We explain radiation heat transfer with examples. The transfer of heat by radiation is the flow of energy through electromagnetic waves. Because these waves can move through a vacuum at the speed of light, they can also transmit heat. example of radiation heat transfer
Electromagnetic waves have a whole continuum of wavelengths, called the spectrum, which ranges from the longest and least energetic wavelengths to the shortest and most energetic ones.
Among them is infrared radiation, a band close to the visible wavelength band or light, but below it. In this way, large amounts of heat from the Sun reach the Earth , crossing millions of kilometers. example of radiation heat transfer
But not only incandescent objects like the Sun emit heat in the form of radiation, in fact any object does it continuously, only when the temperature is low, the wavelength is large and therefore the energy, which is inversely proportional to it. , is small.
How is heat transmitted by radiation?
When electrons vibrate, they emit electromagnetic waves. If the waves are of low frequency, it is equivalent to saying that their wavelength is long and the movement of the wave is slow, therefore it has little energy. But if the frequency increases, the wave moves faster and has more energy. example of radiation heat transfer
An object with a certain temperature T emits radiation with frequency f , so that T and f are proportional. And since electromagnetic waves do not need a material medium to propagate, infrared photons, which are responsible for propagating radiation, can move without problem in a vacuum.
This is how radiation from the Sun reaches Earth and the other planets. However, with distance the waves attenuate and the amount of heat decreases. example of radiation heat transfer
Stefan’s Law and Wien’s Law example of radiation heat transfer
The law Stefan states that the power P irradiated (all wavelengths) is proportional to T 4 , according to the expression:
P = A σe T 4
In International System units, power is in watts (W) and temperature is in kelvin (K). In this equation, A is the surface area of the object, σ is the Stefan – Boltzman constant, which is 5.66963 x10 -8 W / m 2 K 4 ,
Finally e is the emissivity or emittance of the object, a numerical value without units, between 0 and 1. The value is given according to the material, since very dark bodies have high emissivity, the opposite of a mirror. example of radiation heat transfer
Radiation sources, such as the filament of a light bulb or the Sun, emit radiation in many wavelengths. That of the Sun is almost entirely in the visible region of the electromagnetic spectrum.
Between the maximum wavelength λ max and the temperature T of the emitter there is a relationship given by Wien’s law:
λ max ∙ T = 2.898. 10 −3 m⋅K
Radiation from a black body example of radiation heat transfer
The following figure shows energy emission curves as a function of temperature in kelvin, for an ideal object that absorbs all the radiation that hits it and is in turn a perfect emitter. This object is called a black body.
The spaces between the coals of the embers in a furnace behave as ideal radiation emitters, of the black body type, with a fairly close approximation. Numerous experiments have been done to determine the different temperature curves and their respective wavelength distributions. example of radiation heat transfer
As can be seen, the higher the temperature, the shorter the wavelength, the higher the frequency and the radiation has more energy.
Assuming that the Sun behaves like a black body, among the curves shown in the figure, the one that is closest to the temperature of the solar surface is 5500 K. Its peak is at the wavelength of 500 nm ( nanometers).
The temperature of the solar surface is approximately 5700 K. From Wien’s law:
λ max = 2.898 × 10 −3 m⋅K / 5700 K = 508, 4 nm
This result roughly agrees with that seen in the graph . This wavelength belongs to the visible region of the spectrum, however, it must be emphasized that it only represents the peak of the distribution. In reality, the Sun radiates most of its energy between the infrared, visible and ultraviolet wavelengths.
Examples of heat transfer by radiation
All objects, without exception, emit some form of radiation heat, however, some are much more notable emitters:
Electric ranges, toasters and electric heaters
The kitchen is a good place to study the mechanisms of heat transfer, for example radiation is seen by (carefully) approaching the hand to the electric burner that glows orange. Or also to the embers of a grill to grill. example of radiation heat transfer
Resistive elements in electric heaters, toasters, and ovens also become hot and glow orange, also transmitting radiation heat.
Incandescent bulbs example of radiation heat transfer
The filament of incandescent bulbs reaches high temperatures, between 1200 and 2500 ºC, emitting energy distributed in infrared radiation (most of it) and visible light, orange or yellow in color. example of radiation heat transfer
Sun example of radiation heat transfer
The Sun transmits heat by radiation towards the Earth, through the space that separates them. In fact, radiation is the most important heat transfer mechanism in almost all stars, although others, such as convection, also play an important role.
The source of energy inside the Sun is the thermonuclear fusion reactor in the core, which releases large amounts of energy through the conversion of hydrogen into helium. Much of that energy is in the form of visible light, but as previously explained, ultraviolet and infrared wavelengths are also important.
Planet Earth is also an emitter of radiation, although it does not have a reactor at its center, like the Sun.
Earth’s emissions are due to the radioactive decay of various minerals in its interior, such as uranium and radium. That is why the interior of deep mines is always hot, although this thermal energy is of a lower frequency than that emitted by the Sun.
Since the Earth’s atmosphere is selective with different wavelengths, the Sun’s heat reaches the surface without problem, since the atmosphere allows higher frequencies to pass through.
However, the atmosphere is opaque to lower-energy infrared radiation, such as that produced on Earth by natural causes and by human hands. In other words, it does not let it escape to the outside and therefore contributes to the global warming of the planet. example of radiation heat transfer