How to find the intensity of radiation

In summary, the conversation is about a homework problem involving finding the intensity of radiation from a star to a planet at a certain distance. One person suggests using the total power radiated from the star and the other person points out the need to consider the portion of the star's "sky" that the planet subtends. The original person clarifies that the question is asking for intensity, not total power, and the conversation ends with a discussion on the difference between brightness and intensity. Ultimately, the summary concludes that the original answer is correct for finding the intensity of radiation from the star.
  • #1
zaf
Hello Forum members

I need help on a homework problem that I am unable to solve. the problem is as follows

Given the radius of a star of 9.0 * 10^3 m and surface temperature of
10000 k (blackbody)find the intensity of radiation (watts/m^2) incident on a planet that is located 2.4 * 10^11 m from the star

I found the total power radiated from the star by using stefans and taking the area to be 4 pi r^2 so P = sAeT^4

s is the stefan-boltzman constant
and e is emissivity of 1 for a blackbody

then I could also find the brightness for that star as seen from another planet 2.4*10^11 m away from the star by using

b = P / (4*pi*d^2)

d being the distance...but i do'nt think that this is the answer

can someone help ?
 
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  • #2
I'm no great physicist but I don't see how you can answer the question without know what portion of the star's "sky" the planet subtends. That's is a really large planet will intercept more of the stars total radiation (which is what you have calculated) than a small planet will.
 
  • #3
I understand your response...but i guess that issue has not been taken into consideration by the prof...basically it is know that the intensity of radiation is inversely propotional to the distane...I need to somehow incorporate that in my solution
 
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  • #4
Originally posted by HallsofIvy
I'm no great physicist but I don't see how you can answer the question without know what portion of the star's "sky" the planet subtends. That's is a really large planet will intercept more of the stars total radiation (which is what you have calculated) than a small planet will.

This statement is correct, however notice that the question asks for the intensity, not the total power. The intensity of an isotropic radiator, such as a star will be uniform for a given distance, d.

I see no problem with how the problem is worked out, other than you suddenly switch to solving for the brightness (all the equations are still correct, just need to replace b with I (Intensity).

Claude.
 
  • #5
Ok..maybe you are correct...but i am not so sure whether brightness and intensity mean the same thing...brightness is a measure of the maximum wavelength of the color spectrum of a star...intensity is the measure of energy.
 
  • #6
I think intensity = energy per time per surface area.
Or power per surface area.
So your original answer is OK, I think.
 

1. What is the definition of radiation intensity?

Radiation intensity refers to the amount of energy per unit time that is emitted from a source in the form of electromagnetic waves. It is typically measured in units of watts per square meter (W/m^2).

2. How is radiation intensity calculated?

Radiation intensity can be calculated by dividing the power of the radiation source by the surface area over which the radiation is spread. This can be represented by the equation I = P/A, where I is the intensity, P is the power, and A is the surface area.

3. What factors affect the intensity of radiation?

The intensity of radiation can be affected by several factors, including the distance from the source, the type of radiation being emitted, and any barriers or shielding materials present. Additionally, the direction of the radiation and the energy of the particles can also impact the intensity.

4. How does the intensity of radiation change with distance?

As radiation travels through space, its intensity decreases with distance. This is due to the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source. This means that the further away from the source you are, the lower the intensity of radiation will be.

5. How can radiation intensity be measured?

Radiation intensity can be measured using instruments such as Geiger counters, scintillation detectors, and dosimeters. These devices are designed to detect and measure the amount of radiation present in a given area. They can provide readings in real-time or store data for later analysis.

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