Displacement Current

Chapter-8|Electromagnetic Waves|NCERT 12th Physics:

Displacement Current:

Ampere’s circuital law state that the line integral of magnetic field B, around any closed path is equal to μ₀ times the total current threading the closed path i.e

According to Maxwell the above equation is logically inconsistent. With the help of following observation he prove it.

Maxwell’s Experiment to Prove His Point:

He took a parallel plate capacitor and connect it with battery, through a typing key K. After pressing the key K, the conduction current flows through the connecting wires. The capacitor starts storing charge. As the charge on the capacitor grows, the conduction current in the wire decreases. When the capacitor fully charged, the conduction current stops flowing in the wire. During charging of capacitor, there is no conduction current between the plates of capacitor. During charging, Let at any instant I be the conduction current in the wire. This current will produce magnetic field around the wire, which can be detected by using a compass needle.


But in reality it was not correct, because when Maxwell put magnetic needle between two plates it get deflected. He shocked that why this magnetic needle get deflected, while there is no any conduction current between these two plates.

So, he told there is something missing in Ampere’s Circuital Law.

After a variety of experiment Maxwell conclude that at the time of charging of capacitor, there is change in electric field or electric flux, due to this a current is flowing between two plates of capacitor. He named this current as displacement current.

Maxwell said that not only current produces magnetic field but a changing electric field in vacuum/free space also produces magnetic field.

Definition of Displacement Current:

Displacement current is that current which comes into play in the region in which the electric field and the electric flux is changing with time.




 The above expression is called modified Ampere Circuital Law or Ampere-Maxwell Circuital Law.


Properties of Displacement Current:

1. It is not conventional current, as it produces magnetic field, so it is called current.

2. It exists only, when there is a change in electric field(electric flux).

3. It does not exist under steady conditions.

4. Together with the conduction current, displacement current satisfies the property of continuity.

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