---
title: "Displacement vs conduction current"
subject: "Electromagnetism"
chapter: 3
key: "semester_3/electromagnetism:displacement-vs-conduction-current"
source: https://www.bctengineer.com/q/semester_3/electromagnetism/displacement-vs-conduction-current
site: BCT Engineering
---

# Displacement vs conduction current

Electromagnetism · Chapter 3: Magnetic field

- Asked in: 2068 Baisakh, 4 marks; 2067 Shrawan, 2 marks (asked 2 times)
- Video: [What is the difference between Displacement Current and Normal (Conduction) Current?](https://www.youtube.com/watch?v=B0QzjGawlwI) — GRADplus
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electromagnetism/displacement-vs-conduction-current
- Whole chapter: https://www.bctengineer.com/md/semester_3/electromagnetism/chapter-3

## Question

Distinguish between conduction and displacement currents (define displacement current and explain how it differs from 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.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Displacement current density is:
   A. σE
   B. ∂D/∂t
   C. ∂B/∂t
   D. ρ_v v
   Answer: B. J_d = ∂D/∂t = ε∂E/∂t and arises from changing electric flux, not moving charges. (Key point: Definition of displacement current)

2. Which statement is true?
   A. Displacement current exists for DC
   B. Conduction current needs free charges and σ ≠ 0, displacement needs time-varying fields
   C. Both produce no magnetic field
   D. Displacement current produces I²R loss
   Answer: B. Conduction current flows for DC and AC; displacement current only for time-varying E. (Key point: Existence)

3. For sinusoidal fields, the displacement current is ___ E.
   A. In phase with
   B. 90° out of phase with
   C. 180° out of phase with
   D. Unrelated to
   Answer: B. J_d = jωεE leads E by 90°, while J_c is in phase. (Key point: Phase)

4. The ratio |J_c|/|J_d| equals:
   A. ωε/σ
   B. σ/(ωε)
   C. σω
   D. ε/σ
   Answer: B. σE / ωεE = σ/(ωε), so conduction dominates in good conductors at low frequency. (Key point: Dominance)

## Related in Electromagnetism

- [Conduction and displacement current in capacitor](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Aconduction-displacement-current-capacitor) — Electromagnetism, chapter 3
- [Frequency where conduction equals displacement](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adisplacement-current-conduction-ratio-soil) — Electromagnetism, chapter 3
- [Displacement current density at 1 kHz](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adisplacement-current-metal-1khz) — Electromagnetism, chapter 3
- [Maxwell's equations, displacement current](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Amaxwell-equations-explain-displacement-depth) — Electromagnetism, chapter 3
- [Displacement current density in transformer](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adisplacement-current-density-transformer) — Electromagnetism, chapter 3
- [Displacement current density at 60 Hz](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adisplacement-current-metallic-conductor-60hz) — Electromagnetism, chapter 3

## Related in other subjects

- [Source current from parallel branch currents](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Aparallel-branch-currents-time-expression) — Basic Electrical Engineering, chapter 5
- [Induced magnetic field, displacement current](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Adisplacement-current-induced-field-circular-plates) — Engineering Physics, chapter 9
- [Displacement current in parallel plate capacitor](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Adisplacement-current-circular-plates-b-integral) — Engineering Physics, chapter 9
- [Displacement current density, dE/dt, induced B](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Adisplacement-current-density-dedt-plates) — Engineering Physics, chapter 9