Electric Current for Dummies

Discussion in 'General Electronics Chat' started by djsfantasi, Jun 28, 2016.

  1. djsfantasi

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    Apr 11, 2010
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    Let's say I have a loose line of red Solo cups. Some have pong balls in them; some are empty. Pong balls are light and bounce around all over the place.

    Now there is a wind (somewhat possibly laminar) which blows the pong balls out of their cups. Now, being lighter, the red Solo cups are blown down the line, where another loose pong ball falls into it.

    The pong balls are the electrons. The red Solo cup is a charge carrier. The wind is voltage. And if I may extend myself, the somewhat possibly laminar wind is the conductor (being somewhat possibly crystalline).

    So what did I miss?
     
  2. ErnieM

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    Apr 24, 2011
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    Your analogy has the wind as both the voltage or driving force and the conductor or charge carrier.
     
  3. joeyd999

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    What's the difference between an electron and 'charge carrier'?
     
  4. nsaspook

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    You should make a mental distinction in the analogy between the ‛flow of energy’ (called a field or Electric potential/Voltage, electric force, etc ...) and the ‛flow of matter' (normally referred to as current), show their relationship and it must be consistent with Conservation of Energy (~Ohm's law).
     
  5. djsfantasi

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    Woops. Unintentional.
     
  6. djsfantasi

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    I might be using the term 'charge carrier' incorrectly. What I intended was to distinguish between a free electron and the resultant positive 'hole'.
     
  7. nsaspook

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    In the context of a 'regular' electron sea conductor like copper there is no conduction hole so it's an unnecessary requirement.
     
  8. djsfantasi

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    Oh I am so glad I started this post. Now I am more confused than before.

    No conduction hole in copper? I thought current was the motion of either electrons or the virtual motion of 'holes'. I say virtual because there is no actual motion of protons, but a positive charge due to a temporary mismatch in pairings. Thence my understanding of conventional vs something (my memory is going) models of current flow.
     
  9. DGElder

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    Apr 3, 2016
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    Using this rather strained analogy for a wire: the cups and pong balls are atoms sitting in a 2 dimensional array on a table. The balls are negatively charged electrons. The cup is a positively charged nucleus. A slightly more than full cup (conductor) is a neutrally charged atom. In each cup a ball on the top will occasionally be caught by the wind (electric field) and swept into a downwind cup - bumping a ball out of that cup in the general direction of the wind. Meanwhile an upwind ball is swept into the original cup - keeping it electrically neutral. The number of balls that are swept off the end of the table per second - with their associated charge - is the current. The dude that collects these balls as they fly off the end of the table and feeds them back into the upwind side of the table is part of the battery. The number of balls fed into the upwind side equals the number flying off the downwind side, keeping the table neutrally charged.

    In a P type semiconductor a fraction of the cups are not quite full, they have room for one more ball, but they are still neutrally charged. The N type semiconductors have some cups with one extra ball sitting on top but they too are neutrally charged. If you put a table of cups where some are P type next to a table where some are N type, then due to diffusion ( cups are vibrating due to heat), the P type cups near the N type table will have a tendency under this thermal excitation (vibrating table) to accept an extra ball and become negatively charged and the N type cups near the P type table are more likely to donate a ball and become positively charged.

    So in the boundary region where the two tables are butted up against each other you have an electric field due to the now negatively charged P type table and positively charged N type table and this electric field will oppose the movement of any more balls. So if you want current you will have to apply sufficient air pressure (voltage) to overcome the space charge to get the balls to flow. In other words, you have a diode, requiring 0.7 volts (pressure) to get the balls flowing (current).

    This is a loose analogy, constructed after three tall beers, with all the implied limitations. It is best to use analogies/models that are closer to reality. For example http://www.allaboutcircuits.com/textbook/semiconductors/chpt-2/electrons-and-holes/
     
    Last edited: Jun 28, 2016
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  10. nsaspook

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    The most common 'simple' model for the conduction electrons in a metal is a perfect Fermi gas or sea where the electron-electron interaction is simply the Coulomb interaction. There is a rough balance between the electron-nucleus attraction and the electron-ion core repulsion so the conduction electron is free to move at its Fermi velocity between random collisions. In a good conductor there are so many interactions with so many other electrons that the overall effect is they are mainly isolated from proton charge.
     
    Last edited: Jun 28, 2016
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