated” the current is.
Practice
Practice & Exercises
Practice current definitions and microscopic interpretations using drift and current density.
  • Compute I from charge-through-time scenarios
  • Direction questions: conventional vs electron flow
  • Drift speed reasoning (qualitative and simple quantitative)
  • Compute J from I and wire area
  • Exam-style current and drift sets (intro)
Open practice sets Keep signs and directions explicit.