When two bodies have the same temperature, they are said to be in thermal equilibrium. But if their temperature differs, one will yield energy to the other. There will then be a transfer of thermal energy. A transmission of heat which can be done according to several modes.
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[EN VIDÉO] Kézako: what effect does the wind have on the feeling of heat? If a little refreshing breeze is welcome in summer, we would do well without it in winter. The wind seems to have the gift of making the thermometer lie. But the feeling of heat or cold is in fact a skin-deep experience, which Unisciel and the University of Lille 1 intend to explain in this episode of Kézako.
Every time there is a temperature difference between two systems, there is a transfer of energy in the form of heat. Such heat transfers are important in many situations. When it comes to heat a room in winter or in the food industry, for example, which, in the course of its processes, heats, cools or even refrigerates food. Three modes of transmission heat are traditionally distinguished.
It is by thermal conduction that the outside of a cup of tea becomes hot after boiling water has been poured into it. Conduction occurs inside a body or when two bodies come into contact. It does not involve any apparent displacement of the matter. It is based solely on vibes ofatoms which are transmitted step by step, or on the transmission ofelectrons free. It is thus the only mode of transmission of heat within bodies. solid. In this case, the heat flow appears proportional to the gradient temperature and takes place in the opposite direction, i.e. in the direction of decreasing temperatures. The heat flow is also all the more important as the thermal conductivity bodies is high. That of bread, for example, is weak compared to that ofaluminum.
Modes of transmission that combine
When we blow on a hot soup spoon to cool it, we count on the phenomenon of convection which occurs within the fluids. Convection supposes, in fact, the displacement of material elements between regions of different temperatures. There are several modes of heat transfer by convection. When the fluid is put in movement by variations in density due to the heating or cooling of the elements of the fluid, we speak of natural or free convection. This is what happens in the case of ocean currents. When external forces — such as agitators, pumps or fans — intervene, it is about forced convection. One even speaks of strong convection when the fluid is the seat of reactions which release heat, or of endothermic transformations. Note that the heat flux transmitted depends in the case of convection, on the speed of circulation of the fluid and on its properties, in a more general way, but also on the surface characteristics of the solid with which it is in contact and on the geometry of the situation.
Finally, the Sun transmits to us its radiant heat. Radiant heat transmission relies primarily on theepisode and theabsorption ofelectromagnetic waves. The energy they carry is converted into heat. Radiation therefore does not require the intervention of any movement of matter or any contact between objects or media. He is the only one who can do himself in a vacuum.
Note that most heat transfer involves more than one of these modes of transmission. They then intervene simultaneously, in parallel or in series. A saucepan intended for heating water is a good example. First comes the radiation due to the flames, then the convection of the boiling water in particular; finally, the conduction between the handle of the saucepan and the hand of the cook.
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