The energy balance of a system is a consequence of heat transfer.
Heat transfer is the spread of thermal energy from one place to another . It is a physical phenomenon that is analyzed by thermodynamics .
There are three major heat transfer mechanisms: radiation , convection and conduction . These modes of transmission are used in multiple areas.
Development of heat transfer
The development of heat transfer begins when two systems that are at different temperatures come into contact . This causes heat to begin to flow from the system that is at a higher temperature to the one that is at a lower temperature.
Heat transfer ends when both systems reach the same temperature. At that point the so-called thermal equilibrium is achieved.
It is important to note that once heat transfer starts, it cannot be stopped: it only stops when thermal equilibrium is achieved. What can be done is to slow it down through insulators and barriers .
Thermal resistance is the ability of a material to resist heat transfer.
thermal energy
Thermal energy is that which establishes the temperature of a system or a body. Also called heat energy , it increases or decreases through heat transfer.
At this point it should be clarified that heat is never "had" . Objects, depending on how the transfer is developed, can lose or gain it, without it being an intrinsic property of them. That is why it can be said that heat is a measure of change or a variable .
In other words: thermal energy is the internal energy that, when it increases or decreases, modifies the temperature of the system, while heat is the variable associated with the transmission of said thermal energy. The idea of heat, in short, is linked to energy in transit .
It is possible to distinguish between different types of heat. Specific heat is the amount of heat that must be added to a unit of mass of a system or element to increase its temperature by one degree Celsius.
Latent heat , for its part, is that which does not increase the temperature of the receptor since it is intended to promote a change of state. Sensible heat , on the other hand, is what modifies the temperature without generating a change of state.
Calorimetry measures the quantities involved in heat transfer.
Heat transfer by radiation
Heat transfer by radiation occurs through the emission of electromagnetic waves of energy . Except for a body with a temperature of absolute zero, all of them develop electromagnetic radiation that is related to the thermal movement of their electrically charged particles.
Radiation involves the transformation of thermal energy (equivalent to the kinetic energy of atoms) into electromagnetic energy . The movement of particles produces an acceleration of charges, creates electromagnetic fields and emits photons.
The Stefan-Boltzmann law , Kirchhoff's law , Planck's law and Wien's law are scientific postulates that describe how this process occurs and provide formulas to develop calculations in this regard.
convection
Convection is a heat transfer that takes place through two simultaneous processes. On the one hand, the movement of molecules causes propagation by conduction , while the displacement of parts of the fluid driven by an external force also incites transfer by natural convection , forced convection or mixed convection .
It should be noted that convection only takes place through fluids . Heat transfer, in fact, is carried out by the movement of the fluids in question.
When you heat water in a pot, for example, the liquid that is in contact with the base of the utensil begins to rise in the center due to the heat. In turn, the liquid in the superficial part goes down the sides as it cools, taking the position abandoned by the fraction that ascended. This heat transfer occurs by convection.
Heat transfer by conduction
Heat transfer by conduction occurs when there is a temperature difference between the particles of an element or between two objects or substances that are in direct contact .
This heat conduction, which is determined by Fourier's law , depends on the physical properties of the materials. The tendency to allow conduction is known as thermal conductivity ; The resistance to said transfer, on the other hand, is mentioned as thermal resistivity .
A body with high thermal conductivity favors heat transfer. At the other extreme, the object with high thermal resistivity rejects this propagation .
thermal insulation
Thermal insulation is a resource that aims to minimize heat transfer . The use of insulating materials or conductive materials is essential for heating and cooling .
When developing a construction, it is key to think about thermal insulation. Depending on the materials chosen, the heating or cooling of the environment will be favored. The idea is that the building is as least permeable as possible to temperature variations from the outside.
In air conditioning systems and air conditioning equipment, for example, heat exchangers are used. These devices allow heat transfer between two fluids or between a solid and a fluid. Heat accumulators and cooling towers are devices that use exchangers.
The greenhouse effect and heat transfer
Heat transfer is the key aspect of the greenhouse effect . Although the greenhouse effect that occurs naturally is essential for life on our planet, the one that is generated by human action promotes global warming that has multiple harmful effects.
Of the total solar radiation that impacts the planet after passing through the atmosphere, a percentage heats the ground and another part is reflected and returns to space. Due to the so-called greenhouse gases , the thermal radiation emitted by the surface cannot re-emerge, thus increasing the temperature on Earth.
Although there are sectors that question the impact of man on global warming, there is a consensus based on temperature measurements in recent decades that attributes the increase in marks to industrial activity and deforestation.