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Re: [Phys-L] voltage rise/drop terminology



The talk of apt metaphors for an electrical circuit brings me to that great grand daddy of electrical explanatory aids, the water pump, feeding plumbing, with constrictions.
In that context, the head is the work of the pump, the flux is that of water particles not charge, and what is lost is head at a constriction, and the cumulative head lost equates to the pump's head. This model seems not to mesh well with energy source/sinks.

Brian Whatcott Altus OK

On 4/9/2014 11:39 AM, Robert Cohen wrote:
Though it seems straightforward to migrate from their incorrect view to the exactly correct view, I have found little success in my polishing attempts so far.

It is for this reason that I am considering the change in terminology. If no one can see anything wrong with "sources" and "sinks", then I will try that, as it seems more consistent with the view that energy enters the circuit via the battery and exits the circuit via the resistors without necessarily needing to specify that it travels in any particular direction ALONG the circuit.

Robert A. Cohen, Department of Physics, East Stroudsburg University
570.422.3428 rcohen@esu.edu http://www.esu.edu/~bbq

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John Denker
Sent: Tuesday, April 08, 2014 8:05 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] voltage rise/drop terminology

On 04/08/2014 02:53 PM, Robert Cohen wrote:
When describing circuits, I refer to the voltage "rise" across a
battery and the voltage "drop" across a resistor. However, in
discussions with students, I think this may be confusing them. It
makes them think that there is something traveling from the battery,
that they call voltage (and imagine as electrons), that gets "used up"
in each resistor. This leads them to several errors. For example, it
leads them to think that the last item in a line of bulbs may not
light if there is no "voltage left".
To avoid this confusion (at least until they get a better sense of
what is going on), does anyone see a problem with referring to the
voltage "sources" and "sinks", instead of "rises" and "drops"? Or is
there some other language that is clearer?
Let me answer a slightly different question.

The students' intuition is not far wrong, and should be encouraged. With a little bit of polishing, the students' idea can be made exactly correct.

There is a picture here that hangs together beautifully, if you get the details right:

a) The students' intuition can be quantified directly
in terms of /energy/ ... which is only indirectly
related to voltage and electrons.

b) The energy does not get "used up". Energy can
never be "used up" because energy is conserved.
Instead we say the energy enters the circuit via
the battery and exits the circuit via the resistors.

c) There are also entropy issues involved, which is
why not very much energy /enters/ the circuit via
the resistors.

d) Voltage is /energy per unit charge/ ... or equivalently
power per unit current. Therefore the rate of energy
transfer depends on both the voltage and the current.

e) Conservation of energy and conservation of charge
are both conservation laws, but they are *NOT* the
same conservation law. Wildly different things are
being conserved. They are separately and independently
conserved.

f) Electric charge is not electrons. Electric charge
is not protons. Electric charge is /completely/
abstract. Charge is /carried/ by pions and muons
and electrons and lots of other things, yet the
charge is profoundly different from the particles.
-- pions are not conserved
-- muons are not conserved
-- electrons are not conserved
++ charge is conserved.

Different particles, same charge.

Contrary to what it says in most of the PER literature, there is only *ONE* kind of charge. Positive versus negative is a difference in amount, not a difference in kind.
http://www.av8n.com/physics/one-kind-of-charge.htm

The thing that matters to the equations of electro- magnetism is charge. The thing that matters to the conservation law is charge. The charge is /completely/ abstract.

By way of analogy, a wave is carried on a rope, yet
the wave behavior is different from the rope behavior.
Here's a video, along with some discussion:
http://www.av8n.com/physics/reality-reductionism.htm#sec-rope-chain

Different materials, same wave.

g) Similarly, energy is /completely/ abstract. It
may be /carried/ by electrons and/or other things,
but it is not the same thing. Use the wave video
again, or use Newton's cradle. The energy flows
from ball to ball to ball, yet we recognize it as
the same energy.

Different ball, same energy.
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