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Physical Science: Session 7

A Closer Look: Why do We Need Three Scales of Temperature?

Thermometer showing the Fahrenheit
and Celsius scales.

As mentioned in the video, the two scales we are most familiar with, the Fahrenheit and Celsius (also known as centigrade) scales, are defined in terms of events that are universal: a temperature of zero on the Fahrenheit scale is the temperature of a mixture of equal parts ice, water, and salt, and the freezing point of water is what sets the zero point on the Celsius scale. A difference of one degree Celsius is larger than a difference of one degree Fahrenheit.

So why do we need yet another temperature scale?

If we examine the definition of temperature as it relates to our particle model, we can see that there is something more fundamental on which to base our temperature scale. Temperature is related to the average energy of the motion of the particles of the object. Therefore, a natural point for a temperature scale is the point at which all particle motion stops. This point is defined as zero on the Kelvin scale. The unit of the Kelvin scale is referred to as a "Kelvin," and the magnitude of one Kelvin is the same as the magnitude of one degree Celsius. The zero temperature on the Kelvin scale is called absolute zero.

Is it possible to reach absolute zero?

Kelvin thermometer
Thermometer using the Kelvin scale.

In a word, no. As we've seen in previous sessions, successful scientific models often break down when they are applied to circumstances with extreme conditions: for example, mass conservation breaks down when we deal with the high temperatures and pressures inside a star. Very low temperatures are another extreme. As it turns out, the best model for understanding the world of the very small, quantum mechanics, dictates that we cannot completely stop the motion of any particles, no matter how hard we try. However, with certain special techniques (including using the light from lasers to slow down particles), scientists have been able to lower the temperature of matter to just a fraction of a Kelvin above absolute zero.

At these very low temperatures, matter behaves very differently than it does near room temperature. As a result, scientists had to develop further refinements to their particle model. However, since everyday life is experienced between the extremes of high and low temperatures, we do not observe or experience any of these unusual effects.

prev: a closer look intro