20.1. Thermodynamics http://www.ck12.org
FIGURE 20.4
A Celsius thermometer.This choice was entirely arbitrary in any universal sense, but it served physics completely adequately. Any other unit
of distance would have worked as well.
The slightly tricky aspect to scales of measure is that — to completely define one — in addition to picking the unit
size in terms of physical quantities (which is clear from above) one needs to set a zero level. When dealing with
length scales, the zero level is apparent: the complete absence of length. Because of this, 0 miles is the same as 0
feet and 0 meters and conversion between lengths is a matter of multiplication alone (why?). This seems obvious,
but let’s ask a more subtle question: would a length scale where a length of zero corresponds to, say 1 meter on the
SI scale, work consistently? The answer is that yes, it would, but it would be cumbersome in mathematical analysis;
there would be negative lengths: a rather counter-intuitive concept.
As we will later see,the properties of temperature also suggest anabsolutezero for temperature. If all our
scales were set with that zero, conversion between temperature scales would be as easy as between length scales:
simple multiplication (miles to kilometers, meters to centimeters, etc). Unfortunately, the Celsius and Fahrenheit
scales were created before temperature was this well defined, so they assign the value of ’0 degrees’ to arbitrary
points, and, therefore, have negative temperatures and are cumbersome in mathematical analysis. Still, any two
temperature scales can be related through a linear relationship.
The Celsius scale — used throughout most of the world — establishes its unit, or degree Celsius, by defining the
temperature difference between the freezing and boiling point of water as 100 degrees Celsius. This is analogous to
the definition of meter above. However, it assigns a temperature of 0 to the freezing point of water; this temperature
is considerably higher than absolute zero.
Question
Find the conversion relationship between Celsius and Fahrenheit temperature scales. Explain how this is
different from converting miles to kilometers.Thus, scientists generally use the Kelvin temperature scale, which has degree increments equal to the Celsius scale’s
(and so is pretty easy to recognize and interpret — at least for people outside the U.S.), but sets the value of zero
temperature to theabsolute zero— the point at which all molecular motion ceases. On the Celsius scale, this
temperature is -273.15, so to convert between from degrees Celsius to Kelvins (frustratingly, while we call the
Celsius Scale units degrees Celsius, the Kelvin scale units are conventionally referred to simply as Kelvins) we us
the following:
Tk=Tc− 273. 15 [1]For the rest of this chapter, temperature will is assumed to be measured in Kelvins.