Measurement of Thermal Energy
The measurement of thermal energy involves indirect measurement of the molecular kinetic energies of a substance. Rather than providing an absolute measure of molecular kinetic energy, thermal measurements are designed to determine differences that result from work done on, or by, a substance (e.g., heat added to, or removed from, a substance). Temperature differences correspond to changes in thermal energy states and there are several analytical methods used to measure differences in thermal energy via measurement of temperature.
When dealing with the terminology associated with the measurement of thermal energy, one must be mindful that there is no actual substance termed "energy" and no actual substance termed "heat." Accordingly, when speaking of energy "transfer" or heat "flow", one is actually referring to changes in functions of state that can be raised or lowered only within a body or system. Neither energy or heat can really be "transferred" or "flow."
In thermodynamics, temperature is directly related to the average kinetic energy of a system due to the thermal agitation of its constituent particles. In accord with entropic law, temperature-as designated by a value on a scale (e.g., Celsius)-is commonly said to determine the direction of heat "flow" between systems (e.g., heat will always "flow" from a body of higher temperature to a body of lower temperature). More properly, thermal energy will decrease in the initially warmer body and increase in the initially cooler body.
In practical terms, temperature measures heat and heat measures the thermal energy of a system. In meteorological systems, for example, temperature, as an indirect measure of heat energy), reflects the level of sensible thermal energy of the atmosphere. Such measurements utilize thermometers and are expressed on a given temperature scale, usually Fahrenheit or Celsius. The common glass thermometer containing either mercury or alcohol uses the property of thermal expansion of the respective fluid as an indirect measure of the increase or decrease in the thermal energy of a body or system.
Other types of thermometers utilize properties such as resistivity, magnetic susceptibility, or light emission to measure temperature.
Electrical thermometers (e.g., thermoprobes, thermistor, thermocouples) that relate changes in electrical properties (e.g., resistivity) to changes in temperature are extensively used in scientific research and industrial engineering.
A technique known as differential thermal analysis measures the difference in temperature between a sample and a known reference as heat is applied to or lost from a system. Differential thermal analysis is sensitive to endothermic and exothermic processes and is a useful method to characterize various process (e.g., phase transitions).
Thermogravimetry measurements relate changes in the mass of a sample to temperature differences. These types of thermal measurements are often useful in determining the purity of a substance. Thermogravimetry measurements are sensitive indicators of water, carbonate, and organic content in a sample. Thermogravimetry studies are also useful in the study and characterization of various decomposition reactions.
A technique known as differential scanning calorimetry measures the rate of thermal energy change (i.e., heat flow) in a sample and relates that change to a standard at the same temperature (i.e., in thermal equilibrium). Data derived from differential scanning calorimetry are useful in measuring the heat capacities of various substances.
Because energy is commonly defined as the ability to do work, the thermal energy of a system is directly related to a system's ability to translate heat energy into work. Correspondingly, the measurement of the thermal energy of a system must be interpreted as the measurement of the changes in the ability of a system or body to do work. Absolute zero Kelvin (-459.69°F, -273.16°C, 0°R on the Rakine scale) is the lowest temperature theoretically possible. At absolute zero there is a minimum of vibratory motion and, by definition, no work can be done by a system on its surrounding environment. In this regard, such a system (although not motionless) would be said to have zero thermal energy.
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