Re: Current is a vector

From: Bill Hobba (bhobba_at_rubbish.net.au)
Date: 06/03/04 

Date: Thu, 03 Jun 2004 23:33:54 GMT

"Harold Ensle" <heensle@ix.netcom.com> wrote in message
news:uKLvc.20554$Tn6.6720@newsread1.news.pas.earthlink.net...
>
> "Bill Hobba" <bhobba@rubbish.net.au> wrote in message
> news:MBFvc.4055$rz4.1800@news-server.bigpond.net.au...
> >
> > "Mike" <eleatis@yahoo.gr> wrote in message
> > news:c9n5f9$99v$1@usenet.otenet.gr...
> > >
> > > "Bill Hobba" <bhobba@rubbish.net.au> wrote in message
> > > news:FfBvc.3651$rz4.3549@news-server.bigpond.net.au...
> > > >
> > > > "Harold Ensle" <heensle@ix.netcom.com> wrote in message
> > > > news:woyvc.21399$be.15661@newsread2.news.pas.earthlink.net...
> > > > > In Griffiths "Introduction to Electrodynamics" p202-203 (2nd Ed.)
> > > > > One can read:
> > > > > _____________________________________________
> > > > > "The current in a wire is the charge per unit time passing a
> > > > > given point.....
> > > > > .....The current at each point is actually a vector:
> > > > >
> > > > > (bold)I=lambda*(bold)v
> > > > >
> > > > > However, since the path of the flow is dictated by the shape
> > > > > of the wire, most people don't bother to display the vectorial
> > > > > character of (bold)I explicitly....
> > > > > .....When the flow of charge is distributed throughout a
> > > > > three-dimensional region, we describe it by the volume current
> > > > > density, (bold)J, defined thus: Consider a "tube" of
> infinitesimal
> > > > > cross-section da (perpendicular subscript), running parallel to
> > > > > the flow. If the current in this tube is (bold)dI, the volume
> > > > > current density is
> > > > >
> > > > > (bold)J=(bold)dI/da(perpendicular subscript)
> > > > >
> > > > > In words, (bold)J is the current per unit area-perpendicular-
> > > > > to-flow.....
> > > > > .....According to [the above equation], the current crossing
> > > > > a surface S can be written as
> > > > >
> > > > > I=int(S) Jda = int(S) (bold)J dot (bold)da"
> > > > > ______________________________________________
> > > > >
> > > > >
> > > > > Despite the clear and understandable definitions given in
> > > > > this introductory text, Bilge, Franz Heymann, Bill Hobba, and
> > > > > Dirk van de Moortel insist that current is ONLY a scalar.
> > > > >
> > > > > Apparently their education in the basics has been wanting.
> > > > >
> > > > > I do not have the time to give them the remedial help they
> > > > > need to understand these basic things, so I must leave it
> > > > > at this...and hope that they can figure it out for themselves.
> > > >
> > > > You have been refereed to other sources that corrects this oversight
> in
> > > the
> > > > use of terminology by Griffith but for some reason choose to
believer
> > > > Griffith is infallible and people like Feynman are in error. Even
> when
> > > > quotes are given clearly demonstrating your error you refuse to
> > > acknowledge
> > > > it claming when the word 'vector' is put in front of current it does
> not
> > > > modify its context. Such is obviously a failing on your part but
like
> > > most
> > > > crackpots believe they can not be mistaken, they are infallible,
their
> > > view
> > > > is true and beyond question. Harold - grow up. Griffith is not
> > > infallible,
> > > > your interpretation is not the be all and end all, and every other
> > author
> > > in
> > > > existence can not be mistaken.
> > > >
> > > > Bill
> > > >
> > > >
> > >
> > > What about the F-B-I rule?
> > >
> > > in a current carrying conductor placed in a magnetic field B, the
forces
> F
> > > on both negative and positive charges are in the same direction:
> > >
> > > F = lI x B
> > >
> > > where l is the conductor length. Obviously, this is a vector cross
> product
> > > and I, B are treated as vectors.
> > >
> > > Mike
> >
> > Exactly as found on page 13-3 of volume 2 of the Feynman Lectures on
> Physics
> > the I in that equation refers to the vector current in the wire. The
> vector
> > current in a wire is not the same thing as current
>
> hmmmmmm.....explain to me how "current" is different from "vector
> current in a wire" as a physical quantity. Do they have different base
> units? What is physically different between them that would make
> one a scalar and the other a vector?

Easy Harold. If you had bothered to read the Feynmann Lectures page 13.8
volume 2 or simply the quote that was given from that page then you would
see it is JA where J is the current density (a vector) and A is the
crossectional area of the wire. This is equivalent to the definition given
of page 202 of Griffith where the 'error' he made is that the quantity was
not current but rather the vector current of a wire.

>
> >which is defined as I =
> > integral j.ds which means it by definition is a scalar.
>
> Equations do not define the physical quantity. Equations relate
> physical quantities. You will note that Griffiths (with proper
> technique) carefully defines the physical quantities and then
> gives the equations relating them.

Equations can form part of a definition. Current is defined as the time
rate of change of charge which makes it a scalar. You will note that
Griffith did not use the word define. And even if he did, if all (or even
most) other texts by well known and respected authors define and/or use it
differently then guess what, whether you like it or not Griffith made a
mistake. Stupid arguments about appeal to authority notwithstanding.

>
> This equation you give here is in the original post with its
> correct interpretation.
>
> >What this whole
> > silly and stupid thread is about is Harold's inability to admit he is
> > misinterpreting what Griffith said rather than face the awful truth he
may
> > be, horrors of horrors, wrong.
>
> Projection noted.

Your attitude however does not need to be noted - it has been known for a
long time.

Bill

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