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

A little confusion with parallel circuits

  1. Feb 24, 2014 #1
    suppose that you have to wires connected in parallel, one double the length of the other
    now that means that the longer wire has double resistance .
    according to kirchhof's law , the current going in the two wires is the same as the current coming out , but if one is double the length of the other , that means that current will take double the time to reach its end .
    so if we measure the current going in and coming out in 1 sec for instance
    the current coming out should be lower than the current going in since it takes more time for electrons to travel through the longer wire
    unless either electrons travel faster in the longer wire , which i dont think is the case
    or that current intensity has nothing to with the speed of electrons * or charges to be more accurate *
    if it doesn't , then will you please correct me ?
     
    Last edited: Feb 24, 2014
  2. jcsd
  3. Feb 24, 2014 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Hi
    welcome to PF :smile:

    No, doesn't work that way
    The speed of the electrons is very slow ... google ... "electron drift"

    an easier way to look at it is for every electron that enters one end one pops out the other end

    The Electromagnetic field travels at near the speed of light along the wires. just a tiny propagation delay that over even reasonable lengths of wire ... many km's you wouldn't notice the delay.
    only when you get many 100's to 1000's of km's you can start to measure or physically notice the difference

    cheers
    Dave
     
  4. Feb 25, 2014 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Actually, that delay you refer to is much easier to see than that (with modern equipment, of course). A time domain reflectometer uses a step function with a rise time of a few ps. If you repeat this experiment, using a TDR in ' transmission mode', rather than on reflection, you can easily see the pulse arrive on a path through a 20cm wire in half the time that the pulse arrives on a 40cm parallel path.

    It's a shame that 'they' don't point out, in the very first lessons on electricity, that all the simple DC equations only apply after everything has settled down. The 'how does it know?' question is easily answered if students are aware of this settling process, from the very start in their learning.

    Of course, the fashion for starting the learning about Electricity in terms of moving electrons - rather than 'current' - is responsible for a lot of this sort of problem. How I wish each Teacher who tells kids about electrons moving along wires, was just a bit better informed about the nature of electron conduction. The point of 'simplification' is surely to make things more simple and not to open so many trap doors for the uninformed to fall into.

    Absolutely, KingCrimson!!
     
  5. Feb 25, 2014 #4
    Exactly , sadly most of the time simplification is just "give them an analogy of an observable process so they can relate even though it has nothing to do with what we are dealing with "
    anyway , back to my problem , if the two wires connected in parallel , have the same surface area but one of them have double the resistance , that means that the guy with double the resistance would have half the numbers of electrons flowing , right ?
    btw what do you mean by settled down ??because "how does it know "is a question that has bothered me alot with electricity
     
  6. Feb 25, 2014 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    If you consider a circuit consisting of a large Capacitor, being charged by a large resistor, you would have no problem, I imagine, in accepting that, until the capacitor was fully charged, the circuit would be in a state of change. So, make the capacitor very small, the resistance very small (as with lengths of wire) and bear in mind that where will be some inductance due to the leads etc., then wouldn't it be reasonable to expect that to take some time before the circuit reached a steady state.
    What is it about using the word "electrons" that makes you think that an explanation of this is somehow, deeper or better?
    If you look in text books and reputable websites like Hyperphysics, you will not find the way circuits behave, described in terms of electron flow. Why not just go along with the concept of Electric Current and 'charge' and deal with circuit behaviour at that level. I need hardly point out that a current can flow when either positive or negative charged particles, other than electrons, flow. They can all be treated by using 'Current' as the relevant quantity - and pretty much every such situation is studied on the basis of current. No one denies that electrons are the major charge carriers under most circumstances in our lives but they are an unnecessary complication.
    You will be aware that the electrons travel very slowly through the wire and that the impulse of current from one end to the other is almost travelling at c. (btw, I think you mean Cross Sectional Area and not "surface area") There are two issues - one is the amount of current that will flow in the respective wires and the other is the time taken for the change to propagate. Neither need involve the concept of electrons. If you really want to think about "how many electrons" flow, you can convert from Current to Electrons per second whenever you want but why would you want to? You will not find many electrical meters which are calibrated in "Electrons per second". Why do you think that is?
     
  7. Feb 25, 2014 #6

    davenn

    User Avatar
    Science Advisor
    Gold Member

    yep that's true ... have used TDR's for cable fault finding in the telecoms field in years gone by... but generally looking at distances of several km's or more
    In light of the OP context (electrons racing at high speed along wires etc), I decided not to get into TDR measurements

    Dave
     
  8. Feb 25, 2014 #7
    the problem is that i have been taught to think of current in terms of flowing electrons , i know this builds up lots of problems , but i seem to have no other way ,
    even if i think of charges , i have also been taught that a charge unit * coulomb * is a bunch of electrons , so that takes me back to the electron point of view .
    i have tried to think of current in terms of flow of electric potential but that seems to be harder
    , Would you care to explain to me what a charge is ? and how it propagates ?
    i am sick of imagining charges as a bunch of electrons moving because as i stated before it builds lots of problems when you think of it deeper , like Why do electrons split up in a parallel circuit , why don't all of them take the wire with least resistance , but then again its not about just moving electrons .
    if you have a link to a book , article , forum thread or website that explains what a charge is and how it propagates , i would be really grateful if you could supply such link * or if you even explain it yourself if it's okay "


    and yea i did mean cross sectional area , forgive my A.D.D.
     
