Difference Between Conduction Current and Displacement Current
Physics · Electromagnetic Waves · NEET
Conduction current (Ic) is the flow of real charge through a conductor, given by Ic = dQ/dt. Displacement current (Id) is NOT moving charge; it is produced by a changing electric field between capacitor plates, given by Id = ε0 (dΦE/dt). In a charging capacitor circuit both are exactly equal (Ic = Id), which keeps the current continuous. Memory hook: "Conduction = charges MOVE, Displacement = field CHANGES, but the meter reads the SAME number."
In the connecting wires real charge flows as conduction current Ic. In the empty gap the changing electric field acts as displacement current Id. They are exactly equal (Ic = Id = C dV/dt), so the current is continuous through the whole circuit.
Your doubts, answered
Is displacement current a real flow of charge like conduction current?
No. Conduction current is real charge (electrons) drifting through a wire. Displacement current has NO moving charge across the gap between capacitor plates. It exists because the electric field between the plates is changing with time. Maxwell called it a 'current' only because it produces a magnetic field exactly like a real current does. So it is a real physical effect, but not a real movement of charge.
Why does conduction current equal displacement current?
When a capacitor charges, charge dQ/dt piles up on the plates, so the conduction current in the wire is Ic = dQ/dt. That same charge sets up an electric field E = Q/(ε0 A) between the plates. As Q grows, the flux ΦE = EA = Q/ε0 grows, so Id = ε0 (dΦE/dt) = dQ/dt. Both expressions equal dQ/dt, so Ic = Id always. This is why an ammeter reads the same current everywhere in the circuit.
What current actually flows in the empty gap between the plates?
No conduction current flows in the gap (there is no conductor, only vacuum or a dielectric). Instead the displacement current 'flows' there. Its value equals the conduction current in the wires, so the circuit stays continuous. This is the classic NEET point: in the connecting wires it is conduction current, in the gap it is displacement current, and they are numerically equal.
Do both currents produce a magnetic field?
Yes. This was Maxwell's whole reason for inventing displacement current. Ampere's original law used only conduction current and gave two different answers for a charging capacitor. Adding displacement current fixed it: Ampere-Maxwell law is ∮B·dl = μ0(Ic + Id). Both currents produce identical magnetic fields, which is why a compass would behave the same near the wire or near the gap.
⚠️ The NEET trap ✗ Choosing 'conduction current in wires, but ZERO current in the gap between plates.' ✓ In the gap the displacement current flows and it is EQUAL to the conduction current in the wires (Ic = Id). The current is continuous everywhere. 🧠 NTA loves options like 'zero, 60 µA' or '60 µA, zero'. The correct one is always 'equal, equal' because Ic = Id = C(dV/dt).
Real NEET questions
NEET 2019
A parallel plate capacitor of capacitance 20 µF is being charged by a voltage source whose potential is changing at the rate of 3 V/s. The conduction current through the connecting wires and the displacement current through the plates of the capacitor would be, respectively,
A · Zero, 60 µA
B · 60 µA, 60 µA ✓
C · 60 µA, zero
D · Zero, zero
Solution: Step 1: Both currents obey I = C(dV/dt). Step 2: Put values: I = 20 µF × 3 V/s = 20×10^-6 × 3 = 60×10^-6 A = 60 µA. Step 3: Conduction current in the wires = 60 µA. Displacement current in the gap = the SAME 60 µA (because Ic = Id keeps the current continuous). Answer: (B) 60 µA, 60 µA.
NEET 2024
A parallel plate capacitor is charged by connecting it to a battery through a resistor. If I is the current in the circuit, then in the gap between the plates,
A · Displacement current of magnitude equal to I flows in the same direction as I ✓
B · Displacement current of magnitude equal to I flows in a direction opposite to that of I
C · Displacement current of magnitude greater than I flows but can be in any direction
D · There is no current
Solution: Step 1: In the wires the conduction current is I = dQ/dt. Step 2: For the current to be continuous, the gap must carry a displacement current Id = ε0(dΦE/dt) = dQ/dt = I. Step 3: It points the same way as I, completing the circuit. Answer: (A) equal magnitude, same direction.
NEET 2023 Phase 2
To produce an instantaneous displacement current of 2 mA in the space between the parallel plates of a capacitor of capacitance 4 µF, the rate of change of applied variable potential difference (dV/dt) must be
Are conduction and displacement current always equal?
Inside a charging or discharging capacitor circuit, yes: Ic = Id = C(dV/dt). This equality is what makes the total current continuous through the circuit, even across the empty gap.
What is the SI unit of displacement current?
The ampere (A), the same as conduction current. Both are measures of the same physical quantity, current, so they share the unit and produce the same magnetic effect.
Does displacement current exist in a steady (DC) circuit with a fully charged capacitor?
No. Once the capacitor is fully charged, the voltage is constant, so dV/dt = 0 and Id = 0. Displacement current only exists while the electric field (and hence the charge/voltage) is changing with time.
Who introduced the idea of displacement current?
James Clerk Maxwell. He added it to Ampere's circuital law to remove the contradiction at a charging capacitor, giving the Ampere-Maxwell law ∮B·dl = μ0(Ic + Id).
