How do you find the direction of a displacement current?

In summary, the displacement current is defined as the change in electric flux over time and, like regular current, it has a direction despite being a scalar quantity. To find its direction, we can measure it through three perpendicular surfaces or by finding the surface with the greatest displacement current, which corresponds to its direction.
  • #1
Ackbach
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We know that the so-called displacement current is defined as
$$i_d=\varepsilon_0 \, \frac{\partial\Phi_e}{\partial t}.$$
Like regular current which is the movement of charges, $i_d$ has a direction, even though it's technically a scalar. How do we find its direction?
 
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  • #2
Ackbach said:
We know that the so-called displacement current is defined as
$$i_d=\varepsilon_0 \, \frac{\partial\Phi_e}{\partial t}.$$
Like regular current which is the movement of charges, $i_d$ has a direction, even though it's technically a scalar. How do we find its direction?

Hi Ackbach,

From wiki, the displacement current density is:
$$\boldsymbol{j}_d = \pd {\boldsymbol{D}} t = \varepsilon_0 \frac{\partial \boldsymbol{E}}{\partial t} + \frac{\partial \boldsymbol{P}}{\partial t}$$

If we pick some surface, through which we want to know the displacement current, this is:
$$i_d=\iint \boldsymbol{j}_d \cdot d\boldsymbol S
= \iint \Big(\varepsilon_0 \frac{\partial \boldsymbol{E}}{\partial t} + \frac{\partial \boldsymbol{P}}{\partial t}\Big) \cdot d\boldsymbol S
= \varepsilon_0 \iint\frac{\partial \boldsymbol{E}}{\partial t}\cdot d\boldsymbol S + \iint \frac{\partial \boldsymbol{P}}{\partial t} \cdot d\boldsymbol S
$$
For a fixed surface in a medium with constant polarization (such as vacuum), it simplifies to:
$$i_d = \varepsilon_0 \frac{\partial \Phi_e}{\partial t}$$

By doing this, we have "lost" the direction.
If we want to get the direction back, we can measure $i_d$ through 3 perpendicular surfaces of unit size.
The corresponding results are the components of the vector with respect to the normals of those surfaces.
Alternatively, we can search for the surface that has the greatest $i_d$. Its normal is the direction.

Btw, I think this topic belongs in Other Advanced Topics, so I've moved it there.
 

FAQ: How do you find the direction of a displacement current?

How is displacement current different from conduction current?

Displacement current is a form of electric current that is created by a changing electric field, while conduction current is created by the flow of electrons through a conductive material.

What is the equation for calculating displacement current?

The equation for calculating displacement current is Id = ε0 * (dΦE / dt), where Id is the displacement current, ε0 is the permittivity of free space, and dΦE / dt is the rate of change of electric flux.

How is displacement current related to Maxwell's equations?

Displacement current is included in Maxwell's equations as one of the terms in the equation for Ampere's law. It was added by Maxwell to account for the changing electric field in a capacitor and to make the equations consistent with experimental observations.

Can displacement current exist in a vacuum?

Yes, displacement current can exist in a vacuum because it is created by a changing electric field, which can exist in the absence of matter.

How do you find the direction of a displacement current?

The direction of a displacement current is always perpendicular to the changing electric field that creates it. This can be determined using the right-hand rule, where the thumb points in the direction of the electric field and the fingers curl in the direction of the displacement current.

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