[SI-LIST] Re: Direction of displacement curent

  • From: Long Yang <long.0.yang@xxxxxxxxx>
  • To: mervin.jacob@xxxxxxxxxxx
  • Date: Tue, 13 Mar 2012 00:58:18 -0700

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
> >> Aaditya
> >>
> >>
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