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



Almost everybody I know says that "critical thinking" is
super-important, more important than any particular physics
fact.

Unfortunately, I know a lot of people who talk-talk-talk
about critical thinking but never actually *DO* anything
about it.

As an example: Recall that week's discussion started with
Lenz's law. Ask yourself, in all your years of teaching,
how many times did a student stick up his hand and ask
"Please sir, how do we apply Kirchhoff's law to the voltage
in that jumping ring?"

I suggest that every time that /doesn't/ happen it is a sign
that we are not doing our job properly.

Connections are important to thinking in general and learning
in particular. If students cannot see the connection between
the jumping ring and Kirchhoff's laws, there is something
wrong with their learning, and with their thinking.

I mention this because on 04/09/2014 02:14 PM, Bill Nettles
wrote:
Let me first define some terminology:
1) voltage = difference in potential,

It is a Bad Idea to define voltage in terms of potential,
for multiple reasons. For starters, there are a lot of
non-potential voltages in the world. You have no chance
of understanding the jumping ring in particular, or Lenz's
law in general, or transformers, or dynamos, or radios,
or lots of other things if you define voltage in terms
of potential.

I understand that in the introductory course you want
to keep things as simple as possible, but there is no
advantage in cutting corners that didn't need to be cut.
Just define voltage as energy per unit charge. That will
never get you into trouble. And if the students don't
have a good enough grasp of energy to make that work,
then spiral back and reinforce their understanding of
energy. That's more important anyway.

Here is a reasonably careful definition of voltage:
http://www.av8n.com/physics/voltage-intro.htm

People _like_ Kirchhoff's laws, and they often engineer
their circuits so as to uphold Kirchhoff's laws ... but
you cannot take this for granted. It is not a law of
physics. Not even close.

In a situation where you want students to /assume/ Kirchhoff's
laws apply, you should say that explicitly, every time.
Otherwise it becomes a mind-reading exercise.

if you traverse the circuit element (resistor, source, capacitor,
inductor) in the one direction, the potential will rise and in the
other the potential will drop.

This is No Big Deal. This is just like every mechanics
problem on earth, where you get to pick a set of axes and
basis vectors. Current is a vector, and you get to pick
the basis vector. This is discussed at:
http://www.av8n.com/physics/vector-intro.htm#sec-current-vector

You have your choice of basis vectors, so pick one! Then
stick with it. Again I emphasize the spiral approach. This
is an opportunity to reinforce -- and then to exploit --
something they already know about vectors.

It is essential to assign current directions AND values to each
element, too. ----> 3A is exactly the same as (-3A) <-----, so the
direction arrow and value are inseparable.

Exactly so.

Most circuits drawn in introductory physics don't have a reference ground shown

Same gag. Pick one! Also, teach the students to pick one.
Draw it onto the circuit diagram. This is what every engineer
on earth would do. It's also good physics. Call it an
application of gauge invariance.

so you can't talk about either the potential or the voltage at a point.

Again: To talk about "the" voltage at a point, you first
need to assert that Kirchhoff's laws apply. Then you need
to pick a gauge, i.e. pick a ground reference.

With this current and voltage notation, Ohms Law becomes tied to a
sign convention of the voltage across and conventional current
through a resistor: Traditionally, V = IR is true if V is the number
attached to the voltage polarity across the resistor and I is the
number attached to the conventional current flowing in the direction
from the + to the - sign (or entering the + polarity assignment or
leaving the - polarity assignment).

You can say the same thing in a lot fewer words: V_drop = IR.

Choose a direction, i.e. a basis vector for the current. Calculate
the amount of current in the chosen direction. V_drop is the
voltage drop that you see going around the circuit in the same
direction. The mnemonic is simple:
V_drop = IR

That's predicated on the even more fundamental rule:
Pick a basis and use it consistently.

===============

Note that assuming that a battery corresponds to a voltage "rise"
is a Bad Idea for a different reason: Consider what happens if
there are multiple batteries in the circuit, and some of them
are oriented "backwards" relative to the others. This is not
a problem in practice; you just look at the circuit diagram and
write down the ΔV accordingly. It is however a problem for the
all-too-numerous folks who like to play lawyer, who like to
write down rules and then argue over the words.

The physics fact remains: You simply cannot assume that every
battery is oriented according to the basis vector you chose
for that branch of the circuit. In the case of a battery charger,
you probably wouldn't chose the "voltage rise" orientation even
if given the opportunity.

===========

Again: Trying to define "voltage" in terms of "potential drop"
is a Bad Idea for multiple reasons. First of all it is inconsistent
with the fundamental physics, and secondly it is inconsistent with
the way other people use the word. Maybe you think it makes your
job easier, but I'm not convinced it really does ... and even if
it did, it would come at a terrible price, because even if
hypothetically it gave you a 0.01% advantage, it puts the students
at a disadvantage because they will have to unlearn it before they
can take an engineering course or read a book on the subject or
communicate with anyone outside your classroom.