What is temperature?

If someone asks you for the temperature, more often than not, you would respond with the degree or the “feel” of the temperature (hot, warm, or cold). We usually have a good feel for how a temperature is qualitatively different than another temperature. The dictionary has many definitions for temperature, such as: “the degree of hotness or coldness of a body or environment” and “a measure of the warmth or coldness of an object or substance with reference to some standard value.”

What is temperature really measuring? Some numbers on a thermometer? There is definitely more to it than that.

For a better understanding of what temperature really is, we need to review kinetic energy again. Kinetic energy is defined as the energy of motion. An object, or a particle, that is moving has kinetic energy, and includes three common forms--vibrational (motion possessed by an object that is vibrating about a fixed position), rotational (associated with an object that is rotating about an imaginary axis) and translational (movement of an object from one place to another).

The scientific definition of temperature is a measure of the average kinetic energy of the particles in a sample of matter. A sample of matter consists of particles that can be vibrating, rotating and moving through the inside of its container. At the particle level, a sample of matter possesses kinetic energy. A cup of hot chocolate on a countertop may appear to be as still as can be, but it still has kinetic energy.

At the particle level, there are atoms and molecules that are vibrating, rotating and moving through the inside of the cup. Stick a thermometer in the cup and you will see evidence that the hot chocolate possesses kinetic energy. The hot chocolate’s temperature, as reflected by the thermometer’s reading, is a measure of the average amount of kinetic energy possessed by the molecules.

When the temperature of an object increases, the particles that make up the object begin to move faster. Increasing the temperature of an object causes an increase in particle speed. So as a sample of water in a pot is heated, its molecules begin to move with greater speed, and this greater speed is reflected by a higher thermometer reading. Likewise, if a sample of water is placed in the freezer, its molecules begin to move with a lower speed and this is reflected by a lower thermometer reading. In this sense, a thermometer can be thought of as a speedometer!