The quantity of heat in a solid, liquid, or gas, can be measured. The degree of this hotness (conversely, coldness), is known as temperature. Temperature is expressed, with regard to several arbitrary scales. It also indicates the direction of spontaneous flow of heat from a hotter body, of higher temperature, to a cooler body, of lower temperature.
It is common for people to think temperature is the equivalent of the energy contained within a thermodynamic system. This is wrong, as we see in the case of a burning log of wood and an iceberg, in the polar ice cap. The burning log is at a higher temperature, but the total heat energy contained in the iceberg, is far superior.
Temperature is what you call an intensive property – a property that is independent of the quantity of matter in question. Other intensive properties include pressure and density. Of course, there are extensive properties, like mass and volume.
Temperature has extensive implications, in all disciplines of science and technology. Did you know temperature is the most significant determinant, in the distribution of organisms? Well, temperature influences the physical state of water, and most organisms will not survive in environments, where temperatures are below 0 °C or above 45 °C for any length of time.
How to measure temperature
A thermometer is the instrument for temperature measurement. Every thermometer is calibrated according to at least one of the temperature scales mentioned above. The most common temperature scales are:
- the Celsius (°C)scale, formerly known as the centigrade
- the Fahrenheit (°F) scale
- the Kelvin (K) scale, the SI unit of temperature
The Fahrenheit is the least used of these three, being only common in the United States, and some other English-speaking countries. The Celsius has the widest adoption, especially in countries where the metric system of measurement is used, while the Kelvin scale is the absolute temperature scale. It leverages the Celsius scale, such that absolute zero (-273.15 °C, the coolest anything can be), coincides with 0 K.
A fourth absolute temperature scale is the Rankine scale, common in certain fields of engineering, where it is preferred over the Kelvin. The Rankine’s unit of measure is the Fahrenheit, such that the degree Rankine, equals 1 °F.
For historical measure, we will mention the Réaumur (°Re) temperature scale. It was used in many parts of Europe in the 18th and 19th centuries, to measure mixture temperature, during brewing, milk, during cheese making, and syrups, while producing some food products.
Everyday applications make it convenient to use the Celsius scale in measurements. On this scale, 0 °C represents the freezing point of water, and 100 °C is its boiling point at sea-level, atmospheric pressure. Liquid droplets commonly exist in clouds, at less than zero temperatures. This is why 0 °C is better defined as the melting point of ice. On the Celsius scale, a difference of one degree Celsius equals a 1 K (one kelvin) increment, but the scale is offset, by melting temperature of ice – 273.15 K.
By international convention, two fixed points define the Kelvin and Celsius scales. The two points are absolute zero and the triple point, of the Vienna Standard Mean Ocean Water. The Vienna Standard Mean Ocean Water is water specially prepared, with a specific blend of hydrogen and oxygen isotopes (variant forms of an element, with different atomic mass).
Absolute zero is precisely 0K and -273.15 °C. At this temperature, all classical translational motion of particles of matter, stop, and they are at complete rest. One important point to note about absolute zero (-273.15 °C) is that, it is practically impossible to attain. Absolute zero in theory, means a total cessation of all thermal motion.
In terms of quantum mechanics, there is zero-point motion with an associated zero-point energy. Here, matter is in its ground state, with no thermal energy, whatsoever. This leads us to the triple point of water, defined as 273.16 K and 0.01 °C.
The benefit of this definition is that, it establishes 1 K, as exactly the same magnitude as 1 °C. It also determines the distinction of 273.15 K that sets the null points of these scales apart. Thus,
0 K = -273.15 °C and 273.16 K = 0.01 °C.
The Fahrenheit scale that is used in the US and a few other countries has the freezing point of water at 32 °F, and boiling point at 212 °F.
It is common to convert temperature values from one temperature scale to another. To do this quickly, the following formulae can help this endeavour:
|Scale||From Celsius||To Celsius|
|Fahrenheit||F = (C x 9/5) + 32||C = (F – 32) x 5/9|
|Kelvin||K = C + 273.15||C = K – 273.15|
|Rankine||R = (C + 273.15) x 9/5||C = (R – 491.67) x 5/9|
|Delisle||De = (100 – C) x 3/2||C = 100 – De x 2/3|
|Newton||N = C x 33/100||C = N x 100/33|
|Réaumur||Re = C x 4/5||C = Re x 5/4|
|Romer||Ro = C x 21/40 + 7.5||C = (Ro – 7.5) x 40/21|
The table above also includes other less-common scales that are of relevance in certain industries.
Refrigeration, temperature, and food safety
To impede growth of harmful bacteria, it is necessary to chill stored foods, to proper temperatures. This is where a refrigerator thermometer becomes necessary. Even at room temperature, bacteria causing food-borne illness, doubles every 20 minutes. A refrigerator kept at 40 °F or below, is effective to ensure this doesn’t happen. While a few refrigerators are equipped to show actual temperatures, one can monitor temperature with an inexpensive appliance thermometer and adjust the setting of the refrigerator as necessary.
Refrigeration strategies to keep food safe include keeping it covered in containers or sealed storage bags, checking expiration dates on foods, and cleaning the fridge frequently. Others are letting air circulate around the refrigerator, by not over-packing it, and wiping up spills, immediately. This last tip reduces the growth of Listeria, and prevents cross-contamination, where bacteria spread from one food, to another.
Various effects of temperature
Temperature can have varying effects on physical systems. It can alter the physical properties of materials like state – solid, liquid, gas, plasma; electrical conductivity, density, vapour pressure, and solubility. It also impacts the rate and extent of chemical reactions.
Other physical effects include the properties and amount of thermal radiation, emitted from the surface of an object, in addition to sound speed (square root of the absolute temperature), within a particular medium.
A discussion on temperature is incomplete, without any mention of plasma physics. It is the field of physics that deals with electromagnetic phenomena, at extremely high temperatures. Temperature can be expressed as energy in electronvolt (eV) units, or kiloelectronvolts (keV).
The energy is calculated as the product of the Boltzmann constant, and temperature (E = kBT). Each electronvolt unit is the equivalent of 11,605 K, and temperatures in the region of several hundred MeV, are common in the study of QCD matter. This equals 1012 K.
Being a vast and important topic, temperature deserves extensive study, in the fields of manufacturing, electronics, refrigeration, food production and delivery, as well as temperature-controlled systems and vehicles.