Iron Phase Changes and Energy Location
Date: Summer 2013
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?
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.
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Update: November 2011