Side-by-side comparison
| Parameter | Conduction | Convection (EM analogy) |
|---|---|---|
| Charge/heat carrier | Free electrons in a conductor | Moving fluid or gas carrying thermal energy |
| EM analogy | Conduction current (J = σE) | Convection current (moving charged medium) |
| Driving quantity | Electric field E (V/m) | Bulk velocity of charged medium |
| Ohm's law applicability | Applies directly; J = σE | Does not follow Ohm's law |
| Medium requirement | Solid conductor (copper, σ ≈ 5.8×10⁷ S/m) | Moving dielectric or ionised gas |
| Typical example | Current in a 22 AWG copper wire | Electron beam in a CRT; ionic wind |
| Continuity equation | ∇·J + ∂ρ/∂t = 0 applies | Same equation but ρ_v ≠ 0 in medium |
| Temperature dependence | σ drops with temperature rise | Velocity-dependent, not σ-dependent |
| Role in Maxwell's equations | Part of total current density J | Treated as source current J_conv = ρ_v · v |
Key differences
Conduction current obeys J = σE and exists only where σ ≠ 0, such as in copper (σ = 5.8×10⁷ S/m). Convection current needs no conductor — it is ρ_v·v, the product of volume charge density and bulk velocity, seen in cathode-ray tubes and ion thrusters. Conduction current satisfies Ohm's law; convection current never does. In Maxwell's displacement current term, both types feed into the total J, but their physical origins are completely different.
When to use Conduction
Use the conduction current model when analysing current flow in PCB traces, resistors, or any metallic interconnect where σ is defined and Ohm's law holds.
When to use Convection (EM analogy)
Use the convection current model when dealing with electron beams in vacuum tubes, plasma discharges, or electrostatic precipitators where charges move through a non-conducting medium.
Recommendation
For 90% of circuit and EMT problems, choose the conduction model — it directly applies J = σE and links cleanly to Kirchhoff's laws. Switch to convection only when the problem explicitly mentions a moving charged medium or vacuum electron beam.
Exam tip: Examiners frequently ask students to write the expression for convection current density (J = ρ_v · v) and distinguish it from conduction current density (J = σE) — know both forms cold.
Interview tip: Interviewers at core hardware companies expect you to explain why convection current does not obey Ohm's law and give one practical device example such as a cathode-ray tube or electrostatic precipitator.