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Circuits Question : Series

  1. Jan 18, 2014 #1
    Hello all,

    A quick question: I am trying to come up with a valid explanation of a question posed to me regarding series circuits. It was deceptively simple but unsure how to address it. I would very much like any thoughts on this:

    how do the charges 'know' to give only some of their energy to each resistor along a series circuit? Why do they not give all of it to the first one they see?

    many thanks in advance
     
  2. jcsd
  3. Jan 18, 2014 #2
    When you hit the brakes when your vehicle has some non-zero velocity, why is its entire energy not consumed by brakes immediately?
     
  4. Jan 18, 2014 #3
    If you have a river and in several places you put some stones. Some here, more there..
    This stones will brake the flow and it gets slower.
    It will slow more where the stones are more and it will slow less, where they are few.
    So the charges does not now how many energy to give.
    They just give more energy, where they have greater resistance to overcome.

    [itex]\ \textit{}P = I^2R[/itex]
     
    Last edited: Jan 18, 2014
  5. Jan 18, 2014 #4

    phinds

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    I think you completely misunderstand how current works, and I don't really like the answers given so far, so let me try it.

    A good analogy for this question is a linked chain. You put posts of varying strength upright in the ground (some are harder to bend over than others). One end of the chain is on a roller and you pull on the other. The chain links all move at the same speed, it's just that some of them have to pull harder than others to move the post they are on so the response to the total force applied by you to the end of the chain is spread out unevenly over the various links but they still all move at the same speed. Does that help?
     
  6. Jan 18, 2014 #5

    sophiecentaur

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    There's one thing you need to realise about the way circuits work - and no one really makes it clear, early on in your education about electricity. They expect you to accept the steady state conditions without thinking too much about it and, in fact, anyone who 'thinks' about it will find that unsatisfactory. That thing is : When you switch on a DC circuit, there is an initial pulse (step function) that sweeps through the circuit at a bit less than the speed of light, getting reflected and generally bouncing around.. During the first ns or so, there is a lot of jockeying round between the components to decide who gets what and how much current will go where. The components establish their conditions within that short time (that's when they 'get to know' what to do) and the potential differences get shared out appropriate to the various resistances.
    I hate the water analogy for most things but you could imagine opening the valve at the top of a hydroelectric power station and the water, initially rushing down the pipe, unimpeded, then hitting a stationary turbine. This all resolves itself, once the turbine has spun up to operating speed and the flow has settled down.
    If any change is made to the circuit, another rearrangement will take place and the new conditions will then be established.
     
  7. Jan 18, 2014 #6
    I thank all of you for helping me with this.
    I think the initial problem was considering the 'story' of the path of a current as essentially starting from a battery which provides the Energy to each charge - giving it the energy to move in the first place. This, i think, is where the problem manifested. The concept lay in the idea that charges would then 'give out' some energy to a resistor so that it may transform that energy into something else - light/heat. It was a good analogy but it led to this idea that the charges gave off energy and were then subsequently bereft of it.

    I was not aware of the initial 'pulse' situation described by sophiecentaur. I find the analogy of the links as proposed by phinds to be very apt in describing applied force to the linked charge along the loop - i quite like it.
     
  8. Jan 18, 2014 #7

    sophiecentaur

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    IT takes care of the problem about the components 'knowing' what to do. It is also a reality. Nothing happens instantly in all reference frames.

    The problem with the 'journey' approach is how do you identify the start and the finish? Easy when you have a single battery and a loop but what about when you have a number of emfs and circuit branches?
    I don't like the Force (Field) approach as much as the Potential (Energy) approach here (and in many other electrical and non-electrical situations). Potential may take a bit of getting used to (we reckon we 'feel' forces so they are more familiar, perhaps) but the directions of the Fields get a bit difficult to consider because you would need to recalculate the whole thing (in principle) if you just jiggled the wires a bit.
    People seem to shy away from the Kirchoff Laws as if they are too much of a short cut to 'understanding' circuits but, imo, they are a very good half way house to getting there. You do need to be careful when you're not in a conservative field though - but there again, that applies to many of the other approaches (journey included).
     
  9. Jan 19, 2014 #8
    I should make it clear to everyone on this thread that this entire exercise is related to finding a way to teach the idea properly to high school students. Therefore some concepts must necessarily be left out. I have found that certain analogies - the water pipe one for instance - cause more problems then it solves as they tend to simply replace one concept with another equally confusing one [such as fluid dynamics].

    Despite its shortcomings I have found that describing the circuit as having charges that 'piggyback' their energy and then 'give' it to the resistor so they may transform that said energy into another form seems to work very well for them and help them towards proper calculation of circuit problems.

    often they seem to have their difficulties in really understanding voltage. in describing it as energy giving/taking then it settles many things in their minds.

    any further thoughts on this i would be happy to hear.
     
  10. Jan 19, 2014 #9
    This may work somehow if you understand that the charges are pushed by the field and not by other charges. They move in the resistors due to the electric field (or potential difference).
    And they do not get it once and they loose it all. They are accelerated by the electric field and gain some extra kinetic energy between collisions with the lattice. Once they collide they transfer some of this extra energy to the lattice. And then they gain a little bit again, due to the field. And give this to the lattice at the next interaction. And so on.
    So as long as there is a field (or potential difference) the process continues. The charges act more like an instrument of transferring the energy form the field (potential energy) to the lattice (kinetic energy).
    You see that the original question does not make much sense.
    As long as there is some field in the resistor, there is energy to be transferred from the field to the lattice.

    This is not a rigorous model but is better than the image of an electron being endowed by the battery with a large amount o kinetic energy which is somehow lost when travelling through resistor. And you don't need to assume that an electron goes through each resistor, to complete the circuit. Which in AC does not even happen.
     
  11. Jan 19, 2014 #10

    sophiecentaur

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    I really don't see the necessity of using extra, possibly more familiar concepts to put across the basic ideas of Electricity. It smacks of instant gratification rather than useful education. When is it too early for students to learn that some things just don't work like the analogues and metaphors that the School system floods them with?

    That is never done with Maths education and the people who are capable of getting somewhere with Maths just get over it and then get on with it.
     
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