Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page Visit Our Archives Ask A Question How To Ask A Question Question of the Week NEWTON Teachers Our Expert Scientists Volunteer at NEWTON! Referencing NEWTON Frequently Asked Questions About Ask A Scientist About NEWTON Education At Argonne Iron Phase Changes and Energy Location

Name: Nate
Status: educator
Grade: 9-12
Location: OH
Country: USA
Date: Summer 2013


Question:
I am attending a Materials Science workshop in Ohio. We were reviewing heating curves and phase changes. The working definition of energy that we are using is that energy is a conserved quantity that flows when there is a change. Using this, energy is described by location, such as in a field or in motion. During a classic change of phase, say heat of fusion or heat of vaporization, the kinetic energy (vibration) does not change, but energy is stored in the electric field within the system. So far, we understand what is happening. In iron, there are two solid phase changes. Let us take this in heating from room temperature. The curie point is 770 C. When cooling, symmetry is broken at this temperature. Where does the magnetic energy go when above this temperature? At about 912 C, the iron goes from BCC to FCC and is more dense. During the phase transition, the temperature does not change. Does the thermal energy of heating go into the electric field in this closer packing? What is energy transfer and to what location? At 1394 C, the iron goes back to BCC. So where is the energy stored for this phase transition? Why does it go back to BCC, or why is there a middle range of FCC?



Replies:
Despite our loose use of the term, we do not know what energy IS. We know how it behaves, but not what it IS. In the words of Richard Feynman, "It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount." In most standard physics and chemistry texts Energy is defined as the capacity to do work. This is not correct. The first law says:

dE = d q - d w neither heat transfer, d q, nor d w alone is conserved. It is their algebraic sum that is conserved. This is how energy behaves, but not what it is. The second law can be written in terms of the free energy (??): dG = dH – TdS. It is this “free energy” that is the capacity to do work, but even so it refers to changes, not absolute quantities. It is only the entropy, Third Law, that is known in absolute terms.

In phase changes, whether structural or magnetic, it is the change in entropy that “soaks up the thermal and/or magnetic” order/disorder.

The definition of many terms, like energy, where the definition is described by a list of behaviors, not essences!! Defining fundamental terms is a tricky business, more often misused than not.

Vince Calder


Click here to return to the Material Science Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 223
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: November 2011
Weclome To Newton

Argonne National Laboratory