3. Displacement Current
“The current due to the changing electric field is called displacement current.”
Current in a capacitor:
When a capacitor starts charging there is no conduction of charge between the plates.
However, because of change in charge accumulation with time above the plates, the
electric field changes causing the displacement current as below.
ID=JDS= S ∂D/∂t
4. ID= JDS= S ∂D/∂t
where,
* S is the area of the capacitor plate.
* ID is the displacement current.
* JD is the displacement current density.
* D is related to electric field E as D = Εε
* ε is the permittivity of the medium in between the plates.
SI unit of displacement current is Ampere (A)
The current that exists inside the capacitor is Displacement current.
5. Ampere-Maxwell law
The line integral of the magnetic field around a closed loop is equal to
μ0 times the sum of conduction current and displacement current.
6. Displacement current is a quantity appearing in Maxwell’s Equation. It is defined in terms of the rate of
change of electric displacement field. The units of displacement current are same as that of electric
current density. Displacement current is not an electric current caused due to moving of charges(called
displacement current), but it is caused by a time varying electric field.
7. Imagine a wire connected to a charging or discharging capacitor. The area in
the Amperian loop could be stretched into the open region of the capacitor. In
this case there would be current passing through the loop, but not through the
area bounded by the loop.
8. If Ampere’s Law still holds, there must be a magnetic field generated by
the charging E-field makes it loop like there is a current (call it the
displacement current) passing through the plates.
9. Properties of the Displacement Current
* For regions between the plates but at radius larger than the plates, the
B-field would be identical to that at an equal distance from the wire.
* Displacement current arises due to the change in electric flux through an area. If the
change stops displacement current becomes zero. Displacement current also induces
magnetic field just as conduction current.
10. Distinguish between conduction current and displacement current:
* Conduction current is due to the flow of electrons in a circuit. It exists even if
electrons flow at a uniform rate.
* Displacement current is due to the time-varying electric field. It does not exist
under steady conduction.
Concept of displacement current associated in the behavior of electric and
magnetic fields:
According to Faraday’s law of electromagnetic induction, a time-varying magnetic
field induces an emf, According to Maxwell, an electric field sets up a current and
hence a magnetic field. Such a current is called displacement current. It follows that a
time-varying electric field produces a magnetic field and vice-versa. Hence the
behavior of the electric and magnetic field is symmetric.