  9. Feb 25, 2014 #8

    phinds

    User Avatar
    Gold Member
    2016 Award

    I don't know who "you" refers to in this case but your statement is not true for extremely high speed electronics. Modern computers depend on having circuit elements VERY close together to avoid having the delay cause problems. When home computers first came out (build your own from various cards in a card rack), for example, memory would be on a separate card from the CPU and the signals had to travel many inches, possibly even a foot. But that worked just fine because the computers were so slow. If you were to move the memory chips that far away in today's computers, the operations would stop working because the delay would be too great.
     
  10. Feb 25, 2014 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    If you want to find out "what charge is", then google "Electrical Charge Meaning". You will find hits at all levels. If you want a fundamental definition then you are in the same neck of the woods as "What is Mass?", "What is Energy?" and"What is time?", all of which questions can be 'answered' - or at least discussed till the cows come home.
    Would it help to put this electron / current thing to bed if I suggest that you put it the other way round and say "When electrons flow, there is an electric current". That is true but allows other things to be involved in Current. Current, being the flow of charge past a point or through an area of conductor, is more fundamental than the medium it uses. Maxwell's Equations take all the Electrical quantities and show, extremely well, the relationships between them and time - without ever mentioning an electron.
    I think you may be after an answer to this that is too simple to be worth while. If you actually start to use the methods associated with "Electrical Studies' and circuit theory then you will see that it is perfectly possible to use the accepted terms and to arrive at a working system of knowledge. There is not a satisfactory quasi-mechanical explanation for this stuff and I think this is what you are really after. I can only suggest you read lots more about this.
     
  11. Feb 26, 2014 #10

    davenn

    User Avatar
    Science Advisor
    Gold Member

    yes, I agree, and that isnt in dispute

    BUT Once Again .... in the context of what the OP was asking/stating, that isnt the most important factor around what was being stated


    Dave
     
  12. Feb 26, 2014 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    We cannot know what was in the OPs mind, precisely, but I would say that, as soon as you are dealing with non steady state, the delay becomes relevant. Phase shift is a consideration in virtually all circuitry, at some stage. This time delay simply has to be introduced in any argument or explanation about 'how a circuit knows' what to do.
     
  13. Feb 26, 2014 #12

    davenn

    User Avatar
    Science Advisor
    Gold Member

    I give up !!!!

    If you guys want to make a simple thing so complex .... go for it
    he asked something really basic and you guys turned it into rocket science :frown:

    Dave
     
  14. Feb 26, 2014 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I'm afraid that, basically, it really is as hard as rocket science.
    We gave the message that the model of electrons moving through a wire is totally inadequate for dealing with the OP's question. Then the idea of EM fields and waves was introduced - to explain the delay. It then got more complicated. But he who rides the tiger cannot dismount.

    You either have to take things on trust or go with the 'correct' (i.e. the accepted view) explanation and you cannot really pick and choose with Science.
     
  15. Feb 26, 2014 #14

    LURCH

    User Avatar
    Science Advisor

    If you need to think in terms of electron charges, then think of the wire as a tube full of marbles. As you push a marble in this end, one falls out that end (not instantaneously, but as quickly as a sound wave could travel through those marbles). Push two marbles per second into one end, and two marbles per second fall out of the other end. This will happen no matter how tightly the marbles fit into the tube. If the resistance of the tube increases, and you keep pushing with the same amount of force, you will no longer be able to push two marbles per second. Yet, for every marble you push in the entrance, one will fall out of the exit. If the tube is very tight, or very long (increased resistance), you may only be able to push 1 marble per second through the tube. If you push harder (increased voltage), maybe you can get 10 marbles per second through the tube. But in any case, the ratio of input to output remains 1/1, no matter how quickly or how slowly you can push the marbles through.
     
  16. Feb 26, 2014 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You can get away with the idea of the electrons comprising an incompressible fluid of zero density but you have to avoid any ideas of Kinetic Energy being carried by the electrons. The 'marbles' must be massless.
     
  17. Feb 26, 2014 #16
    thanks , well appreciated , i did spend some time alone and it all makes sense now
    @sophiecentaur and @davenn
    i wasn't going for the idea of delay and phase shift , i was just talking about normal circuitry and schematics , and also my confusion about current and velocity of electron did add to my confusion about parallel circuits, but knowing about delay and phase shift due to difference in lengths specially in High Speed Electronics did open up a window for me , so thanks everyone for your elaborate explanations
    @LURCH thanks , thats the conclusion i arrived at after a little while of thinking and reading
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: A little confusion with parallel circuits
  1. Parallel circuits (Replies: 5)

  2. Parallel Circuits! (Replies: 1)

  3. Parallel Circuits (Replies: 4)

Loading...