Radiation Emission and Absorption

In summary, the second equation represents the rate at which the object absorbs energy from the environment through thermal radiation, and it can be used in any environment as long as the temperature of the environment is known.
  • #1
AlexChandler
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Homework Statement



A sphere of radius 0.500 m, temperature 27.0°C, and emissivity 0.850 is located in an
environment of temperature 77.0°C. At what rate does the sphere (a) emit and (b) absorb
thermal radiation? (c) What is the sphere's net rate of energy exchange?

Homework Equations



Pemit=σεATobject^4
Pabsorb=σεATenvironment^4

σ=5.6704 x 10^4 W/(m^2 K^4)=Stefan-Boltzmann constant

The Attempt at a Solution



I am able to use the equations and find the answer that the book is looking for.. but I do not understand the second listed equation. Does this account for only the energy radiated by the molecules in the environment? Will this equation work in any environment...(air/water/concrete...etc)? My book says that this equation gives the energy the object absorbs via thermal radiation. Do we not have to worry about energy transferred by conduction? The collisions between the molecules of the environment and the molecules of the object will tend to transfer energy to the object since its molecules are moving slower on average. How do we account for this energy?
Thank you. I hope I am not horribly mistaken or somehow being stupid.
 
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  • #2


The second equation accounts for the energy that the object absorbs from the environment through radiation. This equation works in any environment because it takes into account the temperature of the environment, which is a key factor in determining the rate of thermal radiation.

You are correct that there are other methods of energy transfer, such as conduction, that can also affect the rate of energy exchange between the object and its environment. However, in this particular problem, we are only considering the exchange of energy through thermal radiation. This is because the emissivity of the object (0.850) indicates that it is a good emitter and absorber of thermal radiation, so the other methods of energy transfer can be neglected in this case.

In general, when analyzing energy transfer in a system, it is important to consider all possible mechanisms of energy transfer and determine which ones are most significant in a particular situation. In this case, thermal radiation is the dominant mechanism of energy exchange, so we can focus on using the equations for thermal radiation to solve the problem.
 

1. What is radiation emission and absorption?

Radiation emission and absorption is the process of energy being released or absorbed in the form of electromagnetic waves, such as light, x-rays, and radio waves. This can occur naturally, from sources like the sun and stars, or from human-made sources like nuclear power plants.

2. How does radiation emission and absorption affect living organisms?

High levels of radiation exposure can be harmful to living organisms, causing damage to cells and DNA. This can lead to health problems, such as cancer. However, low levels of radiation exposure are a natural part of the environment and are not typically harmful.

3. What are some examples of radiation emission and absorption?

Examples of radiation emission and absorption include the emission of light from a light bulb, the absorption of UV rays from the sun by plants for photosynthesis, and the absorption of x-rays in medical imaging.

4. How can we protect ourselves from harmful radiation?

There are several ways to protect ourselves from harmful radiation. These include limiting exposure to sources of radiation, such as x-rays and nuclear materials, using protective equipment, and following safety protocols in industries that use radiation.

5. What are the different types of radiation and their properties?

The three main types of radiation are alpha, beta, and gamma. Alpha particles have a positive charge and are easily stopped by a sheet of paper. Beta particles have a negative charge and can penetrate skin, but are stopped by a few millimeters of aluminum. Gamma rays have no charge and are highly penetrating, requiring several centimeters of lead or concrete to stop them.

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