A question on thermal radiation

In summary, the conversation discusses the concepts of absorptivity and emissivity and how they relate to the efficiency of different colored surfaces in absorbing and radiating heat. The participants also consider the color choices of common heating devices and the impact of cost and design on their efficiency. Ultimately, it is concluded that while black surfaces may be the most efficient at transferring radiant heat, other factors such as cost and design may influence the color choices of heating devices.
  • #1
cfung
17
1
It is common knowledge that black colored surface absorbs light and heat the best. Then it seems to me the next logical leap would to think that black colored surfaces would radiate heat most efficiently.

If my assumption is correct, then why do most heating devices, such as oil heater or common home heater, have silverish finish instead of black?

Thank you for your time reading this.
 
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  • #2
cfung said:
It is common knowledge that black colored surface absorbs light and heat the best. Then it seems to me the next logical leap would to think that black colored surfaces would radiate heat most efficiently.
Very good, your intuition about absorptivity and emissivity (as they are called) is correct.

cfung said:
If my assumption is correct, then why do most heating devices, such as oil heater or common home heater, have silverish finish instead of black?

Am not sure what kind of heater you are referring to. The kind with heating coils that glow orange are usually backed by a polished metal reflector that projects the infrared radiation forward.

A better example is to consider the heat sinks on electronic devices--they are nearly always black for maximum radiative cooling. The only exceptions tend to be the the integrated heat sink/fan units on big computer chips. The color doesn't matter in this case because cooling is achieved via forced air flow.
 
  • #3
Its not always about being the most efficient. Cost comes into play a lot as well. Back anodizing or powder coating costs money which a manufacturer may omit due to keep costs down.

Also, while black does transfer radiant heat the best, it doesn't always conduct it well. In other words, a black powder coating may transfer heat from the air to itself well, but not from itself to the material its been coated to.
 
  • #4
cfung said:
It is common knowledge that black colored surface absorbs light and heat the best. Then it seems to me the next logical leap would to think that black colored surfaces would radiate heat most efficiently.

If my assumption is correct, then why do most heating devices, such as oil heater or common home heater, have silverish finish instead of black?

Thank you for your time reading this.

You are correct, absorption = emission, so a black object would be a better emitter than a reflective object- with the caveat that I mean "black" and "reflecting" within the waveband of interest (8-12 microns for room temperature, 3-5 microns for hot objects).

Radiators are not generally painted black purely for decorative reasons. In addition, those 'decorative' enclosures also act to decrease the efficiency of the radiator. AFAIK, heat fins on circuit boards and in non-visible places (i.e. backs of amplifiers) are indeed black.
 
  • #5
Cool, millions of thanks guys, the answers are reasonably convincing and exactly resolve a thought that's been giving me a headache
 

1. What is thermal radiation?

Thermal radiation is the emission of electromagnetic waves, such as infrared radiation, by a body due to its temperature. It is a form of heat transfer that does not require a medium, unlike conduction and convection.

2. How does thermal radiation work?

Thermal radiation occurs when an object with a temperature higher than absolute zero emits electromagnetic waves. These waves carry energy and can travel through space until they are absorbed or reflected by another object. The rate of thermal radiation depends on the temperature and surface area of the object.

3. What are the effects of thermal radiation?

Thermal radiation can have various effects depending on the temperature and type of radiation. Infrared radiation, for example, is commonly associated with the feeling of warmth from the sun or a fire. However, prolonged exposure to high levels of thermal radiation can also be harmful to living organisms, causing burns and other tissue damage.

4. How is thermal radiation measured?

Thermal radiation is measured in terms of its intensity and wavelength. The intensity, or power per unit area, is typically measured using a radiometer or an infrared thermometer. The wavelength, or color, of the radiation can also be measured using a spectrometer.

5. How is thermal radiation used in everyday life?

Thermal radiation has many practical applications in everyday life. It is used in cooking, as heat from a stove or oven is transferred through thermal radiation. It is also used in heating systems, such as radiators, and in industrial processes such as melting metals. On the other hand, thermal insulation is used to prevent or reduce thermal radiation, such as in building materials to keep homes cool in hot weather.

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