Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Three bulbs.

  1. Jun 18, 2005 #1
    Which of the bulbs shown in the figure will light up first when the switch is closed?
    E: cell
    K: key
    A,B,C: bulbs

    PS: If it tells you that you don't have permission, logout and then login again to see it.

    Attached Files:

    Last edited: Jun 18, 2005
  2. jcsd
  3. Jun 18, 2005 #2
    Not working.

    The Bob (2004 ©)
  4. Jun 18, 2005 #3

    C should light up first?
  5. Jun 19, 2005 #4

    I'm gonna say they all turn on at the same time. Due to the potential difference between the plates, electrons will go through the circuit in a clockwise fashion. However, electrons are not emitted from the negative plate and shot to the positive one, the electrons come from the actual copper wiring itself. The electrons in the wire at all points are drawn towards the positive plate as soon as the switch is closed. Thus current is established everywhere at the exact same time (but not instantly after the switch is closed).
    Last edited: Jun 19, 2005
  6. Jun 19, 2005 #5

    Whozum is right, but can you explain it.
  7. Jun 19, 2005 #6
    More of a physics problem than a brain teaser. I hid my answer for others to try.
  8. Jun 19, 2005 #7
    I don't think this is correct because it would imply that the signal travels faster than the speed of light.
  9. Jun 19, 2005 #8
    Bulb A will light first because it's closest to the switch, and at the instant the switch closes that's where the potential difference is.

    Before the switch closes, there's no potential across the battery (if there were, current would flow through the battery). So it doesn't matter which bulb is closest to which battery terminal.
    Last edited: Jun 19, 2005
  10. Jun 19, 2005 #9

    If you read the line right after it it explains it. Ofcourse its not instant but, as soon as the field is established.
  11. Jun 20, 2005 #10
    I don't think this is correct either because it would also imply that the signal travels faster than the speed of light.
  12. Jun 20, 2005 #11
    Gif is still not working.

    The Bob (2004 ©)
  13. Jun 20, 2005 #12
    Click on the link for the image, then click on the "Log Out" link.
  14. Jun 22, 2005 #13
    no it doesnt. re-read please.
  15. Jun 22, 2005 #14
    What you say, ("current is established everywhere at the exact same time") cannot happen. Not as soon as the switch is closed, and not at any instant after the switch is closed either. The current cannot be established everywhere at the exact same time period because it would imply that a signal travels faster than the speed of light. Try to imagine the circuit to be made of water troughs, water wheels, and a water pump. When the pump is turned on, a pulse of water will propogate along the circuit and the water wheels will not all start to spin at the same time but rather one by one as the initial pulse hits them . Electrical fields aren't that much different, just faster.
    Last edited: Jun 22, 2005
  16. Jun 25, 2005 #15
    Once the field is established (very soon after the swithc is closed) electrons from all over the wire begin moving, not just at the very beginning of the wire. This is why there is current flowing everywhere once the field is established. This is why the bulbs all turn on at the same time.
  17. Jun 26, 2005 #16
    What happens in parallel circuits?
  18. Jun 26, 2005 #17
    What? This isnt a parallel circuit. What's your point?
  19. Jun 26, 2005 #18
    Does the current start to flow at the same instant all along a parallel circuit?
  20. Jun 26, 2005 #19
    Sounds to me like you are saying that electrons in the wire that are submersed in an E field will not accelerate behind each other.
  21. Jun 27, 2005 #20
    No, I was asking a question.
  22. Jun 27, 2005 #21
    Of course there is always a potential accross the battery, where do yo think the potential accross the open switch came from? All closing the switch does is transfer that potential to across all three light bulbs.
  23. Jun 27, 2005 #22
    I like this explaination but it doesn't address "very soon after the switch is closed" who gets the info first to turn on first and what is that info.

    As we figure out what info needs to transfer we must also remember to use the Non-Simultaneous effect of SR in a common reference frame over a distance. So what info needs to be transferred, much more than just the switch is closed. Only after the switch is closed can the info on resistance of each light can go from the light to the batter and the available current or “battery impedance” can be communicated to the other members of the circuit. But with added info required from all parts of the circuit at the same time. However SR simultaneous issue says that cannot be true. So I figure the light closest to the observer doing the measurement comes on first. Other observers will get a different answer.

    Maybe this does belong in another forum, but it sure teased my brain.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook