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Circuit- How is this possible?

  1. Nov 29, 2005 #1
    So my teacher was showing us this circuit he made in class today, and he wont tell us how it works. This is the picture of the circuit:
    http://img517.imageshack.us/img517/7576/trickcircuit1tb.jpg

    Its an AC circuit, one wire going through two switches, and then two light bulbs. He showed us, with both switches closed, both lights went on, as normal. Then he opened both switches, and neither light bulb lit up, as expected. Next he opened the right switch, and closed the left, the left light bulb went on, but not the right. How was this possible?? This also worked vise versa. He wont tell us what he put in the circuit to do this. Does anyone know how the circuit was able to do this, is it a trick? ( I thought it might have something to do with the light bulbs, since both were frosted and you couldnt see inside them, also he wouldnt let us touch the circuit after he demonstrated). He did say there was something that we couldnt see that was important. What was it? :grumpy:
     
  2. jcsd
  3. Nov 30, 2005 #2
    because the circuit was really wired like this:
     

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  4. Nov 30, 2005 #3
    no, there was no wire hidden in the board, the hidden thing is in the switches or bulb.
     
    Last edited: Nov 30, 2005
  5. Nov 30, 2005 #4

    ranger

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    Its because current needs a complete path to flow. IF the switch is open that means there is not a complete path or as some say an open circuit (infinite resistance). Please refer to the circuit diagram in the previous post. Try to trace the current path in both loops. Start from the AC source then trace to either side of the circuit. If the switch is closed, that means current can flow so you contiunue to trace. But if it is open, then you cant trace on that side of the circuit anymore. The key is start at a point and trace back to that same point. If that is accomplised, then you have a complete path and the lamp would light.
     
  6. Nov 30, 2005 #5
    yes I know what a circuit is, but im saying there is no hidden wire outside the circuit I first drew. There is something hidden in either the switches or the bulb, nothing outside that. I came up with this....
    http://img494.imageshack.us/img494/6055/circuittrick1ao.jpg

    Simple? Seems to work...
     
  7. Nov 30, 2005 #6

    ranger

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    That circut is wrong. In that circuit the bulbs will only light when both switches are closed and non will l ight when just one is open. That is what I was trying to explan in my previous reply about a complete path for the current to flow. The circuit that mugsby has is correct.
     
  8. Nov 30, 2005 #7
    look closer... its basically two different circuits, the red wire connects to the cirst switch, goes in the base of the second switch ( not connected) goes under the base of the light bulb( not connected) goes to the second light bulb ( CONNECTS HERE!) and back around (thats 1 complete circuit) The BLUE wire is basically another circuit, goes under base of 1st switch, connects to second switch, connects to left light bulb, goes under base of right light bulb, and back to make full circuit. (2 complete circuits, left swich controlling left bulb, right switch controlling right bulb. Get it now?
     
  9. Nov 30, 2005 #8

    ranger

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    Ah yes, I didnt even notice that.
     
  10. Nov 30, 2005 #9
    I'm not sold on it. I think the OP has the schematic done correctly in the first post since I have seen this trick in the past. The OP says the teacher implied that there is something else hidden in the circuit. There are diodes hidden in the switch bases and light sockets. The diodes in the light sockets point towards each other and the diodes in the switches point away from each other, or the other way around. Either will work. Next time the trick is done look closely at the bulbs. You MAY be able to notice a flicker since each bulb is running on only half of an AC cycle.
     
  11. Nov 30, 2005 #10
    average super nova, congratz, you are exactly right! Too bad you didnt post it this morning when I needed it :cry: Yes the teacher told us how it was done, and your explanation nailed it. O well, next time you will see my posts earlier :tongue2:
    Thanks though
     
  12. Nov 30, 2005 #11
    I saw that trick done over 16 years ago but it was a bit different. There was a clear plexi glass sheet with 2 porcelain light sockets wired in series to a 120 VAC line cord. There were NO switches. Teacher would unscrew a bulb and the other would continue to glow. I nailed it that time without any previous knowledge. I had the advantage of experience this time. The clear sheet that the sockets were mounted on gave the impression that there was nothing hidden anywhere. BUT, the teacher was smart and had soldered diodes in place and then run plaster of paris over the socket base so only the screw terminals showed. I must say he did a FINE job of it. You could not tell the difference between the rough porcelain and the plaster. All of this showed through the plastic so you couldn't really feel the texture of the plaster without disassembly.
     
  13. Nov 30, 2005 #12
    Sounds like that man was very determined to have you guys baffled, he must have been very upset when you figured it out:rofl:
     
  14. Dec 1, 2005 #13

    DaveC426913

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    I'm still missing a piece of the puzzle. I get the diode thing, but in the OP's first diagram, there is still no closed circuit until both switches are closed.
     
  15. Dec 1, 2005 #14
    Then you don't really 'GET' the diode thing. Redraw his circuit, and then draw the diodes in how I described them in my previous post. You will then see how the circuit is indeed closed for half of the AC cycle.
     
  16. Dec 1, 2005 #15
    He was a pretty easy going guy. I can't say how many students figured it out over the years. I'm sure I wasn't the first one. If I were the teacher I'd be delighted to find out that students were figuring stuff like that out. Although it would motivate me to go ahead and create something a little more difficult.
     
  17. Dec 3, 2005 #16

    DaveC426913

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    But the claim was that the OP's FIRST diagram is correct:
    which will not work even with diodes.
     
  18. Dec 3, 2005 #17
    get it now?
     

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  19. Dec 3, 2005 #18
    Dave, the first diagram IS correct. We are told in the first post that the teacher implied there was something hidden in order to make the circuit behave the way it did. Obviously if whatever it is that is hidden were revealed by being drawn on the schematic then it wouldn't be much of a puzzle would it? Please tell us why you think it won't work even with diodes.

    Mugs, the way you drew it will have the left switch controlling the right bulb. Just a technicality. It won't short circuit or anything.
     
  20. Dec 3, 2005 #19
    fixed that. :blushing:
     
    Last edited by a moderator: Dec 3, 2005
  21. Dec 4, 2005 #20

    DaveC426913

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    So, you're saying that, at first appearance, it looks like the OP's diagram in post#1, but underneath things, it actually looks like Grandma's post #5 PLUS the diodes.
     
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