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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.