# [SI-LIST] Re: Direction of displacement curent

• From: Ihsan Erdin <erdinih@xxxxxxxxx>
• To: orphanou@xxxxxxxxxxxx
• Date: Wed, 14 Mar 2012 07:24:18 -0400

```In a PURE TEM case, magnetic field, electric field and conduction current
are all perpendicular to each other with the latter being in the direction
of propagation ONLY. There is no electric field in the direction of
propagation and the displacement current is in the direction of electric
field. Quasi TEM happens due to the voltage drop along the line hence a
small component of the electric field in the direction of propagation. The
voltage drop could be due to the resistivity of the conductor, the impurity
of the dielectric medium as in the case of microstrip, anisotropy, etc.
Ihsan

On Tue, Mar 13, 2012 at 3:01 PM, Antonis Orphanou <orphanou@xxxxxxxxxxxx>wrote:

> There is a more unified assumption to the general current density concept.
> In a hypothetical general volume, charges will move when forces act on
> them. If that force is constant and it's time derivative is zero, then the
> current is only conduction. If that charge accelerates with time (and the
> force time derivative is not zero) then that yields displacement current.
> Free charge will give rise to conduction currents and free charge can only
> exist and move in the direction along the metallic trace. That requires an
> E-field along the trace and hence the quazi-TEM assumption.
>
> Polarization charges can only exists between the trace and it's return
> path and those give rise to displacement current when a force is acted on
> it.
> The polarization current direction is from the trace to the return path.
> When a moving charge can no longer move it behaves just like a polarization
> charge.
>
> Maxwell's equations predict that both the conduction and displacement
> currents are in the same direction as the E-field; Given the fact that we
> have E-field in both the vertical and parallel to trace directions, we
> should also account for the mechanisms that can sustain those two current
> concepts and we will see that in the pcb trace case we can sustain
> displacement current in the vertical and conduction current in the
> longitudinal trace directions.
>
>
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Long Yang
> Sent: Tuesday, March 13, 2012 12:58 AM
> To: mervin.jacob@xxxxxxxxxxx
> Subject: [SI-LIST] Re: Direction of displacement curent
>
> Displacement current is not an electric current of moving charges, but an
> electric field that varies with time.
> For perfect PCB traces, energy propagates in the way of electromagnetic
> wave.
> The trace plays the role of wave guide, not current conduit.
> When TEM mode dominates, electric field is from ground to trace (and
> reversed along propagation).
> This is also the direction of displacement current, flowing by the way of
> capacitance between gnd and driven signal.
>
> On Mon, Mar 12, 2012 at 22:21, Mervin J <mervin.jacob@xxxxxxxxxxx> wrote:
>
> > As per Maxwell's equation, The direction of displacement current is
> > along the conduction current.
> >
> >
> > On 3/13/2012 6:54 AM, Long Yang wrote:
> > > In Maxwell's equations, there are two items about current,
> > > conduction current and displacement current.
> > > Conduction current is sometimes expressed related to electric field
> > > by conductivity.
> > > In the similar way, you can expect that displacement current has the
> > > similar relationship with electric field.
> > >
> > > Regarding PCB trace, the electric field inside copper trace is
> > > driven by driven voltage potential, so conduction current is along the
> copper wire.
> > > While the electric field of TEM mode is from signal trace to ground,
> > > and that implies the direction of displacement current.
> > >
> > > thanks
> > >
> > > On Mon, Mar 12, 2012 at 11:31, Antonis
> > >Orphanou<orphanou@xxxxxxxxxxxx
> > >wrote:
> > >
> > >> On displacement current:
> > >> If you assume that for all the copper surfaces the tangential
> > >> electric field is zero and that there is only a normal E-field
> > >> component (which gives rise to the copper surface charge density)
> > >> then you could
> > visualize
> > >> the displacement current. It is the time derivative of the E-field
> > >> in
> > that
> > >> same direction.
> > >>
> > >>
> > >>
> > >> -----Original Message-----
> > >> From: si-list-bounce@xxxxxxxxxxxxx
> > >> [mailto:si-list-bounce@xxxxxxxxxxxxx
> > ]
> > >> On Behalf Of Aaditya Kandibanda
> > >> Sent: Sunday, March 11, 2012 8:36 PM
> > >> To: si-list@xxxxxxxxxxxxx
> > >> Subject: [SI-LIST] Direction of displacement curent
> > >>
> > >> Hello everyone,
> > >> Can you please let me know what will be the direction of
> > >> displacement current on the traces of PCB, I am trying to
> > >> understand it for analysis
> > of
> > >> near and far end cross talk on PCB traces. the problem is, when
> > >> there
> > is a
> > >> equal distribution of capacitance along the length of 2 PCB traces?
> > >> what will be the direction of displacement current? If possible can
> > >> some one briefly let me know about the near end and far end cross
> > >> talk? I am
> > trying
> > >> to understand it, I see different authors express it in different
> > >> views which is confusing me as a novice.
> > >>
> > >>
> > >> Thanks and regards
> > >>
> > >>
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--
Ihsan Erdin

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