What is Thermal radiation: Definition and 79 Discussions

Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation.
Infrared radiation emitted by animals (detectable with an infrared camera) and cosmic microwave background radiation are examples of thermal radiation.
If a radiation object meets the physical characteristics of a black body in thermodynamic equilibrium, the radiation is called blackbody radiation. Planck's law describes the spectrum of blackbody radiation, which depends solely on the object's temperature. Wien's displacement law determines the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the radiant intensity.Thermal radiation is also one of the fundamental mechanisms of heat transfer.

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  1. Philip Koeck

    I Radiation from solid black bodies

    I understand that Planck's law is derived for a cavity with a hole in it. I haven't found a clear argument that the same result and all results that follow from it also apply to solid surfaces that are black. Can anybody point me to a text that shows this?
  2. Engineer_Kosyakova

    Engineering COMSOL plot of thermal radiation from heated surfaces -- Help please

    There is heating of the surface of the material using an electron beam. It is necessary to calculate how much heat will be released and build a graph of dependence. Please tell me how this can be done, which modules in COMSOL can be used?Thank you!
  3. F

    I Do you think emissivity of air makes sense?

    Hello guys :)In the frame of finding a physical model for the temperature of Earth's surface, talking about the very "idealized" two-layers model of atmosphere, I ask you now the question to the other physicists or engineers: does it make sens to associate an emissivity to a layer of air (+ some...
  4. F

    A Average temperature in a greenhouse

    Hello everyone, since several weeks, no response from the other forums, I tried to compute a simple model for a greenhouse in a garden. First idea was to compute mass transfer, Navier-Stokes and heat equation all together but in my knowledge no analytical solution exists.I need to build a simple...
  5. V

    Heat Radiation question (skin's reaction to UV versus IR radiation)

    I am not sure of the answer. But I am guessing that infra-red rays carry more heat radiation than ultra-violet rays, and the heat is instantly felt on our skin when exposed to both these rays. So what we feel is the heat sensation from infra-red rays and that sensation is missing when it's...
  6. FEAnalyst

    Triangular cavity thermal radiation simulation

    Hi, I'm trying to solve a problem involving radiation in a triangular cavity: As you can see, lengths and emissivities of all surfaces are given. For two of them, the heat flux is known and the temperature has to be found while for the remaining surface it's the other way around. I have the...
  7. P

    Problem on thermal radiation and lenses

    I know that the Sun is at a very far distance from the lens, so I assume the rays are parallel and the image is formed at the focus (500mm away from the centre of the lens). How do I calculate the diameter of the image? The hint to solving this problem says that "The angular diameters of...
  8. carter7gindenv

    Clarification about Kirchhoff's law of thermal radiation

    Hello, I'd like some clarification with the exact wording of Kirchhoff's law of thermal radiation. The law states that at thermal equilibrium αλ=ελ. However sometimes I read that absorptivity= emissivity and sometimes I read that rate of A. = rate of E. Which one is correct? I'm not sure but I...
  9. carter7gindenv

    Thermal Radiation Behavior of Non-Black Bodies: I'm Confused!

    Hello, I've searched a bit about about the thermal radiation behavior of non-black body and I'm confused. I've read that the color is dependent only on the temperature so every thing display the same color as a black body. But at the same time I've read that materials don't always follow the...
  10. J

    Thermal radiation in CMOS transistors

    Inside a CPU, when applying voltage to the gates of the CMOS transistor(high input), are the gate ore another components of a transistor will generate thermal radiation which will be absorbed by the neighboring transistor?
  11. V

    A CMB as thermal radiation of cosmic dust?

