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How does electricity really flow?

  1. Sep 30, 2012 #1
    I'm in grade 11 and we still haven't looked into how electricity really flows. Right now all we're told is that the electrons move from atom to atom, the quantity of electrons dictates the current (in coulombs, however you spell that), and the amount of energy in each electron is the voltage. This seems to work well with all of the things I use it for but I know that's not how it really works.
    Can someone explain how it really works and how that ties in with current, voltage, and other electrical ratings?
     
  2. jcsd
  3. Sep 30, 2012 #2
    Packocrayons,

    "Electricity" is a generic term applied to so many aspects of electrical energy that it almost has lost its definitive meaning. Charge carriers, specifically electrons in metals, do flow. Current is the number of charge carriers moving per unit of time, so saying "current flow" means "charge flow flow", which is redundant and ridiculous. It is on a par with NASA "walking" in space.

    Yes, conductor atoms swap their outer valance electrons very readily with other conductor atoms. When current exists in a conductor, the electrons drift from one end of a wire to the other end. This drift velocity is extremely slow, about the speed of cold molasses. The common analogy is putting marbles at the end of a hose already filled with marbles. A different marble pops out of the other end of the hose almost immediately, but the particular marble first inserted takes a long time to reach the other end.

    For the MKS system of units, current is measured in coulombs/sec or amperes. Voltage is the energy density of the charges (joules/coulomb), whose MKS unit is a volt.

    You have to be more specific about you want to have answered.

    Ratch
     
    Last edited: Sep 30, 2012
  4. Sep 30, 2012 #3
    I meant current, I wasn't really thinking when I said it.
    So what you are trying to say is that it is actually energy being passed from electrons in one atom (valence or all?) to another and the motion of the electrons really doesn't have much to do with it. Is that correct?
    If you think of it like electrons moving, the speed of the motion of current would dictate the current, but in reality, what is actually happenening? For example if you run 1 amp through a conductive wire and then 2 amps through the same wire, what is actually happening differently? The same with voltage, what is actually happening differently in the circuit?
    Thanks for helping me out here.
     
  5. Sep 30, 2012 #4
    electrons move very slow, it is not the flow of electrons that create the current. Against conventional believes. Any varying signal travels are EM wave. The voltage and current you measure is the consequence of the the boundary condition of the EM wave. As

    [tex]\nabla \times \vec B = \mu \vec J +\mu \frac {\partial D}{\partial t} \;\hbox { AND }\; \nabla \cdot \vec E = \frac {\rho_{free}}{\epsilon}[/tex]

    BUT this is too abstract to work with, using current and voltage is just as good as long as you understand that ultimately is the EM wave that propagates in any of the circuit.

    Yes, this is hard to swallow, but if you look at any circuit, there is always a forward path where signal travel to AND a return path that the signal has to return to it's source. The two path form a guided structure. eg. a trace on a pcb with ground plane is actually a microstrip, an electric wire from the hot wire through the light bulb and the neutral forms a two wire transmission line............hopefully they are parallel to each other to form a parallel wire tx line. It might be easier to think of like Ratch said you push in an electron into one end and one got bumped out on the other end. But it is EM wave as it travel with velocity:

    [tex] v=\frac 1 {\sqrt { \mu \epsilon}} = \frac 1 {\sqrt{LC}}[/tex]

    As you can see, the speed ABSOLUTELY obey the EM propagation. Also we use transmission line theory in RF for all the circuit design.

    Yes, people gone out of their way to use magnetics, current and all to make it simpler. But sorry, science is not meant to be simple. I am not good with quantum mechanics, ultimately there is not even magnetic field, it is all electric from my understanding. Magnetic field is only a term given when the electric field of the two wire move at slightly different velocity or something and start to repel............This is way out of my league. But bottom line, electronics always try to use very simplified notation to explain to the point that it really can get thin at times. You ask the question, and this is a very unpleasant answer. Look into it if you really want, go the the classical or quantum physics section and ask this question.

    Believe me, even KVL has holes, I spent a whole Christmas argue in the Classical Physic section against the MIT professor's assertion. You have to be very very careful with all the Thevenin, Norton, Mesh.............not everyone if any can hold up in physics point of view. It is not that simple.
     
    Last edited: Sep 30, 2012
  6. Sep 30, 2012 #5

    dlgoff

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  7. Sep 30, 2012 #6
    Packocrayons,

    Electrons are negative charge carriers, and are predominent in metals. Because of their negative charge, they repel each other. It takes energy to accumulate a concentration of electrons in a defined space. The more electrons you put in a defined space, the more energy it takes. The smaller the defined space for the same number of electrons, the more energy it takes. The number of electrons divided by the energy it took to accumulate them is the energy density of the charge measured in volts. In the MKS system, 1 coulomb/1 joule is one volt. When one end of a wire has more volts or energy density than the other end, the electrons will want to move to the other end of the wire where the energy density is lower, and they are not so crowded together. In doing so, they will lose some of their energy in the form of heat in a conductor, and arrive at the other end of the wire with a lower energy density or voltage. So the energy carried by the electons is not due to their speed in a conductor, it is determined by the path they move within the electric field.

    Current is determined by the amount of charge carriers moving past a point, not be how fast the the charges are traveling. If I have a huge amount of electrons moving slowing past a point, that could equal the same current as a trickle moving very fast. If you send 1 amp through a wire, you will have to double the voltage (energy density difference ) between the ends of the wire to realize twice the current. So doubling the voltage will double the already very slow drift velocity of the electrons.

    Ratch
     
    Last edited: Sep 30, 2012
  8. Oct 1, 2012 #7
    Wow! Lots of great information here, more than enough to help me understand. Thanks for all the help guys!
     
  9. Oct 1, 2012 #8

    sophiecentaur

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    It is more accurate to describe the valence electrons as 'dissociated'. They are, in effect, shared by all their neighbouring atoms and are attracted to them all. This accounts for the physical properties of (most pure) metals which can be stretched and stretched without breaking. This is because the bonds never 'let go' but grab onto a new atom as it comes near. The energy needed to move an electron from one place to another is extremely small - hence the low electrical resistance of metals and the good thermal conductivity due to high speed electrons being able to travel easily around inside the metal and carry their kinetic energy through it. This is not to be confused with the average drift speed of an electric current which is, as stated earlier, very low.
     
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