Everything about Temperatures totally explained
Temperature is a
physical property of a
system that underlies the common notions of hot and cold; something that's hotter generally has the greater temperature. Specifically, temperature is a property of matter. Temperature is one of the principal parameters of
thermodynamics. On the microscopic scale, temperature is defined as the average energy of microscopic motions of a single particle in the system per
degree of freedom. On the macroscopic scale, temperature is the unique physical property that determines the direction of heat flow between two objects placed in thermal contact. If no heat flow occurs, the two objects have the same temperature; otherwise heat flows from the hotter object to the colder object. These two basic principles are stated in the
zeroth law and
second law of thermodynamics, respectively. For a solid, these microscopic motions are principally the vibrations of its atoms about their sites in the solid. For an
ideal monatomic gas, the microscopic motions are the translational motions of the constituent gas particles. For a multiatomic gas,
vibrational and
rotational motion should be included too.
Temperature is measured with
thermometers that may be
calibrated to a variety of
temperature scales. In most of the world (except for the
United States,
Jamaica, and a few other countries), the
degree Celsius scale is used for most temperature measuring purposes. The entire scientific world (the U.S. included) measures temperature using the Celsius scale and thermodynamic temperature using the
kelvin scale, which is just the Celsius scale shifted downwards so that 0 K= −273.15 °C, or
absolute zero. Many engineering fields in the U.S., especially high-tech ones, also use the kelvin and degrees Celsius scales. However, the United States is the last major country in which the
degree Fahrenheit temperature scale is used by most lay people, industry, popular
meteorology, and government. Other engineering fields in the U.S. also rely upon the
Rankine scale (a shifted Fahrenheit scale) when working in thermodynamic-related disciplines such as
combustion.
Overview
Intuitively, temperature is a measure of how hot or cold something is, although the most immediate way in which we can measure this, by feeling it, is unreliable, resulting in the phenomenon of
felt air temperature, which can differ at varying degrees from actual temperature. On the molecular level, temperature is the result of the motion of particles which make up a substance. Temperature increases as the energy of this motion increases. The motion may be the translational motion of the particle, or the internal energy of the particle due to molecular vibration or the excitation of an
electron energy level. Although very specialized laboratory equipment is required to directly detect the translational thermal motions, thermal collisions by atoms or molecules with small particles suspended in a
fluid produces
Brownian motion that can be seen with an ordinary microscope. The thermal motions of atoms are
very fast and temperatures close to
absolute zero are required to directly observe them. For instance, when scientists at the
NIST achieved a record-setting cold temperature of 700 nK (1 nK = 10
−9 K) in 1994, they used
optical lattice laser equipment to
adiabatically cool
caesium atoms. They then turned off the entrapment lasers and directly measured atom velocities of 7 mm per second in order to calculate their temperature.
Molecules, such as O
2, have more
degrees of freedom than single atoms: they can have rotational and vibrational motions as well as translational motion. An increase in temperature will cause the average translational energy to increase. It will also cause the energy associated with vibrational and rotational modes to increase. Thus a
diatomic gas, with extra degrees of freedom rotation and vibration, will require a higher energy input to change the temperature by a certain amount, for example it'll have a higher
heat capacity than a monatomic gas.
The process of cooling involves removing energy from a system. When there's no more energy able to be removed, the system is said to be at
absolute zero, which is the point on the
thermodynamic (absolute) temperature scale where all kinetic motion in the particles comprising matter ceases and they're at complete rest in the “classic” (non-
quantum mechanical) sense. By definition, absolute zero is a temperature of precisely 0
kelvins (−273.15
°C or −459.67
°F).
Details
The formal properties of temperature follow from its mathematical definition (see below for the zeroth law definition and the second law definition) and are studied in
thermodynamics and
statistical mechanics.
Contrary to other thermodynamic quantities such as
entropy and
heat, whose microscopic definitions are valid even far away from
thermodynamic equilibrium, temperature being an average energy per particle can only be defined at thermodynamic equilibrium, or at least local thermodynamic equilibrium (see below).
As a system receives heat, its temperature rises; similarly, a loss of heat from the system tends to decrease its temperature (at the--uncommon--exception of negative temperature; see below).
When two systems are at the same temperature, no heat transfer occurs between them. When a temperature difference does exist, heat will tend to move from the
higher-temperature system to the
lower-temperature system, until they're at thermal equilibrium. This heat transfer may occur via
conduction,
convection or
radiation or combinations of them (see
heat for additional discussion of the various mechanisms of heat transfer) and some ions may vary.
