Archive for the ‘thermochemistry’ Category

Ice Skates

Tuesday, November 1st, 2011

In the book, there is an example of the phenomenon that occurs when a thin wire with weights attached to both ends is placed on top of a block of ice.  This wire pass through the block of ice without leaving a path because the ice melts with the increase in pressure and then refreezes over top once the wire passes through.

This example made me think of a property of ice skates.  This is the concept that the pressure of the blade on the ice melts the ice directly below it, not because the blades are at a warmer temperature, but because of the increase in pressure.

Ice skating blades are not exactly what people assume them to be;  each blade consists of a

http://www.everglides.co.uk/about_ice_skates.html

rocker and a hollow.  The rocker is a curve that is lengthwise along the bottom of the blade and has its lowest point at the ball of the foot, where the hollow is a curve of radius ranging from 1/2” to 15/16” on the bottom of the blade.  It is common knowledge to ice skaters that the hollow allows for turns by “biting” into the ice upon changes in center of gravity, but what is not know is that the hollow also allows for the gliding property.  This article discusses how this occurs when the pressure from the weight of the person is exerted

http://noicingsports.com/employee_bios1_2.htm

downwards, melting the ice at the two points of contact.  These points of contact then sink in further and the water melted is pushed up into the hollow of the skate.  The water in the hollow acts as a cushion and reduces the friction between the blade and the ice.  According to the article, the more water that is in the hollow, the easier it is to achieve higher speeds. Therefore, with a flatter rocker, more gliding properties are observed.  It is for this reason that speed skaters can achieve speeds of 30mph (also, their hollow is flatter and thus does not cause as much drag).

The statements made by this article make sense for the most part.  The water would appear in the hollow because the volume of water is smaller than the volume of ice. Therefore  with increase in pressure  and moderately low temperatures, more water will form.  The concept that the longer blades make gliding more probable may not be entirely correct however.  This is because pressure is (mass x acceleration)/area and a longer blade would decrease the pressure, decreasing the amount of ice converted to water.  Gliding still may increase however, because the downwards force may be large enough that the increase in area is negligible.

 

Another source referenced:

http://www.jce.divched.org/journal/Issues/1988/Feb/jceSubscriber/JCE1988p0186_1.pdf

Phase diagram

Sunday, October 30th, 2011

An interesting phase diagram is the diagram of carbon as can be seen below or in this article.  Carbon, at standard temperature and pressure, is in the form graphite.  This solid state is crystalline hexagonal shape.  Another common form of carbon is diamond.  As seen in the diagram, diamond only forms at extremely high pressures thus the solid form of carbon has a more compact structure and is tetrahedral.  The article also says that there is evidence that another solid phase that is metallic and cubic in structure exists at extreme temperatures and pressures.

In the diagram, there are several intermediate phases that occur in between graphite and diamond. These phases are actually consisting of two types of structures; a metastable diamond form with graphite at lower pressures (carbyne), and the metastable graphite with diamond at higher pressures (chaoite). The triple point of the graphite, diamond, and liquid phases is at about 4300-4700K and 10 GPa which is also where coexistence lines for the metastable forms intersect.  Therefore, if the authors are correct, there are 5 phases in equilibrium at this point.  The second triple point for liquid, graphite and vapor occurs at almost the same temperature, but at much lower pressures.

http://lbruno.home.cern.ch/lbruno/documents/Bibliography/LHC_Note_78.pdf

Also, in viewing the diagram, one can see that graphite will sublime to form the vapor form of carbon at standard pressure and elevated temperatures.  The liquid form on the other hand requires both higher temperatures and pressures (at least 4×103K and about .2 GPa).  Another characteristic of carbon is that the appearance of the gas phase at all temperatures ends rather abruptly with increase in pressure.

Carbon is strange in the fact that all of these transitions occur at such high temperatures and pressures, but other than that, it’s just another element.