Blackbody Radiation: Why Bodies Absorb & Radiate Heat

In summary, bodies emit and absorb radiation at the same rate due to the second law of thermodynamics, as stated by Kirchhoff. If a body were to absorb more radiation than it emits, it would become warmer and the surrounding objects would become colder, which goes against the second law. Bodies radiate heat because the electromagnetic field is coupled to other forms of energy and the spectrum of wavelengths follows the Planck distribution.
  • #1
Bassalisk
947
2
In fact, we can be much more precise: a body emits radiation at a given temperature and frequency exactly as well as it absorbs the same radiation. This was proved by Kirchhoff: the essential point is that if we suppose a particular body can absorb better than it emits, then in a room full of objects all at the same temperature, it will absorb radiation from the other bodies better than it radiates energy back to them. This means it will get hotter, and the rest of the room will grow colder, contradicting the second law of thermodynamics. (We could use such a body to construct a heat engine extracting work as the room grows colder and colder!)

If I may dare to ask the question: WHY are bodies absorbing and radiating heat in same proportion?

I can't find the answer anywhere on the internet. And why are bodies radiating heat in the first place? And why whole spectrum of wavelengths?
 
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  • #2
Bassalisk said:
If I may dare to ask the question: WHY are bodies absorbing and radiating heat in same proportion?

For the reason stated in the quote you supplied? If something is a net absorber or emitter of radiation, then it's gaining or losing energy relative its surroundings, meaning it's getting hotter or colder. As the quote says, something at the same temperature as its surroundings can't spontaneously heat up or cool down without violating the second law of thermodynamics.

I can't find the answer anywhere on the internet. And why are bodies radiating heat in the first place?

Because the electromagnetic field is coupled to all the other forms of energy in the system. A vibrating molecule can give off its vibrational energy as EM radiation, and absorb EM radiation as vibrational energy (or rotational or other degrees of freedom).

And why whole spectrum of wavelengths?

It's not the whole spectrum; it follows the Planck distribution, so the probability of very high energy radiation quickly becomes negligible.
 
  • #3
Thanks, I understand now.
 

Related to Blackbody Radiation: Why Bodies Absorb & Radiate Heat

1. What is blackbody radiation?

Blackbody radiation is the phenomenon where a body absorbs and radiates heat energy. It is also known as thermal radiation or heat radiation. All objects with a temperature above absolute zero emit blackbody radiation, regardless of their color or composition.

2. Why do bodies absorb heat?

Bodies absorb heat because of the vibration of their atoms and molecules. When energy in the form of heat is transferred to an object, its particles start to vibrate faster, causing an increase in temperature. This is known as thermal energy and is responsible for the absorption of heat by bodies.

3. How does blackbody radiation affect temperature?

Blackbody radiation plays a crucial role in regulating the temperature of a body. When a body absorbs heat energy, it also radiates heat energy at the same rate. This balance between absorption and radiation helps to maintain a constant temperature within the body.

4. What is the relationship between blackbody radiation and color?

The color of an object does not affect its ability to absorb or radiate heat energy. However, the temperature of an object does affect the color of its blackbody radiation. As the temperature of a body increases, the color of its radiation shifts from red to orange, yellow, white, and eventually blue as the temperature continues to rise.

5. How is blackbody radiation used in everyday life?

Blackbody radiation has several practical applications in our daily lives. For example, it is used in the design of thermal insulation materials to reduce heat transfer. It is also used in technologies such as infrared cameras and heat lamps. Additionally, blackbody radiation is essential in understanding and predicting weather patterns and climate change.

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