- Asked 2/13
- 2068 Baisakh · 4 marks
- 2067 Shrawan · 2 marks
Displacement vs conduction current
Short answer
Conduction current is due to drift of free charges (J_c = σE) in conductors and produces I²R loss. Displacement current is due to a time-varying electric flux (J_d = ∂D/∂t) in dielectrics or vacuum. Both produce magnetic fields; at frequency ω, the ratio |J_c|/|J_d| = σ/(ωε) determines which dominates.
Distinguish between conduction and displacement currents (define displacement current and explain how it differs from conduction current).
Conduction current is the current due to the drift of free charge carriers (electrons or ions) under an electric field in a conductor. Its density is J_c = σE (A/m²).
Displacement current is the current in a dielectric (or free space) due to a time-varying electric flux density. Its density is J_d = ∂D/∂t (A/m²). It was introduced by Maxwell so that Ampere's law agrees with the continuity equation: ∇×H = J_c + ∂D/∂t. It flows, for example, between the plates of a capacitor in an ac circuit.
| Conduction current | Displacement current |
|---|---|
| Due to actual flow of free charges | No flow of charge; due to changing electric flux |
| J_c = σE (Ohm's law) | J_d = ∂D/∂t = ε ∂E/∂t |
| Exists in conductors (needs σ ≠ 0) | Exists in dielectrics, vacuum and conductors |
| Flows for steady (DC) as well as AC fields | Exists only for time-varying fields |
| Heat is produced (I²R loss) | No power loss in an ideal dielectric |
| In phase with E | 90° out of phase with E (for sinusoidal fields) |
Both produce a magnetic field. In a medium, the ratio |J_c|/|J_d| = σ/(ωε) shows which dominates at frequency ω: conduction for good conductors at low frequency, displacement for good dielectrics and high frequency. In phasor form J_d = jωεE.
✓ Answer checked by Claude Opus 5.5 (high effort)
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