Heat transfer . Process by which energy in the form of heat is exchanged between different bodies, or between different parts of the same body that are at different temperatures .
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- 1 General
- 2 Driving
- 3 Convection
- 4 Radiation
- 5 Units of measurements
- 6 Sources
Heat transfer always occurs from a hotter body to a colder one, as a result of the Second Law of Thermodynamics . Heat transfer occurs until bodies and their environment reach thermal equilibrium.
Heat transfers through convection, radiation, or conduction. Although these three processes can occur at the same time, it can happen that one of the mechanisms predominates over the other two.
When there is a temperature difference between two objects in proximity to each other, the heat transfer cannot be stopped; it can only slow down.
It is the mechanism of heat transfer on an atomic scale through matter by molecular activity, by the collision of some molecules with others, where the most energetic particles deliver energy to the least energetic, producing a flow of heat from the highest temperatures. high to the lowest.
Conduction is a transfer of heat between solid bodies. Heat conduction only occurs if there are temperature differences between two parts of the conductive medium.
The best conductors of heat are metals. The air , Styrofoam, cork, wood , ice , wool , paper or plastic , are poor heat conductors. These substances are called insulators.
If a person holds one end of a metal bar, and brings the other end into contact with a candle flame, so that its temperature increases, the heat will be transmitted to the coldest end by conduction. Atoms or molecules from the end heated by the flame, they acquire a greater energy of agitation, which is transmitted from one atom to another, without these particles undergoing any change in position, thus increasing the temperature of this region. This process continues along the bar and after a certain time, the person holding the other end will notice a rise in temperature there.
Convection is the mechanism of heat transfer by mass movement or circulation within the substance. If there is a temperature difference inside a liquid or gas, a movement of the fluid will almost certainly occur. This movement transfers heat from one part of the fluid to another.
Convection transfers heat through the exchange of hot and cold molecules : it causes the water in a kettle to heat evenly even though only its bottom is in contact with the flame.
This process called convection can be natural produced only by differences in the densities of matter; or forced, when matter is forced to move from one place to another, for example air with a fan or water with a pump. It only occurs in liquids and gases where atoms and molecules are free to move in the medium.
In nature , most of the heat gained by the atmosphere from conduction and radiation near the surface, is transported to other layers or levels of the atmosphere by convection.
Thermal radiation is energy emitted by matter at a given temperature, produced directly from the source outwards in all directions. This energy is produced by changes in the electronic configurations of the constituent atoms or molecules and transported by electromagnetic waves or photons , therefore it is called electromagnetic radiation .
Electromagnetic radiation is a combination of oscillating and perpendicular electric and magnetic fields that propagate through space transporting energy from one place to another.
Unlike conduction and convection, or other types of waves, such as sound , which need a material medium to propagate, electromagnetic radiation is independent of matter for its propagation, in fact, the transfer of energy by radiation is more effective in a vacuum. However, the speed, intensity, and direction of your energy flow are influenced by the presence of matter. Thus, these waves can traverse interplanetary and interstellar space and reach Earth from the sun and stars .
Units of measurement
The same as for energy and work is used to measure heat transfer in the International System of Units : the joule.
Another widely used unit to measure the amount of heat energy exchanged is the calorie (lime), which is the amount of energy that must be supplied to a gram of water to raise its temperature by 1 ° C. Different initial conditions give rise to different values for the calorie. Calorie is also known as small calorie, compared to kilocalorie (kcal), which is known as large calorie and is used in nutrition.
- 1 kcal = 1000 cal
Joule, after multiple experiments in which the movement of shovels, driven by a set of weights, moved inside a container with water, established the mechanical equivalent of heat, determining the increase in temperature that occurred in the fluid as a consequence of the friction produced by the agitation of the blades:
- 1 cal = 4.184 J 
The BTU, (or British thermal unit) is a measure of heat widely used in the United States and in many other countries in America. It is defined as the amount of heat that must be added to a pound of water to increase its temperature by one degree Fahrenheit , and is equivalent to 252 calories.