Electric Current & Drift Velocity
Electric current definition I = ΔQ/Δt, mobile charge carriers across metals, semiconductors, and electrolytes, microscopic drift velocity derivation I = nAv_de, and current density J = I/A.
Marking a lesson complete represents finishing your study of the topic; it does not denote tested mastery.
Learning Objectives
- Define electric current and state the SI unit (Ampere), distinguishing conventional current from electronic flow.
- Derive the microscopic current relation I = n A v_d e and calculate electron drift velocities in metallic conductors.
- Define current density J = I/A and explain how an applied electric field establishes collective drift motion across thermal agitation.
Core Theory & Concepts
Electric current represents the organized, net transfer of electrical charge through a defined cross-sectional surface. Although individual conduction electrons engage in chaotic, high-speed thermal collisions at hundreds of kilometres per second, an electric field superimposes a steady, collective drift at fractions of a millimetre per second.
Microscopic Current Equation
For mobile carrier density n, cross-section area A, carrier charge e, and drift speed v_d: I = n A v_d e.
Current Density
Current per unit normal area: J = I / A = n e v_d (unit: A m⁻²). At the microscopic level, J = σ E, where σ is electrical conductivity.
Common Misconceptions
Electrons race through circuit wires at the speed of light.
Syllabus Topic Outline
- 1Electric charge flow and macroscopic current definition (I = dq/dt)
- 2Charge carriers in metallic conductors, electrolytes, and semiconductors
- 3Free electron model and thermal velocities (~ 10⁵ m s⁻¹)
- 4Derivation of drift velocity equation: I = n A v_d e
- 5Current density vector J = I / A and local Ohm’s law J = σE