WHAT IS DISPLACEMENT CURRENT?

Normally a current through a capacitor is explained in terms of dipole movement in dielectric. However, when dealing with vacuum capacitor, there is no physical material present between capacitor plates, hence dipoles do not exist as in other capacitors. The component is factually a capacitor with no physical dielectric medium. The concept of displacement current was introduced to overcome such anomalies when dealing with vacuum and space. The term “displacement” refers to movement of electric dipoles induced by an electric field, (analogous to the polarization of a dielectric material).

James Clerk Maxwell introduced this concept in 1861 to fix a gap in the existing laws of electromagnetism, and it filled a missing gap in understanding how electromagnetic waves (light, radio signals, microwaves) travel through vacuum or space. Maxwell’s coined this name to unify the principles of electricity and magnetism into a single theory.

Displacement current describes the current arising from a time-varying electric field, which is the result of a change in polarization of a medium, including vacuum (and not due to movement of charges). This concept made Ampere circuit law logically consistent and the inconsistencies in explaining charging process of capacitors.

Displacement current is the current produced by the rate of change of electric displacement field. It differs from the normal current that is produced by motion of electric charges. Displacement currents are produced by a time-varying electric field rather than moving charges.

Displacement current – a Misleading term

The term “displacement current” is one of the most confusing names in physics. It is not a current in any conventional sense. No charges are displaced, and nothing flows. Maxwell originally conceived it on the basis of of a mechanical model involving an elastic medium filling all of space, and the name stuck even after this model was abandoned.  In reality, it is  the magnetic effect of changing electric field. Displacement current signifies a changing electric field doing the work, not moving charges.

Importance of displacement current

In a normal wire, current is the charges moving through a medium (conduction current). No real charges can flow through the insulation between capacitor plates, so no real charges flow across this gap. Even though no charges move, there is still a magnetic field “as if” a current were flowing; this “as‑if” current is displacement current.​

Conduction Current vs. Displacement Current

Feature	Conduction current	Displacement current
Current mechanism	Charge movement (electrons, ions) ​	Changing electric field
Medium	Conductors	Gaps/ spaces around changing fields
Driving force	Voltage difference	Time-varying electric field
Magnetic field	As per Ampère’s law ​	Present, as if it were a real current
Steady‑state behaviour	Can be constant in wires ​	Zero if electric field is constant ​​
Energy dissipation	Due to atomic collisions	No heat as no charges move.

Both types generate magnetic fields in exactly the same way. From the perspective of surrounding magnetic field, displacement current is indistinguishable from conduction current.

Inside a capacitor being charged:

• Real / conduction current carries charge to the plates.​
• Electric field between plates keeps changing as charge builds up.
• Changing electric field in space behaves like current to create magnetic field.

This quantity is displacement current, and its value is proportional to rate of change of electric field (or electric flux).

Physical analogy

If a rubber band is being stretched by hand:

• Conduction current is like hands pulling the band (visible movement).​
• Displacement current is like the “tension” build up in the band as it is stretched. It is not a moving object, but it still affects the system.

In similar way, a changing electric field (even without moving charges) “carries” a displacement current and completes the closed loop of current. It helps explain electromagnetic wave (including light) travel through space.​

Larger relevance: electromagnetic waves

By including displacement current, Maxwell showed that a changing electric field can generate a changing magnetic field, and vice versa, so the two fields can mutually sustain each other and propagate through space as electromagnetic waves (including light).

Physical meaning and role in capacitors

In a parallel‑plate capacitor connected to an AC source or being charged by a battery, conduction current   flows in the wires, delivering charge to the plates. Between the plates, however, there is no continuous flow of free charges; instead, the electric field   between the plates grows or oscillates as the charge on the plates changes. The displacement current   in the gap is numerically equal to the conduction current   in the wires, so the total effective “current” remains continuous across the entire circuit.

This continuity is crucial because it allows us to treat the capacitor region as if a current still flows through it, even though the physical mechanism is different. The displacement current accounts for the magnetic field that is observed around the capacitor gap, which otherwise would have no explanation in the old Ampère’s law.

In simple terms

Displacement current is Maxwell’s way of saying: ‘Even if no charges move, a changing electric field can act like a current for the purpose of creating a magnetic field’. He introduced it to fix the inconsistency in Ampère’s law when applied to capacitors, and in doing so unified electricity, magnetism, and optics into a single coherent theory based on four equations now called Maxwell’s equations. Without displacement current, we could not explain how capacitors work in AC circuits and, most importantly, how electromagnetic waves—including radio waves, microwaves, and visible light—can travel through vacuum.

Displacement Current Equation

A charging capacitor has no conduction of charge but the charge accumulation in the capacitor changes the electric field link with the capacitor that in turn produces the current called the Displacement Current.

I_D = J_DS = S(∂D/∂t)

where

• S is the area of the Capacitor Plate
• I_D is the Displacement Current
• J_D  is the Displacement Current Density.
• D is related to Electric Field E as, D = εE
• ε is the Permittivity of material between plates.

In practice, an ideal insulator does not exist, and some conduction current through the dielectric is always present, depending on nature of material. Therefore  a capacitor will also carry a conduction current between plates, in addition to displacement current. This factor contributes to dielectric loss component in capacitors.