Energy

[Brian McInnes <bmcinnes@PNC.COM.AU>]

Let's look at this variance in the description and the terms used by us in
this debate.

We are looking at situations where processes occur involving changes in the
energy within a system or between adjacent systems.

It seems to me that, depending on our approach,  we describe what happens to
energy in one of three ways.

The first and most parsimonious approach is to talk (write) about changes in
the energy.

The  second approach is to talk (write) about energy interchanges.

The third approach is to talk (write) about the flow of energy.

Now, all three ways are based on  conservation of energy.

Take a very simple case: a boy falls out of a tree and lands on the ground
below.
No matter which of the three approaches  (and sets of words) are used, there
will be agreement (I hope) that between the initial and final situation the
gravitational potential energy of the boy-earth system has been reduced and
the internal energy of the boy and the ground have increased and that the
magnitudes of the reduction and the increase are equal

All groups agree that changes have occured; the first group wonder why it is
necessary to use the word flow to describe what is happening.

I agree with John Denker that flow means that any positive change in a given
region is balanced by a simultaneous negative change in an adjacent region.
My brief description  of the word is that it means " move smoothly and
continuously".

What then is my problem with its use.  I believe its use carries the
pedagogical danger that our students, especially at an introductory level,
move simply and quickly from the word flow to the misconception that the
property flowing is a fluid.  In the everyday world of these students things
that flow are fluids and fluids are stuff.  I argue strongly that energy is
not stuff, and is not a fluid.  Surely we all agree that energy is an
attribute of a system and not an independent stuff.

What do we gain from this description and this use of the term fluid that we
can take this chance of leading students down the garden path?

Finally, for this contribution, I'd like to comment on one argument put
forward by John during the week.  He wrote
> Now, when *you* are writing about the energy change in region A, you may
> decide that you are not interested in what's going on in the adjacent
> region B.  That's fine.  You are free to say nothing about region B, so
> long as you don't say wrong things about it.  Your description of the
> universe may be incomplete, but it is not wrong.

> However, suppose *I* am doing the writing.  Suppose I choose to give a
> more-complete description, mentioning the whole flow, not just one end of
> the flow.

John , That is just so wrong. Those in the first camp do not give an
incomplete description.  When we write about change, we are looking at the
change in both region A and region B.  We, like you, use the conservation of
energy.  We just don't see the need to talk about flow. We say this is what
the energy situation was at the start of the process, this is what it is at
the end of the process.  Work was done (on a macroscopic and microscopic
level - some would call the latter heat) during the process and the result
was the change in energy in both System A and B.  We don't see that anything
extra is gained by the use of the word "flow" and we are frightened of the
baggage that accompanies the word.

Brian McInnes