We explain what heat transfer is and how it is produced by conduction, convection and radiation. Also, insulators and measurements.
What is heat transfer?
It is called heat transfer, thermal transfer or heat transmission to the physical phenomenon that consists of the transfer of heat energy from one medium to another.
This occurs when two systems that are at different temperatures come into contact, allowing the flow of energy from the point of highest temperature to the point of lowest, until a thermal equilibrium is reached, in which the temperatures are equal.
The heat transfer process It is unstoppable (it cannot be stopped) although it can be slowed down (it can be decelerated) using bars and insulators. But whenever there is a heat difference in the universe, heat will tend to transfer through the available media. Depending on them, said transfer can occur through three modes: conduction, convection and radiation.
Driving
Driving is called heat transfer through direct contact of the particles of one material with those of another without transferring matter between the bodies. It occurs in all states of aggregation: solid, liquid or gaseous, although in the latter two convection is usually preferred.
The amount of heat transferred through conduction is determined by Fourier's Law, according to which the rate of heat transfer through a body is proportional to the temperature gradient that exists in it.
A simple example is seen in an electric stove: the burner heats up due to the electric resistances and that heat is transferred by conduction to the pan that we place on it and, in turn, the pan will do the same with the food we cook.
It also happens when we accidentally touch the hot pan with our hand: the heat will be transferred to our skin upon contact, causing a burn.
Convection
Convection is similar to conduction, except that it occurs in cases where a fluid receives heat and moves to transmit it within a space where it is contained. Convection is the transport of heat through the movement of a fluid whether gaseous or liquid.
This transfer occurs in the terms set forth by Newton's Law of Cooling, which establishes that a body loses its heat at a rate proportional to the temperature difference between the body and its surroundings.
A clear example of this occurs when we heat water in a container. The heat transferred by conduction from the container to the liquid will heat the portions that are in direct contact with it, which will rise and force other cold portions of the liquid to take their place, thus heating the water uniformly.
Radiation
The last type of heat transfer is also the only one that can occur in the absence of contact and, therefore, also of a physical medium, that is, in a vacuum.
This is because his origin is in the thermal movement of particles charged with matter, which triggers the emission of electromagnetic particles, that is, thermal radiation, its intensity being dependent on its temperature and the wavelength of the radiation considered.
Generally, bodies in this situation emit ultraviolet radiation, but above certain temperatures they can emit radiation in the visible spectrum, that is, light. The amount of heat radiated in this way can be determined by the Stefan-Boltzmann Law.
Every day we observe the best example of thermal radiation: the Sun. Despite being 149.6 million kilometers from our planet, the Sun's temperature is so high that it radiates enormous amounts of light and heat into space.
Both things reach the Earth's surface and keep it warm and illuminated, with wavelengths ranging from ultraviolet to infrared, obviously passing through the entire visible spectrum.
See also: Electromagnetic spectrum
Insulators and radiation barriers
As we have said, heat transfer cannot be prevented, but it can be slowed down, through the use of certain materials. This is because All materials transfer heat in one way or another, but not at the same rate not even with the same ease.
Those that transmit it quickly and efficiently are called thermal conductors. On the contrary, those that do it slowly and laboriously are called thermal insulators (conduction and convection) or barriers (radiation).
A clear example of insulating materials are those that make up a thermos, which allow a liquid to be kept hot or cold for a longer amount of time, by slowing down its heat exchange with the environment.
Units of measurement of heat transfer
According to the International System of Measurements, the conductivity of a body is expressed in joules (J), the same as for work and energy. However, there are other commonly used units for measuring heat transfer:
- Kilocalories (Kcal) A calorie is defined as the amount of heat necessary to raise the temperature of one gram of water by one degree Celsius. It is a measurement often used in nutrition to measure the chemical energy contained in foods. One kilocalorie is equivalent to 1000 calories.
- BTU British Thermal Unit or British thermal unit) It is defined as the amount of heat necessary to raise the temperature of one pound of water by one degree Fahrenheit, which is equivalent to 252 calories. This measure is commonly used in English-speaking countries, mainly the United Kingdom and the United States.
Continue with: Specific heat
References
- “Heat transfer” on Wikipedia.
- “Radiation, conduction and convection: three forms of heat transfer” in Nergiza.
- “Heat transfer” at the National Experimental University of Táchira (Venezuela).
- “Heat Transfer: Crash Course Engineering #14” (video) on Crash Course.
- “Heat Transfer (Physics)” in The Encyclopaedia Britannica.
