In this group of resource materials we are looking at a number of properties of materials which reflect their structure and the interactions between molecules:
For both these topics, we will be making some simplifying assumptions which do not represent the whole truth! Be aware that real materials exhibit both non-linearity and anisotropy – in other words, their properties are not the same under all conditions and in all directions.
Specifically, conductivity and other properties of materials vary with temperature, and that variation may not be linear. For example, the tungsten filament of a lamp may change in resistance only a relatively small amount at around room temperature, but the hot resistance of a lamp can be 10 times that of the cold filament1 . And water first contracts when you cool it, and then expands, having its maximum density at around 4°C.
1 That is why lamps tend to fail when a lamp is switched on. The thermal shock from the current surge causes filament rupture.
There may also be differences in all kinds of characteristics depending on whether one is measuring through the thickness of a material or across its surface. For example:
These differences with axis are referred to as anisotropy, as distinct to the materials being ‘isotropic’, that is the same in all directions (from the Greek).
If the general topic interests you, and you would like to get a
different perspective, as well as more information, you may enjoy
a visit to Georgia University’s Hyperphysics site at http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html.
Explore the Heat and Thermodynamics section.
[back to top]
Adhesion and flow have a number of common elements, because both are associated with interactions between molecules, both within a liquid and between a liquid and solid surface.
We therefore start our brief tour of the associated physics by looking at the phenomena of surface tension and wetting, which are familiar in our everyday experience, but generally poorly understood, and then examining why things stick together, and how fluids and semi-fluids move.
Just in case you wonder why these issues are important, just reflect on what would happen if the layers of a printed circuit board didn’t stick together, or solder paste didn’t flow properly!
Again, the Hyperphysics site (http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html) has a wealth of information, and gives a different perspective. Look up Fluids in the Mechanics section. The site also has sections on chemistry and electrochemistry that will be relevant elsewhere in this course.
We suggest that you follow the sequence below. After you have read Atoms and bonding, you have a choice of routes.
|Conductors and insulators||Effects at liquid surfaces|
|Conducting heat||Viscosity and flow|
|Coefficient of Thermal Expansion||Flow in practice|
Don’t forget to ask for assistance if you find difficulty with any of these concepts.