What Is Thermal Radiation? Definition, Principles & Real-Life Uses
Thermal Radiation
Heat is a very peculiar form of energy. It assists us in staying warm, makes hot and tasty food, but when it is applied in daily life, it far exceeds the domestic uses mentioned here. By understanding the properties of heat, we can find the key to many branches of science. Thermodynamics is a vast field which deals only with the flow of heat through a system. Even nuclear energy which uses the heat, developed by the atom to create electricity. So it is crystal clear that heat is significant to us. This makes it much vitally essential for us to take a closer look at heat. Thermal Radiation and heat transfer are very important for us to understand the heat energy. Its transmission will help us to understand the characteristics of heat transfer and thermal Radiation.
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What is Heat Transfer?
Heat transfer is defined as the process of transfer of heat from a high-temperature state of the body to the low-temperature state of the body. By following the modes of heat transfer, we can say that thermal Radiation can be of three types:
Conduction: It can be defined to be the heat transfer between two solids.
Convection: It can be defined to be the heat transfer between solid and liquid.
Radiation: It can be defined to be heat transfer when there are no media involved.
What is Thermal Radiation?
Thermal Radiation is the process of transfer of heat by the help of the Electromagnetic Radiation, which is generated by the thermal motion of particles in matter. For the maximum bodies on Earth, this particular electromagnetic Radiation lies in the invisible part of the spectrum, which is known as the Infrared region. Each and every particle with a temperature above is the absolute zero, which emits Thermal Radiation. Thermal Radiation is generally caused by the motion of the particles inside the body. At the absolute zero, this type of motion is wholly stopped, which is the reason why a body at absolute zero does not allow any radiation and everything above absolute zero does. The difference between the state of energy which was there before and the energy which is radiated out after it is called radiation heat loss. Thermal Radiation is very much responsible for the glowing nature of hot objects where iron is often termed as being red hot because at that temperature most of the thermal energy emitted by the body falls in the red band of the spectrum. At very even higher temperatures, it will start emitting a different colour. The invention of heat radiation is a fascinating one. It was invented by the English Astronomer, William Herschel. He noticed that when a thermometer is moved from one aperture of a prism spectrum to another, this will result in a temperature change. The highest temperature which was observed was below the red band of the visible light spectrum. Therefore, the name Infrared. Infrared waves, though, should not be allowed to be confused with Heatwaves. Hence, all forms of electromagnetic radiation transfer energy take from place to place; they could all be coined the term as Heat Waves.
Solved Examples
How is Radiation Different from Other Two Modes?
Unlike the processes of Conduction and Convection, Thermal Radiation does not require any medium to transfer heat. The heat which the Earth receives from the sun is by the process of Radiation. This is because the Radiation can be provided through electromagnetic waves, and they do not require a medium for transmission. Thermal Radiation has another fascinating property. It has been discovered that darker bodies can absorb and have to emit heat radiation better than bodies which are constituted with lighter colours.
Did You Know?
Autoclaving is a very general method of heat sterilisation in which steam is applied under pressure at a temperature of 121 °C for 15 minutes. This technique applies only to items which are not at all heat sensitive or would have their properties to change during the process. The procedures of Sterilisation or filtration can also be achieved for the case of liquids and gases which are heat generally sensitive, which is used to an appropriate size filter for the contaminant, which is to be excluded. This is a very general method of using heat transfer which is used to isolate viruses.
FAQs on Thermal Radiation Heat Transfer Explained for Students
1. What is thermal radiation as a mode of heat transfer?
Thermal radiation is a fundamental process of heat transfer where energy is emitted by matter in the form of electromagnetic waves due to the thermal energy of its particles. Unlike conduction or convection, thermal radiation does not require any medium to propagate and can travel through a vacuum, which is how the Sun's heat reaches Earth.
2. How does thermal radiation fundamentally differ from conduction and convection?
The primary difference lies in the mechanism and medium requirement. Here’s a breakdown:
- Thermal Radiation: Transfers heat via electromagnetic waves and needs no medium. It can occur in a vacuum.
- Conduction: Transfers heat through direct molecular collision and requires a solid, liquid, or gas medium.
- Convection: Transfers heat through the bulk movement of fluids (liquids or gases) and requires a fluid medium.
3. What is the Stefan-Boltzmann law for thermal radiation?
The Stefan-Boltzmann law states that the total radiant heat energy emitted per unit area from a surface is directly proportional to the fourth power of its absolute temperature (in Kelvin). For an ideal emitter, known as a blackbody, the formula is E = σT⁴, where E is the energy radiated per unit area, T is the absolute temperature, and σ is the Stefan-Boltzmann constant.
4. What is a 'blackbody' in the context of thermal radiation, and why is this concept important?
A blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. It is also a perfect emitter of thermal radiation. This concept is crucial in physics because it provides a theoretical standard for measuring and comparing the radiative properties of real objects. Real objects are often characterized by their emissivity, which is a ratio of their radiation to that of a blackbody at the same temperature.
5. What are some real-world examples of thermal radiation heat transfer?
Thermal radiation is a common phenomenon observed in daily life. Key examples include:
- The warmth you feel from the Sun across the vacuum of space.
- The heat radiating from a campfire or an electric heater.
- A hot stove burner glowing red and radiating heat to its surroundings.
- The use of thermal imaging cameras, which detect infrared radiation emitted by objects to create a heat map.
- The Earth absorbing solar radiation and re-radiating it, which is central to the greenhouse effect.
6. Why is it more comfortable to wear light-coloured clothes in summer and dark-coloured clothes in winter?
This is a direct application of thermal radiation principles. Dark-coloured surfaces are good absorbers of radiant energy, including sunlight. In winter, they absorb more heat, helping to keep you warm. Conversely, light-coloured surfaces are poor absorbers and good reflectors of radiation. In summer, they reflect most of the incoming sunlight, absorbing less heat and helping you stay cool.
7. How does Wien's Displacement Law relate to the changing colour of a hot object?
Wien's Displacement Law explains the relationship between an object's temperature and the peak wavelength of its emitted radiation. The law states that as an object's temperature increases, the peak wavelength of its emitted light becomes shorter. This is why a piece of metal, when heated, first glows dull red (longer wavelength), then progresses to bright orange-yellow, and eventually to bluish-white (shorter wavelength) at very high temperatures.
8. What is the role of thermal radiation in the Earth's greenhouse effect?
The greenhouse effect is a natural process driven by thermal radiation. The Sun radiates high-energy, short-wavelength radiation that passes through the Earth's atmosphere and warms the surface. The Earth, in turn, re-radiates this energy as lower-energy, long-wavelength infrared radiation. Greenhouse gases like carbon dioxide and methane in the atmosphere are effective at absorbing this outgoing infrared radiation, trapping the heat and keeping the planet warmer than it would otherwise be.
