The Physics of Heat

Lesson 1: Temperature


When you have completed this lesson and the homework, you will be able to:

  • understand the historical development of the measurements of temperature

  • understand the difference between the relative temperature scales and the absolute temperature scale.

  • convert temperatures between the Celsius, Fahrenheit and Kelvin scales

In this lesson we will address the following standard from the Indiana Academic Standards for Physics I: 


Describe and apply the kinetic molecular theory to the states of matter.


Understand that the temperature of an object is proportional to the average kinetic energy of the molecules in it and that the thermal energy is the sum of all the microscopic potential and kinetic energies.


Describe the Laws of Thermodynamics, understanding that energy is conserved, heat does not move from a cooler object to a hotter one without the application of external energy, and that there is a lowest temperature, called absolute zero. Use these laws in calculations of the behavior of simple systems.

The Lesson


An excellent introduction to the measurement of temperature has been compiled by professor Beverly T. Lynds. at the University of North Carolina. Check out her website at  

Don't rush through this site: Pay careful attention to the first two sections, What is Temperature and The Development of Thermometers and Temperature Scales.  You will be responsible for the information in these two sections.  (This document is very long: just read the first two sections)

Celsius and Fahrenheit

From your reading you probably reached the conclusion that the Celsius temperature scale makes more sense than the Fahrenheit scale.  It is nice to have a scale where water freezes at 0.0 C and boils at 100 C.  Most Americans are not very familiar with this temperature scale, but it is the standard temperature scale for the metric system, so we need to know how to convert back and forth between the two scales.

The relationship between Celsius and Fahrenheit is expressed in the mathematical equation

If you are give the temperature in  C, you can solve this equation for  F.

So to convert back and forth between the two temperature scales, you can use the two forms of this relationship.  We will be using these many times in this unit, so make sure you write them down in a handy place.  



Relative vs. Absolute temperature scales

Temperature is a measure of how hot or cold an object is.  We all know that a temperature of 32 F is pretty cold.  We also know that 0 F  is even colder, but not the coldest possible.  How cold is the coldest possible?  Can you keep decreasing the temperature forever?   Some of the first experiments to address this issue were done on what we now call "ideal gasses".  If some gas, like Nitrogen, is placed in a container of constant volume, it was found that there was a linear relationship between the pressure and the temperature.  If you decrease the Temperature, the Pressure drops.   A sample graph of the data might have looked like this:

No you can see that as the temperature increases, so does the pressure.  But you can also see that there is a lot of room for the pressure to decrease.  If the temperature was lowered below  0 C, the pressure would continue to drop.   It makes sense that the pressure cannot drop below zero, so scientists wondered how cold would it have to be before the pressure actually got to zero.  It is a rather simple exercise in algebra to find the x-intercept of this graph.  This x-intercept could be interpreted as the coldest it could possibly ever be.   

This "coldest possible temperature" was found to be approximately -273  C. This coldest possible temperature was called absolute zero, and a new temperature scale was created that used this absolute zero.  This new scale was called the Kelvin scale, and it is what we refer to as the absolute temperature scale, since 0 Kelvin  is the lowest possible temperature.  There are no negative temperatures on the Kelvin scale. (0 Kelvin is written as 0.0 K - note that there is no symbol on the Kelvin scale).

The conversion between Celcius and Kelvin is rather simple.  To find the temperature in K, you simply add 273 to the temperature in  C.   This is expressed by the relation

Here we have used the variable T to represent temperature.( lowercase t is reserved for the quantity time). Usually subscripts are not needed, since it is possible to tell which temperature scale is used by the units that are given with the problem.


We will be working with three different temperature scales, and it is important to understand the difference between them and how to convert from one to the other.  While it is possible to use web-based converters to do this for you, it will be faster in the long run, and more useful, for you to practice the conversions manually.  

Links for further study

Need an alternate explanation? Try this site:

Homework Hint:  Recall that the symbol D means "change of" . When you are asked to find DT, it means that you must find the quantity Tfinal - Tinitial.  For example, if the oven starts off at 85 F and later it heats up to 300 F, then the change in the temperature, DT, will be 215 F. 

This is the end of the lesson. You can return to the unit main page or start the homework assignment


(c) 2003 Mark Blachly
Indiana Online Academy

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