Temperature is a measure of heat energy of the particles in a substance
Temperature is an objective measurement of how hot or cold an object is. It can be measured with a thermometer or a calorimeter. It is a means of determining the internal energy contained within a given system.
Because humans easily perceive the amount of heat and cold within an area, it is understandable that temperature is a feature of reality that we have a fairly intuitive grasp on. Consider that many of us have our first interaction with a thermometer in the context of medicine, when a doctor (or our parent) uses one to discern our temperature, as part of diagnosing an illness. Indeed, temperature is a critical concept in a wide variety of scientific disciplines, not just medicine.
Heat Versus Temperature
Temperature is different from heat, although the two concepts are linked. Temperature is a measure of the internal energy of a system, while heat is a measure of how energy is transferred from one system (or body) to another, or, how temperatures in one system are raised or lowered by interaction with another. This is roughly described by the kinetic theory, at least for gases and fluids. The kinetic theory explains that the greater the amount of heat is absorbed into a material, the more rapidly the atoms within that material begin to move, and, the faster atoms move, the more the temperature increases. As atoms begin to slow down their movement, the material becomes cooler. Things get a little more complicated for solids, of course, but that's the basic idea.
Temperature Scales
Several temperature scales exist. In the United States, the Fahrenheit temperature is most commonly used, though the International System of Units (SI unit) Centigrade (or Celsius) is used in most of the rest of the world. The Kelvin scale is used often in physics and is adjusted so that 0 degrees Kelvin is equal to absolute zero, which is, in theory, the coldest possible temperature and at which point all kinetic motion ceases.
Measuring Temperature
A traditional thermometer measures temperature by containing a fluid that expands at a known rate as it gets hotter and contracts as it gets cooler. As the temperature changes, the liquid within a contained tube moves along a scale on the device. As with much of modern science, we can look back to the ancients for the origins of the ideas about how to measure temperature back to the ancients.
In the first century CE, the Greek philosopher and mathematician Hero (or Heron) of Alexandria (10–70 CE) wrote in his work "Pneumatics" about the relationship between temperature and the expansion of air. After the Gutenberg Press was invented, Hero's book was published in Europe in 1575, its wider availability inspiring the creation of the earliest thermometers throughout the following century