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What is Temperature?

Temperature is a way to measure the warmth or coolness of an object, medium, body, or system. Warmth and coolness are things we can feel, but what are we feeling? On an atomic level we are feeling vibrations of atoms. In holding an item we can sense the motion of its component parts.

Image of an old thermometer.

Before the Enlightenment, scientists believed that temperature and heat were pretty much the same thing. Heat was thought to be a fluid-like substance they called caloric contained within things that could flow from one object to another. Things with more of this fluid would allow it to flow into things with less of it. They did not differentiate between the amount of heat and the intensity of the heat1.

As far-fetched as their notions seem to us today, these were actually crucial insights. For heat does move from one body to another. Yet it does so by sub-microscopic particles bouncing off other submicroscopic particles. The particles that are vibrating faster or have greater velocity, hit those that are vibrating less. In the course of this the particles with higher vibrations lose some of their motion, and those with less receive some. It is very like a que-ball hitting another ball on a pool table. Some of the energy of the que-ball is imparted to the other ball. During this process, the temperature of the warmer object is lowered, and of the cooler object is raised. The energy from these vibrations is called kinetic energy.

Temperature quantifies this movement. It is done by placing a calibrated instrument within a system and allowing it to come to equilibrium with that environment. When it does so, the change in motion of some component part of it, relative to a more stable part, will indicate to the viewer how much vibrating is going on in the whole system. This calibrated instrument is generally called a thermometer. An old fashioned thermometer will have a glass tube filled with an element such as mercury. The tube is relatively stable and will be marked with even gradations that have been predetermined.

Many scales have been developed to calibrate temperature. In most of the world, the Celsius or centigrade scale is used. For centigrade, the freezing point of water is considered to be zero degrees, the boiling point is 100 degrees, and each degree in between is an equal 1/100th of the distance between freezing and boiling. Fahrenheit is a scale still used in the United States. It was developed with less thought to the decimal system. On the Fahrenheit scale freezing is 32 degrees, and boiling is 212 degrees (180 degrees difference). The Kelvin scale was created to be more scientific. When all vibrating and motion stops within a substance it is called "absolute zero". This is 273.15 degrees below zero on the Celsius scale, and the same standard of 1/100th of the distance between freezing and boiling of water is used as the size of a kelvin. (A degree on the Kelvin scale is called simply a "kelvin".)

We have created a tool to help you convert between Celsius, Fahrenheit, and Kelvin Scales.

To learn all about temperature, continue reading through this site by clicking on the "next page" links at the bottom of each page. To focus on a particular topic, use the links in the text above, or on the navbar near the top of this page.

Fahrenheit and His Scale -->

  1. About Temperature - UCAR

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