    Recently, the origin of the cosmic microwave background as a relic radiation of the Big Bang was questioned and an idea of the CMB as thermal radiation of cosmic dust was revived and revisited. Under this theory, the temperature of the cosmic dust is predicted to be 2.776 K which differs from...
  12. Benhur

    Question about thermal radiation

    Hello everybody, I have a question about thermal radiation: Imagine a body at a certain temperature T, in an environment with nothing around it. Theoretically, this body emits radiation according to its temperature (the maximum radiation is described by the Wien relation); If it emits, it must...
  13. katelr

    When/How does thermal radiation stop (if it stops) and conduction

    Consider two solid objects in the vacuum (of different materials, if you will) at different temperatures approaching each other until they make "perfect contact" through flat surfaces (no gaps or defects, so that thermal contact conductance effects are absent, even though interfacial thermal...
  14. jdawg

    Engineering Heat and Mass Transfer: Thermal Radiation Circuit

    Homework Statement 1.) A piece of bread (ε = 0.85) is being toasted in a toaster, as shown below. The heating element of the toaster consists of a wire 2 mm in diameter that is wound to approximate cylinders spaced 2 cm apart. This element acts as a blackbody. The housing for the toaster is...
  15. T

    B Does Thermal Radiation from Ordinary Materials Span All Wavelengths?

    Hi. I want to know does thermal radiation contain all the wavelengths (from very near zero to almost infinity) or not? I want the thermal radiation of normal things like hot Tungsten (wolfram) or hot Iron. I don't want the black body radiation of a star. I think, theoretically it must contain...
  16. G

    Thermal radiation and charged particle acceleration

    A non ionized neutral gas should still emit thermal radiation.,say xenon. Is this solely because of electron transitions ? I ask this because a lot of sources attribute the emission to charge particle acceleration,so neutral atoms should not do this,is this right?
  17. D

    Thermal radiation in transparent objects

    Every objects emit thermal radiation. Now consider this case: Sun emits waves in a certian electromagneic spectrum (UV + thermal radiation + ecc...); glass is transparent to thermal radiation that usually reach Earth's surface. UV and other radiations are mainly absorbed by Ozone and other...
  18. Biker

    Questions About Stephan's Law of Thermal Radiation

    I took Stephan's law for thermal radiation and I have a couple of questions about it. 1) The law states that the full energy radiated in 1 sec is equal to c T^4 where c is 5.67*10^-8 and T in kelvins and In the book they said if it has surroundings then the net energy emitted would be q = c A...
  19. P

    Thermal radiation inside a cooler

    I am trying to calculate how much a foil liner helps keep the inside of a package cool. I have calculated the rate of conduction but am now concerned with radiation. Above is the equation I am using. The emissivity for the material is 0.05, the area is 1 m^2, the outside air temp is 295...
  20. lonelypancreas

    Calculating Net Heat Flow for an Aluminum Disk in a Room

    Homework Statement What is the net heat flow of an aluminum disk (emissivity = 0.05) with radius 10 cm and temperature 293K placed inside a room where the temperature is 300K? Asurface = π*r2 = 0.01π m2 Homework Equations Hnet = A*e*σ(Tradiate4-Tabsorb4) The Attempt at a Solution By simply...
  21. W

    Problem involving thermal radiation and specific heat

    Homework Statement A satellite to reflect radar is a 3.5-m-diameter, 2.0-mm-thick spherical copper shell. While orbiting the earth, the satellite absorbs sunlight and is warmed to 50 °C. When it passes into the Earth's shadow, the satellite radiates energy to deep space. You can assume a...
  22. V

    Solve Thermal Rad. Homework: Box Painted Black, 500 W Light

    Homework Statement A wooden box is painted completely with black paint. The interior dimensions are 18”x24”x3.5”. The bottom and sides are made with 9 pieces of 2x4 douglas-fir-larch dimension wood lumber. The top of the box is open and is covered with 0.118” thick clear acrylic. The acrylic is...
  23. Kaneki123

    Two Questions about thermal radiation....

