1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Using the equation intensity = power/cross-sectional area

  1. Mar 15, 2015 #1
    Let's say we have the problem: A 100W lamp emits light in all directions. Assuming that the lamp is a point source, calculate the intensity of the radiation 1m away from the lamp.

    The surface area of a sphere is :4*Pi*r2
    intensity = power/cross-sectional area
    The answer is intensity = 100w/4Pi = 7.96

    Now what confuses me is that the answer is derived by intensity = power/surface area of the shape the wave makes when it spreads out.

    Shouldn't cross-sectional area be the cross-sectional area of a sphere?
    I don't see how 4*Pi*r2 is the cross-sectional area in this case.
     
  2. jcsd
  3. Mar 15, 2015 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    4 pi r2 is the surface area of a sphere. Your light will be distributed uniformly over this area.
     
  4. Mar 15, 2015 #3
    But then why does the formula say cross-sectional area instead of surface area?
     
  5. Mar 15, 2015 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    This is the cross-section of the light with your sphere. It happens to be the whole sphere as you have a point-charge radiating in all directions.
     
  6. Mar 15, 2015 #5
    If the question were to ask what the intensity of the light was at a screen 1m away from your light source, then you would use the surface area of that screen, not the surface area of your light source (just to be clear on what surface area you are using -- not the surface area of your light source). I just want to ensure you know that the equation uses the cross-sectional area (of the light) interchangeably with the surface area of the *thing* that the light is hitting.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Using the equation intensity = power/cross-sectional area
  1. Cross sectional area (Replies: 2)

Loading...