Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot. Quantitatively, temperature is measured with thermometers, which may be calibrated to a variety of temperature scales.
Much of the world uses the Celsius scale (°C) for most temperature measurements. It has the same incremental scaling as the Kelvin scale used by scientists, but fixes its null point, at 0°C = 273.15K, the freezing point of water.[note 1] Few countries, most notably the United States, use the Fahrenheit scale for common purposes, a historical scale on which water freezes at 32 °F and boils at 212 °F.
For practical purposes of scientific temperature measurement, the International System of Units (SI) defines a scale and unit for the thermodynamic temperature by using the easily reproducible temperature of the triple point of water as a second reference point. For historical reasons, the triple point is fixed at 273.16 units of the measurement increment, which has been named the kelvin in honor of the Scottish physicist who first defined the scale. The unit symbol of the kelvin is K.
Temperature is one of the principal properties studied in the field of thermodynamics. Particularly important in this field are the differences in temperature between regions of matter, because such differences are the driving force for heat,[1] which is the transfer of thermal energy. Spontaneously, heat flows only from regions of higher temperature to regions of lower temperature. If no heat is transferred between two objects, the objects have the same temperature.
In the classical thermodynamic approach to temperature, temperature of an object varies with the speed of the particles it contains, raised to the second power. Therefore, temperature is tied directly to the mean kinetic energy of particles moving relative to the center of mass coordinates for that object. Temperature is an intensive variable because it is independent of the bulk amount of elementary entities contained inside, be they atoms, molecules, or electrons. In order for the temperature of a system to be defined, the system must be in thermodynamic equilibrium. Temperature may be considered to vary with position only if, for every point, there is a small neighborhood around that point can be treated as a thermodynamic system in equilibrium (i.e. local thermodynamic equilibrium). In the statistical thermodynamic approach, degrees of freedom are used instead of particles.
In a more fundamental approach, the empirical definition of temperature arises from the conditions of thermodynamic equilibrium, expressed as the zeroth law of thermodynamics.[2] When two systems are in thermal equilibrium, they have the same temperature,[3] which is also a matter of common experience. The extension of this principle as an equivalence relation between multiple systems fundamentally justifies the use of a thermometer and prescribes the principles of its construction to measure temperature.[4][5] While the zeroth law permits the definition of a set of many empirical scales of temperature, the second law of thermodynamics selects the definition of a single preferred, absolute temperature function,[6] whence called the thermodynamic temperature. This function is the variation of the internal energy with respect to changes in the entropy of a system. Its natural, intrinsic origin or null point is absolute zero at which the entropy of any system is at a minimum. Although this is the lowest absolute temperature described by the model, the third law of thermodynamics postulates that absolute zero cannot be attained by any physical system.
Much of the world uses the Celsius scale (°C) for most temperature measurements. It has the same incremental scaling as the Kelvin scale used by scientists, but fixes its null point, at 0°C = 273.15K, the freezing point of water.[note 1] Few countries, most notably the United States, use the Fahrenheit scale for common purposes, a historical scale on which water freezes at 32 °F and boils at 212 °F.
For practical purposes of scientific temperature measurement, the International System of Units (SI) defines a scale and unit for the thermodynamic temperature by using the easily reproducible temperature of the triple point of water as a second reference point. For historical reasons, the triple point is fixed at 273.16 units of the measurement increment, which has been named the kelvin in honor of the Scottish physicist who first defined the scale. The unit symbol of the kelvin is K.
Temperature is one of the principal properties studied in the field of thermodynamics. Particularly important in this field are the differences in temperature between regions of matter, because such differences are the driving force for heat,[1] which is the transfer of thermal energy. Spontaneously, heat flows only from regions of higher temperature to regions of lower temperature. If no heat is transferred between two objects, the objects have the same temperature.
In the classical thermodynamic approach to temperature, temperature of an object varies with the speed of the particles it contains, raised to the second power. Therefore, temperature is tied directly to the mean kinetic energy of particles moving relative to the center of mass coordinates for that object. Temperature is an intensive variable because it is independent of the bulk amount of elementary entities contained inside, be they atoms, molecules, or electrons. In order for the temperature of a system to be defined, the system must be in thermodynamic equilibrium. Temperature may be considered to vary with position only if, for every point, there is a small neighborhood around that point can be treated as a thermodynamic system in equilibrium (i.e. local thermodynamic equilibrium). In the statistical thermodynamic approach, degrees of freedom are used instead of particles.
In a more fundamental approach, the empirical definition of temperature arises from the conditions of thermodynamic equilibrium, expressed as the zeroth law of thermodynamics.[2] When two systems are in thermal equilibrium, they have the same temperature,[3] which is also a matter of common experience. The extension of this principle as an equivalence relation between multiple systems fundamentally justifies the use of a thermometer and prescribes the principles of its construction to measure temperature.[4][5] While the zeroth law permits the definition of a set of many empirical scales of temperature, the second law of thermodynamics selects the definition of a single preferred, absolute temperature function,[6] whence called the thermodynamic temperature. This function is the variation of the internal energy with respect to changes in the entropy of a system. Its natural, intrinsic origin or null point is absolute zero at which the entropy of any system is at a minimum. Although this is the lowest absolute temperature described by the model, the third law of thermodynamics postulates that absolute zero cannot be attained by any physical system.
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