    Okay... I have two questions regarding thermal radiation...The first one is that, it is a constant fact that heat travels from hot to cold body, and all things give out thermal radiation, So arent these two facts contradictory in the sense that "everything is heating every other thing regardless...
  24. Carlos de Meo

    Thermal radiation, emissivity and Kirchhoff

    Hi Guys Studying thermal radiation here and I am kinda stuck at one point Kirchhoff law states that at thermodynamical equilibrium, the amount of energy absorbed must be reemited. But before the system reaches equilibrium, is there a model to predict how much energy of these photons absorbed...
  25. S

    Thermal Radiation of a steel ball

    Why does a steel ball appear hotter than a charcoal piece at the same temperature when picked up by bare hands?
  26. M

    I Thermal radiation and light pump

    Hello, I read a lot at this forum about thermal radiation but still i have questions. Consider black body at some temperature T. It means that inside that body we activate many modes of quantum oscillators with some probability(Bose-Einstein) depend on temperature T. Now let's have some bulk...
  27. W

    Does thermal radiation occur within a material?

    Homework Statement I am doing an assignment with multiple heat transfers, and have to investigate the effects of including thermal radiation vs saying that thermal radiation is negligible. I know that the equation for thermal radiation is q = A*σ*(T14 - T24) (*) In my case the equation...
  28. A

    Thermal Radiation Homework: Snow-Covered Surface Absorption

    Homework Statement Consider a snow-covered surface. Assume that the amount of solar radiation that reaches the top of the atmosphere is 1367 Wm-2. Given that 50% of this solar radiation is either absorbed in the atmosphere or reflected by clouds before reaching the the snow covered surface...
  29. S

    Heat transfer by thermal radiation

    Homework Statement The ice is placed in water, we know the surface area A, emissivity of both ice and water, Stefan-Boltzmann constant and the temperature of both ice and water. What is the equation for heat transfer rate between water and ice? Homework Equations Heat transfer rate by...
  30. PytrTchaikovsky

    Thermal Radiation: Calc Total Emitted Joules in Certain Temp Range

    Dear forum I am working with thermal radiation. This is the specific formula: P = σ ⋅ A ⋅ T4 P = emitted effect (W, J/s) σ = Stefan-Boltzmann constant (5,67 ⋅ 10-8) A = area of object (m2) T = temperature of object (K) How can I get to know the...
  31. Sunny Singh

    Apparent violation of kirchoff's law of thermal radiation

    suppose there are two bodies A and B with emissive powers e1 and e2 respectively and with absorptive powers a1 and a2 respectively... now both bodies are at the same temperature T. Hence e1/a1 = e2/a2 = const by kirchhoffs law. for the body A, energy emitted is e1 and...
  32. L

    Thermal Radiation: Measurement & Conversion Help

    hallo to all, i use a Radiation Pyrometer to measure temperature and Radiation Trough a datalogger i get those measurements in Volt i am able to convert the Volt back in the temperature but in Radiation it is more difficult i found the Plancksche function, but i do not really understand that...
  33. A

    The peak of the thermal radiation power spectrum (dR/dλ)

    Homework Statement The peak of the thermal radiation power spectrum (dR/dλ) is at a wacelength of about λm=hc/5kT. Why is the peak of the same power spectrum plotted as dR/df not at fm=c/λm= 5kT/h? Homework Equations dR/dλ= 2πhc2/(λ5(e(hc/λkT)-1)) f=c/λ The Attempt at a Solution
  34. H

    How to calculate thermal radiation intensity without surfaces?

    Imagine there is a semi-infinite semi-transparent body--such as a glass--at some temperature T. No surfaces exist. What is the energy intensity of thermally emitted light from any point in the body? If you use an absorption or emission coefficient (as I suspect is correct), can you explain how...
  35. G

    Iron Bar Thermal Radiation Calculation

    Homework Statement Energy can be transmitted via radiation, the rate at which this happens is H=σAT4. A bar of iron is 0.5m long, 0.2m wide and 0.2m high. Iron has a density of 7900 kg/m3, a heat capacity of 400J/kgC, and a coefficent of linear expansion of 1.2x10-5 What is the rate at which...
  36. J

    Black/white radiator vs white/black wall: radiation

    Hi, Situation A: Black radiator in a room with white walls. Situation B: White radiator in a room with black walls. Which one is the most efficient situation? I know that convection is dominant if you look at a radiator, but i would like to know the answer in terms of radiation. Thank you :)...
  37. robotpie3000

    Improving the Efficiency of Light Bulbs: Possible Solutions?

