Multiple wire connections

Discussion in 'General Electronics Chat' started by wes, Oct 19, 2011.

  1. wes

    Thread Starter Active Member

    Aug 24, 2007
    242
    2
    I was thinking about circuits and how fast a signal (say just a on signal) can get from one end of a wire to the other.
    Also Let's just say this is a 2 ft long wire capable of huge Amperage connected between the 2 ends of a battery (again capable of whatever the circuit needs)

    For example, you have a wire that is 2 feet long and the time for the signal to start at point A and reach B is 2-nanoseconds because information can't travel faster then light and each foot at light-speed is about 2-nanosecond.

    A X----------------------------------------------X B


    Now is it possible to shorten the time needed for the signal to reach point B by just adding another wire to the circuit in the middle. So now the time needed between each point is 1ft = 1-nanosecond.

    A X--------------------------B X ----------------------X C (old B)

    But since the signal we are sending is basically just a on signal. Then wouldn't the time needed to power up this wire be half what it would before?

    Also let's say A and B are at 12v (doesn't really matter what it is) and C is 0V or Ground.

    So wouldn't the current start flowing between A and B while also start flowing Between B and C even though A and B are at the same Voltage. If you think about it and the timescales involved then I would think so. Since the current flow at B is just as much as at A, then when they meet it would be as if there was no B and there was just A and C. But the time needed for the current to rise would be half what it used to be? sound right?

    I think it makes since if you think about the way it works.
    1.Voltage applied at point A and B
    2. current starts moving from Point A toward B
    3. at the same time the current from Point B moves both toward A and C
    4. Voltage reaches B and C and voltage from B reaches both A and C

    This is where I get a little confused, the current flowing between A and B are moving towards each-other (Current from B is also moving toward C but that is the correct direction.) What happens when the current from B reaches A, does the current just change direction back towards B. If so how does this effect the total turn-ON time for the wire.

    Could I just put a diode in-between A and B to stop this, obviously as close to B as possible though , lol.
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    I think the only way your going to have a faster turn on time is to either have a shorter wire or to have a larger wire.
     
  3. colinb

    Active Member

    Jun 15, 2011
    351
    35
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    If you apply the input to a central point, then you could reduce the time taken to distribute the potential along the total length of the wire, compared to feeding it from one end only. There is however no sense in which this speeds communication between the two far ends.

    If the aim is to get power rapidly applied to a network, central feeding might possibly be an advantage, particularly in some DC systems (submerged cables?) where propagation speeds of the DC voltage rise are very much below light speed. (This results mainly from large values of distributed capacitance and resistance, effects which made the operation of early international telegraph cables very difficult. )

    Feeding in the middle also does not help if the requirement is to establish current flow in the same direction along the whole cable length. In the latter case, the most rapid powering is achieved by applying opposite voltages to the two cable ends with respect to earth potential simultaneously.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    Or get a faster than light neutrino.

    (Sorry, I could not resist.)
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    Wes, I have no idea what you are talking about.
    And I have no idea where this is going.

    An electrical signal will travel at about 66% - 75% the speed of light in a coaxial cable, higher in a plain piece of copper wire.

    The electrical current (or electrons) don't travel anywhere near that speed.
    Sometimes we think of electricity as water flowing through a pipe. But here the analogy breaks down. Electricity is more like a pressure wave. It is the wave that travels at a speed close to the speed of light, not the actual charge carriers.

    Hence the time to send information from A to B will be the same regardless of the number of cables you string between A and B. The only way to speed the signal is to use a shorter cable or get a faster cable. Coax cables have different propagation velocities.
     
    Last edited: Oct 20, 2011
  7. wes

    Thread Starter Active Member

    Aug 24, 2007
    242
    2
    Well I might have made it a little confusing I guess, What I meant by signal was just to a power on state, no real signal except that the coil is on needs to go from A to B.

    What this is really for is for a coil, I thought it might be easier to figure out if it was a wire but it would probably help if you knew exactly what this is meant to solve.

    Ok, so let's say you have a coil and these are some of the specs,

    L = 500 uh
    Turns = 100
    Coil Diameter = 10 inches
    resistance is negligible


    So, supposedly this coil with 500uh could reach 1 amp in 10 Nanoseconds if 50KV was applied to it according the formula L(micro)/V=Rstime in microseconds to 1 amp, 500/50,000 = .01 usec = 10 nanoseconds.

    But the since the diameter is 10 inches, then the circumference is 31.4 inches and since their is 100 turns then the total wire length is 31.4 inches x 100 = 3140 inches / 12 inches = 261.6 Feet. Since the speed of light is approximately 1ft per nanosecond, then the total time to travel the coil length is around 261 nanoseconds.

    As you can see even though the inductance is low enough and the voltage high enough to allow for a very fast turn on time, the length of the wire is the main limiting factor here.

    So I think you can see why I was wondering if adding more leads to the coil at certain intervals would allow for a faster turn on time. For example if you added a lead to each turn then the total time in nanoseconds between any two leads would be 31.4 inches or around 2.6 nanoseconds which is well within the time that it would take for the current to reach 1 amp.


    So if this isn't possible then what would you be able to do to decrease the total turn on time of the coil since obviously decreasing the inductance won't solve the problem. Also I know you could just change the physical size but for the sake of argument, let's say that isn't possible in this case.
     
  8. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    Those darn neutrino's! Ya got me. My statements were not well founded...
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    I would think that the fact that the coil windings are magnetically and electrostatically coupled together in a relatively small space effectively reduces the "speed of light" propagation delay limit down closer to a value depending on the coil dimensions.

    If this were not so, the behaviour of even modestly sized inductors would require heavy corrections for pure time delay. I am not aware of this being required in practice until the coil size becomes an appreciable fraction of a wavelength.
    (In other words, where the time taken for the fields to traverse the dimensions of the coil is significant compared to the signal period.)

    I have seen large antenna coils of the type used for AM radio before the advent of ferrite rods, of larger dimensions than the OP describes and a similar inductance. As far as I know, they were designed on a basis of inductance only, with no "fiddle factors" to account for delay.

    Perhaps some field theory guru could comment - I would be delighted to see a definite answer to this.
     
  10. wes

    Thread Starter Active Member

    Aug 24, 2007
    242
    2
    I would be very delighted if someone could have an answer to this too, lol.

    I have thought about maybe the electric field doesn't necessarily have to move through the wire itself to get the current moving too. Maybe instead of moving through the total wire length, since it radiates outward in all directions, it just moves through all the wires without following the path of them. so instead of needing to wait 261 nanoseconds in the coil I talked about, it would only take say 10 nanoseconds if the coil was 10 inches in length and since the coils diameter was 10 inches, it would 10 nanoseconds to intersect them as well, so the total time would only be 10 nanoseconds, since both are happening simultaneously

    But like Adjuster said "Perhaps some field theory guru could comment".
     
Loading...