Temperature and Heat

Temperature is usually measured using one of three units -- degrees Fahrenheit, degrees Celsius, or degrees Kelvin. In the United States the Fahrenheit scale is most common. Using this scale water boils at 212 degrees and freezes at 32 degrees. The Celsius scale is more common in the rest of the world. Using this scale water boils at 100 degrees and freezes at zero degrees. Either of these scales is comfortable for everyday life but for scientific purposes the Kelvin scale is better.

The following formulas can be used to convert from one scale to another.

Degrees Kelvin = Degrees Celsius + 273

Degrees Celsius = Degrees Kelvin - 273

                      9 
Degrees Fahrenheit = --- * Degrees Celsius + 32
                      5
                   5
Degrees Celsius = --- * (Degrees Fahrenheit - 32)
                   9

Whichever scale you use, temperature reflects the random motion of molecules. We feel fast moving molecules as hot and slower moving molecules as cold. But temperature is a bit more complex than just the speed of random motion -- it measures the kinetic energy of moving particles. You may know that it takes more energy to accelerate a car from 50 to 60 miles per hour than from 10 to 20 miles per hour. The energy required to accelerate an object of mass m from rest to the speed v is


All three temperature scales measure energy rather than raw speed. It is almost correct to say that takes the same energy to raise an object one degree regardless of its initial temperature. The reason it is not completely correct is that there are other factors involved besides kinetic energy. One such factor, for example, is the intermolecular forces. For our purpose we will ignore these other factors.

The calorie is a unit of energy that is roughly the amount of energy required to raise one gram of water one degree Celsius. Because of intermolecular forces the initial temperature does make a small difference. So we define the calorie more precisely as the energy require to raise one gram of water from 14.5 degrees Celsius to 15.5 degrees Celsius.

The difference between the Celsius scale and the Kelvin scale is that in the Celsius scale zero represents the freezing point of water but in the Kelvin scale zero represents zero energy -- the temperature of an object whose molecules have no random motion.