|MadSci Network: Physics
I wish the answer to your son's question was simple, but it is not. Thermal conductivity of solids, as well as thermal expansion, heat content, etc. are based on how thermal energy is stored in solids at the atomic level. To understand this you must also understand a little about the atomic structure of different types of solid materials. I will provide you with a simplified discussion which I suspect you will be able to understand. I don't know of any on-line references, but a good book that we used for basic materials structure is called "The Structure and Properties of Materials" published by John Wiley & Sons. Volume 4 of this four book set (edited by R. Rose, L. Shepard, and J. Wulff) focuses on electronic properties, and covers the subject of thermal properties at an easily understandable level. Its Library of Congress catalog card number is 66-16132.
To begin with there are several different types of solid materials which are distinguished by how their atoms are arranged. Metals and ceramics have orderly atomic arrangements, the atoms are all lined up and stacked in an orderly fashion. Metals are different from ceramics in that each atom in the metal is electrically neutral; the atoms in metals do not have an electrical charge. Ceramics (or nonmetallic solids), such as salts, oxides, etc. (sodium chloride is a common salt) have similar orderly structures as metals but each of the atoms has a unique electrical charge. In such nonmetallic solids, such as salt, all the sodium atoms carry a plus 1 charge, and each chloride atom carries a minus 1 charge. These respective charges come from the fact that an electron is transferred from one atom to the other, leaving one atom with an extra electron making it negative and leaving the other atom minus one electron making it positive. Together the entire grouping is electrically neutral because the charges cancel out. Another property of metals is that a certain number of the electrons of each atom is shared among all the atoms. Metals are therefore sometimes referred to as having an electron cloud. It is this fact that gives metals their good electrical conductivity, and ceramics their poor electrical conductivity. What I want to leave you with, is that metals have electrons which can freely move around the solid and ceramics do not.
Another type of solid, called plastics or polymeric materials, also do not have free electrons. Plastics also do not generally have an orderly atomic structure like ceramics and metals. Instead plastics can be thought of as a mixture of long molecules all chained and tangled together. Although the atoms in each molecule are tightly bonded to each other. There is little bonding between the different molecules, and no general order to the structure. The lack of an orderly structure in plastics contributes to it poorer thermal conductivity as you will see.
Now given this knowledge of the structure of the different classes of solids, (metals, ceramics, and plastics), we will look at how heat energy is stored in solids. There are two main ways thermal energy is stored in solids. One way is in higher vibrational energy of the atom around its normal position and the other is in higher kinetic energy (or energy of motion) of any free electrons. In metals, heat energy is mostly transferred by the free electrons, which are free to easily move about the solid. This is why metals have the highest thermal conductivity. Here the thermal energy is picked up by the free electrons and rapidly transferred from atom to atom.
In ceramics, because there are no free electrons, the thermal energy is mostly stored as more energetic vibrations of the atom while it sits in its normal structural position. Atoms are not stationary, they are always vibrating and trying to move. Only at the temperature of absolute zero, does atomic motion cease. Therefore heat transfer in ceramics is by vibrational waves induced because one atom starts to vibrate more vigorously, which transfers some of this energy to surrounding atoms, which in turn does the same to the atoms surrounding it. These waves are called phonons, or vibrational waves at the atomic level. These waves exist and can travel only because of the orderly structure at the atomic scale. In plastic materials, there are no free electrons, there is no real atomic order to support phonon transport or energy, so heat transfer is the slowest. In plastics molecular rotation, vibration, or movement transfers heat.
Things become more complex when you try to explain why one metal transfers heat faster than another. It's based upon differing numbers of available free electrons for the different metals. Such a discussion would easily go over your son's head and is far more than a 7th grade science project should involve. Have your son find out what he can about the atomic structure of the solids he is testing, and start from that point. All material properties begin with the atomic structure and the properties of the atoms involved.
Good Luck, I hope this discussion wasn't too complex. If you have any questions about what I said here, feel free to email me at "Greg- Dries@worldnet.att.net". I also hope these are the types of materials you are asking about. If your concern is with engineering type materials, like textiles, brick, etc. then the discussion involves the issue of conduction, convection and radiation types of heat transfer.
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