Temperature is also related to the amount of
internal energy and
enthalpy of a system: the higher the temperature of a system, the higher its internal energy and enthalpy.
Temperature is an
intensive property of a system, meaning that it doesn't depend on the system size, the amount or type of material in the system, the same as for the
pressure and
density. By contrast,
mass,
volume, and
entropy are
extensive properties, and depend on the amount of material in the system.
The role of temperature in nature
Temperature plays an important role in almost all fields of science, including physics, chemistry, and biology.
Many physical properties of materials including the
phase (
solid,
liquid,
gaseous or
plasma),
density,
solubility,
vapor pressure, and
electrical conductivity depend on the temperature. Temperature also plays an important role in determining the rate and extent to which
chemical reactions occur. This is one reason why the human body has several elaborate mechanisms for maintaining the temperature at 37 °C, since temperatures only a few degrees higher can result in harmful reactions with serious consequences. Temperature also controls the type and quantity of thermal radiation emitted from a surface. One application of this effect is the
incandescent light bulb, in which a
tungsten filament is
electrically heated to a temperature at which significant quantities of visible
light are emitted.
Temperature-dependence of the
speed of sound in air
c, density of air
ρ and
acoustic impedance Z vs. temperature °C
| Impact of temperature on speed of sound, air density and acoustic impedance |
| T in °C |
c in m/s |
ρ in kg/m³ |
Z in N·s/m³ |
| −10 |
325.4 |
1.341 |
436.5 |
| −5 |
328.5 |
1.316 |
432.4 |
| 0 |
331.5 |
1.293 |
428.3 |
| 5 |
334.5 |
1.269 |
424.5 |
| 10 |
337.5 |
1.247 |
420.7 |
| 15 |
340.5 |
1.225 |
417.0 |
| 20 |
343.4 |
1.204 |
413.5 |
| 25 |
346.3 |
1.184 |
410.0 |
| 30 |
349.2 |
1.164 |
406.6 |
Temperature measurement
Main article: Temperature measurement, see also The International Temperature Scale.
Temperature measurement using modern scientific
thermometers and temperature scales goes back at least as far as the early 18th century, when
Gabriel Fahrenheit adapted a thermometer (switching to
mercury) and a scale both developed by
Ole Christensen Rømer. Fahrenheit's scale is still in use, alongside the
Celsius scale and the
kelvin scale.
Units of temperature
The basic unit of temperature (symbol:
T) in the
International System of Units (SI) is the
kelvin (Symbol: K). The kelvin and Celsius (Centigrade) scales are, by
international agreement,
defined by two points:
absolute zero, and the
triple point of
Vienna Standard Mean Ocean Water (water specially prepared with a specified blend of hydrogen and oxygen isotopes). Absolute zero is defined as being precisely 0 K
and −273.15 °C. Absolute zero is where all
kinetic motion in the particles comprising matter ceases and they're at complete rest in the “classic” (non-
quantum mechanical) sense. At absolute zero, matter contains no
thermal energy. Also, the triple point of water is defined as being precisely 273.16 K
and 0.01 °C. This definition does three things: 1) it fixes the magnitude of the kelvin unit as being precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water; 2) it establishes that one kelvin has precisely the same magnitude as a one degree increment on the
Celsius scale; and 3) it establishes the difference between the two scales’ null points as being precisely 273.15 kelvins (0 K = −273.15 °C and 273.16 K = 0.01 °C). Formulas for converting from these defining units of temperature to other scales can be found at
Temperature conversion formulas.
In the field of
plasma physics, because of the high temperatures encountered and the
electromagnetic nature of the phenomena involved, it's customary to express temperature in
electronvolts (eV) or kiloelectronvolts (keV), where 1 eV = 11,604 K. In the study of
QCD matter one routinely meets temperatures of the order of a few hundred
MeV, equivalent to about 10
12 K.
For everyday applications, it's very often convenient to use the
Celsius scale, in which 0 °C corresponds to the temperature at which water
freezes and 100 °C corresponds to the
boiling point of water at sea level. In this scale a temperature difference of 1 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the kelvin scale, but offset by the temperature at which water freezes (273.15 K). Thus the following equation can be used to convert from degrees Celsius to kelvins.
» (8)
ie. the reciprocal of the temperature is the rate of increase of entropy with respect to energy.
Further Information
Get more info on 'Temperatures'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://temperature.totallyexplained.com">Temperature Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
