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Why does a desk lamp often have small holes at the top of the Metal Lampshad

  1. Sep 16, 2007 #1
    Question is Why does a desk lamp often has small holes near the top of the metal lampshade. How do these holes keep the lamp cool.

    This is what i think. Heat travels upward by air convection. Since air is a poor conductor, very little heat travels sideways.

    That is my reason what do u guys think.
     
  2. jcsd
  3. Sep 16, 2007 #2
    Sounds about right.
     
  4. Sep 16, 2007 #3
    same here. warm air rises becasue its less dense. the lamp warms the surrounding air, then that air rises while cool air which is more denses comes down.
    ps whats up. guess who i am
     
  5. Sep 16, 2007 #4

    dynamicsolo

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    I think there's more to this question, though, no? Convection would draw air up from below the lampshade and pass it through the open top, anyway. We're being asked why that circle of little holes going around near the upper edge is drilled there.

    (Sounds like a Jearl Walker kinda question to me...)
     
  6. Sep 18, 2007 #5

    dynamicsolo

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    OK, I'm going to take a guess at this, then we can all have a good laugh...

    I'm thinking that this is referring to the metal lampshades that are open over the bulb, yet still have this set of holes below the top margin of the shade. (A picture with the problem would have been nice...) I'm guessing that the turbulent flow at the mouth of the shade is great enough that, without the holes, the convective transfer of heat would be impeded. The holes perhaps open an alternate avenue for the air flow in a way that reduces the amount of turbulence at the large opening, thereby making the rising flow somewhat faster and improving the rate of cooling. Sound plausible? Silly?

    And if I find out the holes are just there for decoration, I'm gonna slug somebody... >:-/
     
  7. Sep 18, 2007 #6

    mgb_phys

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    I don't think you have to consider turbulence - the holes are simply to let the hot air out, so drawing cooler air in at the bottom to increase the cooling.
    Instead of convection purely inside the shade you are then using convection in the entire room.
     
  8. Sep 18, 2007 #7

    dynamicsolo

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    I'm supposing in that case that the lampshade in question is closed at the top (there are metal ones like that). Otherwise, why *small* holes? Why not just make one big opening, the way most lampshades *are* made?

    I know that convection is the concern here. I wanted to know what the lampshade looks like because I was wondering why the question calls attention to the small holes. (If you've seen Jearl Walker's book, his questions revolve around various subtle physical effects, rather than just the "broad-brush" explanations given in introductory -- and sometimes more advanced -- physics courses and texts...)
     
  9. Sep 18, 2007 #8

    mgb_phys

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    A desklamp normally has both the arm and the lamp holder attached to the shade, unlike a pendant lamp holder where they are both attached to the cord.
    You need to make the shade reasonably stiff where the lampholder joins while keeping the amount of material to a minimum - so no big hole.
    A large hole would allow too much light upwards distracting the user - after all the point of the shade is direct light only down onto the desk. Finally from a safety point a large hole makes it too easy to touch the hot bulb.
     
  10. Sep 18, 2007 #9

    dynamicsolo

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    I'm thinking of pedestal lamps and floor lamps, a lot of which do have open shades. There are many sorts of lamp designs, which is why I felt the question is a bit ill-defined. I think I'll just agree at present that the holes have something to do with improving convection around the bulb...
     
  11. Sep 18, 2007 #10

    DaveC426913

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    I think there's a more practical factor not being considered. Traditional lampshades are secured to the lamp with wire braces around the edge - there's no conflict between airflow and mechanical structure of the lamp. But in a desklamp, the shade has to both provide ventholes AND provide the structural integrity to the lamp proper. i.e. Instead of looking at them as holes, look at them as spokes that attach the shade to the lamp proper.

    Another thing that makes desklamps different from traditional lamps: desklamps are effectively upside-down by comparsion: the small hole (the convective bottleneck) is at the top, rather than the bottom.
     
  12. Sep 18, 2007 #11
    Is it possible that it could be as simple as a left-over of the manufacturing process?
     
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