What is meant by temperature and heat
Lexicon> Letter T> Temperature
Definition: a measure of the ability of an object to give off heat
Categories: Basic Terms, Physical Basics, Heat and Cold
Formula symbol: T
Unit: ° C, Kelvin (K)
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: 03.06.2010; last change: 10.06.2020
URL: https://www.energie-lexikon.info/ Temperatur.html
Although the concept of temperature seems absolutely familiar to practically everyone, understanding it is considerably more difficult than z. B. that of thermal energy. Since here not on quite complicated considerations from the thermodynamics should be used, we only consider a few central aspects of the term, without being able to give a clear definition of the temperature:
- When two objects are in thermal contact with each other, thermal energy flows from the object with the higher temperature (i.e. from the warmer one) to the other (the colder one). This heat flow is irreversible (irreversible), i. H. Heat never flows “voluntarily” back in the other direction.
- Generally, when heat is applied to an object, it becomes warmer; H. its temperature increases. The temperature decreases accordingly when heat is extracted. In some cases, however, the addition or removal of heat does not result in a change in temperature, e.g. B. when the thermal energy is used to melt ice or is released when it solidifies (→latent heat).
- Due to the circumstances mentioned, the heat flow between two bodies will in the long run cause the temperatures to converge more and more. When the same temperatures are reached, the heat flow ceases.
The temperature of a body can therefore be interpreted as a measure of its ability to give off heat to another body. The higher the temperature of a body, the more heat is stored in it - although this amount of heat depends not only on the temperature, but also on the size and composition of the body, possibly also on its physical state.
A heat flow from a colder to a warmer body can be “forced” through the use of a heat pump, which, however, requires drive energy (exergy).
Celsius temperature and absolute temperature
In everyday life, temperatures are usually given using the Celsius scale. Here 0 ° C (0 degrees Celsius) is defined by the melting point of water and 100 ° C by the boiling point (at normal pressure). The Fahrenheit scale is still used in the USA, although this unit does not belong to the international SI system. 0 ° F corresponds to −17.8 ° C, 100 ° F corresponds to 37.8 ° C.
In principle, the temperature of an object can be as high as desired, apart from the fact that the object will be destroyed sooner or later. On the other hand, there is no such thing as low temperatures, but one absolute temperature zero, which is around −273.15 ° C. At this temperature a body can no longer give off any heat; In simplified terms, one can imagine that there is no longer any microscopic vibration of the individual atoms of the object. According to quantum theory, however, this picture is not entirely correct; there are still quantum fluctuations even at absolute zero.
The absolute temperature of an object is, so to speak, the distance between its temperature and absolute zero. They are usually given in Kelvin (K), whereby the Kelvin steps are the same as the steps on the Celsius scale. The absolute zero point is therefore at 0 K = −273.15 ° C, the melting point of water at 273.15 K = 0 ° C, the boiling point of water at 373.15 K = 100 ° C, etc. Absolute temperatures are relevant z. B. in connection with the Carnot efficiency of a heat engine.
Temperature changes or differences should in principle always be stated in Kelvin, not in degrees Celsius, for technical correctness. Therefore z. B. the unit of the heat transfer coefficient (U-value) W / (m2 K) = watts per square meter and Kelvin. Heat flows caused by conduction z. B. through a house wall or a thermal insulation system are usually proportional to the temperature difference between the two sides (outside and inside).
Temperature, exergy and entropy
The temperature at which heat is delivered determines the exergy content of this heat. High temperature heat has a high exergy share, and this means that it could in principle be converted into mechanical energy with a high degree of efficiency. This is not the case for low-temperature heat (e.g. heating); it is energy of a lower valence.
The thermodynamically optimized heating of buildings relies on heating systems that work with the lowest possible flow temperature, because this keeps the exergy content of the required heating energy low. In this way, the heating can be provided in a particularly energy-efficient manner using heat pumps. It can be seen that not only the amount of heat required plays a role for energy efficiency, but also the temperature level. The same applies to other heat applications.
When a body is exposed to an amount of heat in a reversible manner Q is supplied, its entropy increases Q / T. The lower the temperature of the body, the greater the increase in entropy.
Measurement of temperatures
The temperature of objects is often measured by bringing them into thermal contact with a thermometer, which is then brought to the same temperature in a short time. Common types of thermometers use z. B. the temperature dependence of the volume of a liquid for display on a temperature scale. Bimetal thermometers are based on the temperature-dependent bending of a rod made of two components with different thermal expansion. There are also various types of electronic temperature sensors that can use different physical effects, e.g. B. the temperature-dependent electrical conductivity of certain substances.
Temperatures can also be measured without contact, usually by measuring the thermal radiation emanating from an object. This principle is used in Infrared thermometers (see Figure 1 on the right). It is also the one Thermography with an infrared camera based on z. B. can display the temperature distribution on a building envelope and thereby helps, z. B. to recognize thermal bridges. A house with high-quality thermal insulation has a low temperature on the outer surface in winter because hardly any heat can escape from the warm rooms.
Temperature and well-being
The human body, at least in its core, is kept at a fairly constant temperature of approx. 37 ° C. If this doesn’t work anymore - z. B. by excessive heat supply or dissipation - and the body temperature deviates from this target value by only a few degrees, there is a serious problem that can quickly lead to death. This in itself is astonishing, since the “comfort zone” and the temperature range compatible with survival corresponds to only a tiny range of absolute temperatures.
Since heat is constantly being generated in the body and this has to be dissipated in order to maintain the temperature, the environment should be a little cooler - around 25 to 30 ° C without clothing, 20 ° C with clothing. The body then gives off more heat radiation than it absorbs from the environment and also gives off heat to the surrounding air. In addition, the evaporation of water on the skin and in the lungs consumes more heat; this effect can be intensified by sweating, which helps the body to regulate its temperature. This form of heat dissipation no longer works well in very high humidity - hence the tendency to sweat more in “humid” (warm-humid) weather. An air conditioning system that both cools and dehumidifies the air is then used for wellbeing.
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See also: thermodynamics, heat, heat capacity, flow temperature, thermography, Kelvin, heat pump, Carnot efficiency, thermostat
as well as other articles in the categories basic concepts, physical principles, heat and cold
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