Download Explain Thermal Expansion

Unit 2: explaining thermal expansion
Compilation. Unit 2: explaining thermal expansion
Date: Thu, 2 Mar 2006
From: Glen M. Carter, Broward Schools, Florida
Subject: Re: Internal Energy
Just about to complete Unit 2 and need an explanation on why the height (volume) in a
thermometer column rises. The kids have more or less figured that as energy is applied to a
liquid that affects a temperature change, that applied energy increases the speed of particles (KE
account). If the heat applied creates a phase change, e.g. boiling, then that applied energy
reduces the attraction between particles (IE account) and ultimately creates a volume increase.
The question presented by a few kids is, "if the increased energy (temperature increase) only
affects the particles’ speed and not the attraction to one another, should the particles not be the
same distance from each other and ultimately not affect a volume change?"
I realize the volume change is extremely small, hence the reason for a capillary tube to
see the change. So how do you relate the increased energy that affects the speed of the particles
to this small volume increase?
-------------------Date: Fri, 3 Mar 2006
From: Darrell Rahn, a Department of Defense Education Agency high school.
My students raised the same question. Here is what I did. I threw a ball up in the air and
talked about the gravitational and potential energy account changes. Is there ever a time when
one is changing and the other isn't? No. In the case of the particles both accounts are changing
all of the time but not as uniformly. During phase changes the interaction energy changes dwarf
the KE changes, and during warming the KE changes are much more noticeable than the IE
changes. One student’s interpretation: "it's a sig figs issue."
Talking about a possible mechanism seemed to help, too. Something like: During
warming the particles are moving around a lot more but because of the attractions between
particles they are not getting much further away; therefore the IE changes are small. During
phase changes the particles are initially gaining KE but now have enough speed to overcome the
attractive forces and move large distances from neighboring particles. As they move away the
forces attracting them slow them just as the gravitational force slowed the ball. The net effect is a
transfer of KE to IE and the overall KE account has changed little while the IE account has
changed a lot.
-------------------Date: Sun, 5 Mar 2006
From: Larry Dukerich, Dobson HS, Arizona State University
To me, this is yet another argument for why one should do physics before chemistry. The
physics people are lucky because in their initial studies they can reduce almost every situation to
one or two point particles in the system. The simple phenomena we chemists try to explain are
much more complicated because there are so many particles interacting with one another.
For years I have struggled with how to tell a manageable story about energy storage and
transfer in such simple situations like phase change. I looked at the heating curves and wondered
why we have to go through such great pains in the real laboratory to obtain data that yield such
perfect graphs. Well, this is just another example of modeling: these representations are
idealized. We need to remind our students that the real world is so messy that it is difficult to
explain.
So, in order to make sense of it, we make all sorts of simplifying assumptions. If we don't
keep reminding ourselves that we are describing a model of the situation (and not the situation
itself) we can end up with statements that are incredible. Take for example, our description that
the energy supplied to the system is stored as Ei (and not Ek) during phase change. Of course
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Unit 2: explaining thermal expansion
that's a simplification; as anyone who has heated a sample of icy water knows, it's easy to have
an ice-water mixture in which a thermometer shows a temperature above 0.
We should be careful not to imply that the energy supplied to our system of a myriad of
particles somehow "knows where to go." Students should see that in certain groups of particles in
the system the Ek CAN increase, but then, as energy is transferred through collisions of particles,
the Ek of that particular set of particles decreases as the Ei of another collection increases. The
reason we heat gently and stir frequently is to allow these transfers to even things out during
phase change so that we can see more than one energy storage account.
If you want a silly $$ analogy, you can have two kids representing the energy accounts in
the system: Bob represents Ek and Bill is Ei. Someone else (the surroundings) passes dollar bills
to Bob who promptly hands them to Bill. The net effect is that Bill has a greater balance than
before, but at a number of instants in the process, Bob was temporarily richer, too.
I think that both Darrell (and Andy after him) have provided credible explanations of the
phenomenon of thermal expansion of a solid or liquid that should satisfy one who wants to probe
more deeply. I think we tell the kids that we sometimes pretend that only one step is occurring at
a time because that way it's easier to represent simpler changes with such tools as energy bar
graphs.
-------------------Date: Mon, 6 Mar 2006
From: Carmela Minaya, Hanalani School, Hawaii
Also another thought, every book has those fraction of particles storing kinetic energy vs.
kinetic energy of system graphs for students to get an idea of the big picture. Reading those
graphs and interpreting physical meaning is a feat in and of itself. I think that with particle
diagrams and energy bar graphs should help to fill in the picture at different levels.
-------------------Date: Tue, 7 Mar 2006
From: John Barrere, Fresno, CA
I'm presently taking Bruce Sherwood's online course Matter & Interactions. It is a great
course and I highly recommend it. Lots of emphasis on basic principles and connecting the
macro & micro worlds. Modeling matter at the micro level as balls and springs allows good
prediction of many macro properties. In this regard, energy inputs are stored as KE of the balls or
PE in the springs with shifting back and forth. Adding more energy increases both the average
KE and the average PE (and therefore the average disps of the balls).
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