    I just finished an experiment on blackbody radiation, and one of the post lab questions asked me on the possible ways in which a light bulb can be made more efficient such that the majority of the light it produces falls in the visible spectrum. After some quick research I think one of the...
  38. T

    Calculating Heat Transfer (Thermal Radiation) to a Shipping Container

    Hello, I am trying to calculate the thermal radiation heat transfer to a shipping container which would be painted white. The purpose is to size an air conditioner for equipment, not living conditions. I have already calculated heat from the other sources. The desired internal temperature is...
  39. Entanglement

    Where does thermal radiation come from?

    Where does thermal radiation come from? vibration of the molecules or the excitement of electrons ?
  40. T

    Does thermal radiation involve ultraviolet x ray and jamma ray

    does thermal radiation involve ultraviolet x ray and jamma ray or just heat of light...
  41. S

    Thermal radiation lab experiment

    Hi, I have to plan and carry out an experiment on thermal radiation as part of my physics lab module. The description of the experiment is as follows: Investigate the radiation from a bulb filament as a function of input power, using a pyrometer to measure the temperature of the filament...
  42. J

    Do Neutrons Emit Infrared Radiation at Room Temperature?

    All bodies with a temperature emit electromagnetic radiation also known as thermal radiation or blackbody radiation. If I understand it correctly, the origin of this is the thermal motion (acceleration) of the atoms and their included positive and negative charges leads to EM radiation...
  43. K

    Thermal radiation and emission spectra

    I have some questions regarding thermal radiation spectra of different objects. Is thermal radiation of objects due to electromagnetic radiation alone? Does my desk which is at room temperature emit as a blackbody with the proper factor for emissivity? Where is the "cavity" that the radiation is...
  44. M

    How can I calculate the cooling time of a steel ball using thermal radiation?

    Homework Statement Hi! I need some help by my phisycs homework at universty. Here is the problem: We have a steel ball, with density of 7800kg/m^3, with speficic heat capacity 460J/kgK. Temperature of the ball is 1700K. How much time does the ball need to cool down to half of it's temperature...
  45. J

    What is the Mechanism Behind the Emission of Thermal Radiation?

    I'm trying to understand the origin of thermal radiation. All things with a temperature emit electromagnetic radiation. Every discussion I've found starts with that as a premise and then goes on to Wien's displacement law and Planck's law, etc which I follow. But *why* do all things...
  46. E

    Two questions about thermal radiation heat transfer

    Hi guys, could you concretely explain me (also with a simple example) the difference between blackbody emissive power (sometimes found as e'λb) and blackbody radiation intensity (i^{'}_{λb})? and which the difference between a diffuse surface and a surface that follows the Lambert law? Thank...
  47. E

    Origin of thermal radiation in humans.

    hi, i m new to this forum,this is my first post,i have recently started studying heat transfer. i m am little bit confused about the origin of thermal radiation in human beings.i mean radiations are emitted when an electron jump from a shell/orbit which is far from the nucleus to a shell which...
  48. S

    What Makes Aluminium an Effective Thermal Radiation Shield?

    Hi all I'm doing a project for energy efficient buildings with thermal insulation.(Just a leisure project and I am not technically that strong!) This is about thermal radiation shielding. My understanding is that Aluminium has higher reflectivity (Hence lower emissivity ) so that it...
  49. O

    Calculating Thermal Radiation: Power and Wavelength of a 47000K Object

    Consider a 47000K object that emits thermal radiation. How much power does it emit per square meter? What is its wavelength of peak intensity?What formula do I use to solve these questions?
  50. Q

    Thermal radiation between monatomic gases

    Suppose we have two separate containers of helium gas in thermal equilibrium and completely isolated from the rest of the universe. Let's assume the containers are ideal in the sense that they are completely tranparent to all electromagnetic radiation. Since helium is monatomic, the only...
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