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What happens when we close the switch?

  1. Oct 21, 2006 #1
    Hello,
    I have a physics midterm on monday and this question is on the study guide so help with this problem would be greatly appreciated.
    (ok, since this diagram isn't showing up properly, here is a pdf file that includes the diagram I am asking about http://www-physics.mps.ohio-state.edu/~klaus/f06-112/midterm_options.pdf
    |--------------|
    | |
    | | --------------|
    | | |
    _ X X
    __ | |
    | |-----X-| s|---|
    | | |
    | | |
    | X X
    | | |
    |----------------------

    So What happens when we close the switch (labeled s)? The X marks represent light bulbs. What lights are on/off before closing the switch and what lights are on/off when after we close the switch? Also how would I find the total resistance?

    Thank you!:cool:
     
    Last edited: Oct 21, 2006
  2. jcsd
  3. Oct 21, 2006 #2
  4. Oct 21, 2006 #3

    DaveC426913

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    The four hot cross buns continue to smoke?
     
  5. Oct 21, 2006 #4
    thanks for the reply...but why the question mark? I'm taking from your reply that the four "corner" lamps will light up before closing the switch and they will continue to light up after it is closed...but what about the middle lamp?

    Thanks? lol
     
  6. Oct 21, 2006 #5

    DaveC426913

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    The question mark reads as:

    Diagram too ambiguous to understand what's happening. Labels good.
     
  7. Oct 22, 2006 #6
    what diagram is too ambiguous? what needs to be clarified? the one in the pdf file should be fine according to the professor?

    help????? lol
     
  8. Oct 22, 2006 #7

    OlderDan

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    The answer depends on the bulbs being used. Since nothing was said to the contrary, I think you should assume they are identical, having some constant resistance R. Figure out how the potential on one side of the branch with the switch compares to the potential on the other side of that branch when the switch is open. How is that going to caange (or will it change) when the switch is closed?
     
  9. Oct 22, 2006 #8
    so when the switch is open the four bulbs/resistors should be treated as parallel. (1/sum of left side) + (1/sum of right side) = (1/R total)

    and when the switch is closed it should be treated as a series? (sum of left side) + (middle) + (sum of right side).

    So the light bulbs on the outside will stay on with or without the switch but the bulb in the middle will only be on if the switch is closed?

    correct? :-(
     
  10. Oct 23, 2006 #9

    OlderDan

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    Yes, but since you don't know any of the resistances you cannot find the actual R. What is important here is that the same current flows through the left branch as through the right branch.
    No. There is no way to treat this combination as series or parallel
    No. Go back to the first situation and figure out how the potential at a point between the two bulbs on the left compares to the potential at a point between the two bulbs on the right. Then ask yourself how much current will flow if you close the switch.
     
  11. Oct 23, 2006 #10
    so when we close the switch the two upper lights remain same voltage but the two lower ones have lower voltage and the middle light has the same voltage as the upper lights? I'm not sure if this makes sense considering "same voltage acorss all elements in parallel" but it's 2 am here...so i'm tired :-(
    Lets assume the resistance of each lamp is the same.

    I have an exam at 4:30...but i'll probably be w/o internet at 2:30..so i'd really appreciate any last minute advice regarding chapter 18 (electric forces and electric fields), chp 19 (electrical potential energy and electric potential) and chp 20 (electric circuits).

    Thank you!
     
  12. Oct 23, 2006 #11

    OlderDan

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    I have no idea what chapters you are talking about, but I will tell you this. There is no potential difference between the points I asked you to compare. When you close the switch nothing happens. The light in the middle remains dark and the others have exactly the same current they had when the switch was open.
     
  13. Oct 23, 2006 #12

    DaveC426913

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    Yep. Nothing happens. There's no potential across the middle bulb.
     
  14. Nov 4, 2006 #13
    Well, just wanted to stop by and say THANK YOU!
    There was a 5 point (out of 100) problem regarding what happens to the midle bulb and I got it correct..not to mention I scored 98/100 on the test which is amazing considering I've never done that well in Physics before. The class average was a 78. Too bad I don't do so well on the weekly quizzes ...so yea...I need to improve on those. Well once again THANK YOU...you guys made by day when I saw your last repies!:cool:
     
    Last edited: Nov 4, 2006
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