THERMAL RADIATION: PROPERTIES, EXAMPLES, APPLICATIONS - PHYSICAL - 2024

The thermal radiation is the energy transmitted by a body with its temperature and by the wavelengths of infrared electromagnetic spectrum. All bodies without exception emit some infrared radiation, no matter how low their temperature.

It happens that when they are in accelerated motion, electrically charged particles oscillate and thanks to their kinetic energy, they continuously emit electromagnetic waves.

Figure 1. We are very familiar with the thermal radiation that comes from the Sun, which is in fact the main source of heat energy. Source: Pxhere.

The only way that a body does not emit thermal radiation is for its particles to be completely at rest. In this way, its temperature would be 0 on the Kelvin scale, but reducing the temperature of an object to such a point is something that has not been achieved yet.

Thermal radiation properties

A remarkable property that distinguishes this heat transfer mechanism from others is that it does not need a material medium to produce it. Thus, the energy emitted by the Sun, for example, travels 150 million kilometers through space and reaches the Earth continuously.

There is a mathematical model to know the amount of thermal energy per unit of time that an object radiates:

This equation is known as Stefan's law and the following quantities appear:

- Thermal energy per unit time P, which is known as power and whose unit in the International System of Units is the watt or watt (W).

-The surface area of the object that emits heat A, in square meters.

-A constant, called Stefan - Boltzman constant, denoted by σ and whose value is 5.66963 x10 ^-8 W / m ² K ⁴,

-The emissivity (also called emittance) of the object e, a dimensionless quantity (without units) whose value is between 0 and 1. It is related to the nature of the material: for example a mirror has low emissivity, while a very dark body has high emissivity.

-And finally the temperature T in kelvin.

Examples of thermal radiation

According to Stefan's law, the rate at which an object radiates energy is proportional to the area, the emissivity, and the fourth power of the temperature.

Since the rate of emission of thermal energy depends on the fourth power of T, it is evident that small changes in temperature will have a huge effect on the radiation emitted. For example, if the temperature doubles, the radiation would increase 16 times.

A special case of Stefan's law is the perfect radiator, a completely opaque object called a black body, whose emissivity is exactly 1. In this case, Stefan's law looks like this:

It happens that Stefan's law is a mathematical model that roughly describes the radiation emitted by any object, since it considers emissivity as a constant. Emissivity actually depends on the wavelength of the emitted radiation, the surface finish, and other factors.

When e is considered constant and Stefan's law is applied as indicated at the beginning, then the object is called a gray body.

The emissivity values ​​for some substances treated as gray body are:

-Polished aluminum 0.05

-Black carbon 0.95

-Human skin of any color 0.97

-Wood 0.91

-Ice 0.92

-Water 0.91

-Copper between 0.015 and 0.025

-Steel between 0.06 and 0.25

Thermal radiation from the Sun

A tangible example of an object that emits thermal radiation is the Sun. It is estimated that every second, approximately 1,370 J of energy in the form of electromagnetic radiation reaches Earth from the Sun.

This value is known as the solar constant and each planet has one, which depends on its average distance from the Sun.

This radiation perpendicularly passes through each m ² of the atmospheric layers and is distributed in different wavelengths.

Almost all of it comes in the form of visible light, but a good part comes as infrared radiation, which is precisely what we perceive as heat, and some also as ultraviolet rays. It is a large amount of energy enough to meet the needs of the planet, in order to capture it and use it properly.

In terms of wavelength, these are the ranges within which the solar radiation that reaches Earth is found:

- Infrared, what we perceive as heat: 100 - 0.7 μm *

- Visible light, between 0.7 - 0.4 μm

- Ultraviolet, less than 0.4 μm

* 1 μm = 1 micrometer or one millionth of a meter.

Wien's law

The image below shows the distribution of radiation over wavelength for various temperatures. The distribution obeys Wien's displacement law, according to which the wavelength of the maximum radiation λ _max is inversely proportional to the temperature T in kelvin:

λ _max T = 2.898. 10 ⁻³ m⋅K

Figure 2. Graph of radiation as a function of wavelength for a black body. Source: Wikimedia Commons.

The Sun has a surface temperature of approximately 5,700 K and radiates primarily at shorter wavelengths, as we have seen. The curve that most closely approximates that of the Sun is that of 5000 K, in blue and of course has the maximum in the range of visible light. But it also emits a good part in infrared and ultraviolet.

Applications of thermal radiation

Solar energy

The large amount of energy that the Sun radiates can be stored in devices called collectors, to later transform it and use it conveniently as electrical energy.

Infrared cameras

They are cameras that, as their name suggests, operate in the infrared region instead of in visible light, like common cameras. They take advantage of the fact that all bodies emit thermal radiation to a greater or lesser extent depending on their temperature.

Figure 3. Image of a dog captured by an infrared camera. Originally the lighter areas represent those with the highest temperature. The colors, which are added during processing to facilitate interpretation, show the different temperatures in the animal's body. Source: Wikimedia Commons.

Pyrometry

If temperatures are very high, measuring them with a mercury thermometer is not the best option. For this, pyrometers are preferred, through which the temperature of an object is deduced knowing its emissivity, thanks to the emission of an electromagnetic signal.

Astronomy

Starlight is very well modeled with the black body approximation, as well as the entire universe. And for its part, Wien's law is frequently used in astronomy to determine the temperature of stars, according to the wavelength of the light they emit.

Military industry

The missiles are aimed at the target using infrared signals that seek to detect the hottest areas in aircraft, such as engines for example.

References

1. Giambattista, A. 2010. Physics. 2nd. Ed. McGraw Hill.
2. Gómez, E. Conduction, convection and radiation. Recovered from: eltamiz.com.
3. González de Arrieta, I. Applications of thermal radiation. Recovered from: www.ehu.eus.
4. NASA Earth Observatory. Climate and Earth's Energy Budget. Recovered from: earthobservatory.nasa.gov.
5. Natahenao. Heat applications. Recovered from: natahenao.wordpress.com.
6. Serway, R. Physics for Science and Engineering. Volume 1. 7th. Ed. Cengage Learning.