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Homework Help: Energy/Light-Some problem set questions

  1. Mar 18, 2010 #1

    Two lamps illuminate a screen equally. The first lamp has an intensity of 12.5cd and is 3.0m from the screen. The second lamp is 9.0m from the screen. What is its intensity?

    I know I need to use P=4*pi*(I), but there is no variable included for distance (d). Another formula given is: Eill=P/(4*pi*d^2). How should I go about solving this problem now?

    A 15cd point source lamp provide equal illuminations on a wall. if the 45cd lamp is 12m away from the wall, how far from the wall is the 15cd lamp.

    Again, there is no variable for distance in the equation: P=4*pi*I.
    Last edited by a moderator: Mar 20, 2010
  2. jcsd
  3. Mar 19, 2010 #2
    d = v/t is wrong. It should be d = vt. Where did you get 343? you should be using the speed of light
  4. Mar 19, 2010 #3
    For #1: d=vt.

    What value would I use for t? I've tried everything...
  5. Mar 19, 2010 #4
    bump. Please help me.
  6. Mar 19, 2010 #5
    for #1, after you do the d = v * t using the speed of light, don't forget to halve the distance, since that's round trip.
  7. Mar 19, 2010 #6
    What would my value for t be?
  8. Mar 19, 2010 #7


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    If the signal takes 0.003 seconds for the round trip, how long does it take to go up and how long does it take to come back down? :rolleyes:
  9. Mar 19, 2010 #8
    half of 0.003?
  10. Mar 19, 2010 #9
    The answer I have is: 450,000 or 4.5x10^5m

    But, my instructor tells me the answer is 4.5x10^8
  11. Mar 19, 2010 #10
    And is my answer for #2 correct?
  12. Mar 19, 2010 #11


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    Then your instructor is wrong.

    It's useful to have a sense of scale. Earth is 40 000 km in circumference. Low Earth orbit (where the Space Shuttle orbits) is around 300 km from the ground. Geosynchronous orbit, the highest useful orbit, is 36 000 km from Earth's center.
  13. Mar 19, 2010 #12
    So, my answer is correct?
  14. Mar 19, 2010 #13


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    First, a lumen is not equal to a candela. A candela measures power per steradian, whereas a lumen measures total power.

    Are you aware of the formula I=x/r^2, where x is power/steradian?
  15. Mar 19, 2010 #14
    No, I'm not familiar with the latter formula. Is my answer to the Cape Canaveral question correct?
  16. Mar 19, 2010 #15


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  17. Mar 19, 2010 #16
    Could you help me with my NEW #1 and 2 questions? Those are the last in the problem set our Physics teacher gave us.
  18. Mar 19, 2010 #17
    Thank you so much :smile:.
  19. Mar 19, 2010 #18
    bump. Please help me with questions 1 and 2.
  20. Mar 20, 2010 #19
    bump. Help please..
  21. Mar 20, 2010 #20


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    You should start a new thread if you want to ask new questions.
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