2021.08.01
Educational Materials

Get to the Main Point ! Heat Radiation in a Nutshell

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快速抓重點! 簡單了解熱輻射Heat is transmitted in three ways: by conduction, convection and radiation. Heat radiation, when acting, is transmitted without any medium, just like the electromagnetic waves that transmit heat energy, or the heat of the sun transmitted through space to the earth.

Daily application of heat radiation:
1. Commonly used metal heat sinks are usually black in color, as they are more effective at radiating heat.
2. Stainless steel kettles have a glossy surface to reduce heat radiation.
3. Oil tanks are usually painted white to avoid high temperatures, which reduces heat radiation and prevents the tank from overheating.
4. Graphene, with its good conductivity in the planar direction and good thermal radiation coefficient, is often used for electronics cooling.

When charged particles within an atom move, heat is generated. When heat energy is converted into electromagnetic radiation, it is known as thermal radiation. All substances with a temperature above absolute zero emit thermal radiation, the frequency of which depends on the temperature of the substance.

All objects radiate energy in the form of photons, and there is no fixed direction of movement of these photons. When radiant photons from the surface of one object reach the surface of another, they are emitted, reflected and transmitted.


(1) Emissivity (α): Emissivity (absorptivity) is a measure of the relative strength of an object to emit energy in the form of thermal radiation from its surface, and generally speaking the darker the material, the rougher the surface, the closer the emissivity is to 1.
(2) Reflection (ρ): Reflection is the ability of a substance surface to reflect radiation.
(3) Transmittance (τ): Transmittance is the proportion of incident radiation transmitted at a given wavelength.
Thus, the reflection, emissivity and transmittance of a substance sum up to 1, i.e. α + ρ + τ = 1.


According to the Stefan-Boltzmann law, we can know
The total radiated power per unit area of the blackbody surface per unit time
The formula is:M=
σT4

1. Radiation emissivity:M

2. Radiation coefficient of the object: ∈

3. Stefan-Boltzmann constant: σ =5.67 × 10-11 kW/m2 K4
4. Absolute temperature of the object: T
The above formula shows that the emissivity is proportional to the fourth power of the temperature of the object, so the higher the temperature, the higher the emissivity and the more effective the radiation.

 


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