Date: Wed, 22 Jan 2003 18:18:31 -0500

Author: "Roger Feeley"

Subject: Re: LH & RH rules for E&M

Post:

On Wednesday, January 22, 2003, at 04:55 PM, erikh@email.arizona.edu
wrote:
> -Electrons flow from - to + on a circuit outside a battery. (I knew
> that
> but wrote it backwards in my last e-mail.)

You can get positive charge flow too! This happens in many
semiconductors and is shown by the Hall Effect. Sometimes you can also
get a mixture of the two.

Picture a flat thin conductor. Put it in a magnetic field pointing
normal to it's surface. If run a current through the conductor in one
direction, you will observe a potential difference in the other
dimension. This is because the charge carriers are moving in the B
field and tend to be pushed toward one side. Note that the potential
difference perpendicular to the charge flow will flip depending on the
polarity of the charge carrier. In the case of a positive charge
carrier, what actually "moves" is called a "hole" - the absence of an
electron. At the interface with another conductor, holes and electrons
may "annihilate."

Don't know who invented this left-hand-rule thing. This is the first
I've heard of it. Sounds pretty confusing to me. And annoying that
some group of high school teachers decided to change a convention
that's been in use for a hundred+ years.

Paul
From Roger_Feeley@umit.maine.edu Wed Jan 22 18:10:41 2003
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Date: Wed, 22 Jan 2003 18:18:31 -0500
Subject: Re: LH & RH rules for E&M
To: tap-l@listproc.appstate.edu
From: "Roger Feeley"
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tap-l@listproc.appstate.edu writes:
>Hi Folks,
>
>It's been a while since I took E&M. I wanted to teach my HS kids about it
>in a way that was less confusing than when I learned it. But all the literature
>I'm finding: texts, web, etc. seems to be saying something different.
>
>Please help
>
>
>Secondly, they are in disagreement about which direction current flows.
>It's my understanding that the negative charges (electrons) flow from the
>+ of a battery to the - . Correct?

Current is defined as a flow of charge from + to -. So, electron flow (by definition) is the opposite direction of current flow. When Ben Franklin named the charges, he was totally arbitrary and defined current to be a flow from + to -. It wasn't
until later that they found out that electrical current was [most of the time] the flow of electrons.

When I was in the Navy, they defined current to be the flow of electrons. So, diodes worked backwards (flow was against the arrow), and they had a left hand rule AND a right hand rule, depending on if you were talking about motors or generators.


>
>
>Third, when using the above model of current, it is a LEFT HAND rule that
>describes what actually happens to the magnetic flux. In other words, when
>I run a wire across a compass, my thumb points in the direction of the current
>and my fingers point in the direction of north. I've tried this and it works,
>but then I read that it is really a RIGHT HAND rule (and that's what I remember
>from E&M). What's the confusion here?


It is and always will be the RIGHT HAND RULE.




>
>I appreciate any clarification on this.
>
>Erik Herman
>Aztlan Academy
>Tucson, AZ
>



Roger Feeley
roger.feeley@umit.maine.edu
From polands@pop.sonic.net Wed Jan 22 18:16:06 2003
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