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Help me understand electricity a little better

  1. Sep 23, 2009 #1
    Hi, I'm brand hew here, I've heard lots of good things about this forum and thought I'd take a couple questions of mine here. I am trying to learn about electricity and electronics, as it would be incredibly beneficial to my desired job of audio engineer. Most resources about electricity are horribly counter-intuitive and I feel I need an interactive explanation for this.

    Firstly, what is voltage? It's defined as the "potential difference" between two points, does this mean literally the difference in the amount of electric charge between two points; that is, Coulombs A minus Coulombs B equals voltage (figuratively speaking)?

    My second question is, what exactly is a watt? Is it a property of electricity or is it an abstract measurement of the amount of work that SHOULD be done, given a voltage and amperage? This book I'm reading describes a watt as the "heat lost when a current flows through resistance," which doesn't...sound right. And obviously most resources define a "watt" as the amount of work done when one amp flows through one volt. That's great if I want to conquer the math aspect of it all, but it doesn't help me understand the "picture" of the way electricity works.

    And my last question is, what is the criteria for determining whether electrons or holes are the "charge carriers?" Obviously a hole doesn't "carry a charge" but we make the distinction in semiconductors regardless - for what reason (electrically) is a material defined as P-type? A hole is simply the space where an electron isn't, correct? And the backwards "flow" of a hole is just low-energy movements of many electrons, correct?
     
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  3. Sep 23, 2009 #2

    russ_watters

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    Staff: Mentor

    Welcome to PF.

    Much of what you are looking for can be found in Wikipedia articles as a starting point to learning. But here are some brief definitions, with links:

    Voltage is a similar concept to pressure. It isn't a difference in charge, it is what pushes charge from one place to another. A common analogy water pressure. http://en.wikipedia.org/wiki/Volt

    A watt is a joule per second: a measure of power. A joule is a newton-meter of work. Watts are not strictly an electrical measure of power, they also apply to other forms (such as and mechanical work). Electrically, a watt is the voltage times the amperage, or the potential difference times the amount of charge moved. Using the water in a pipe analogy, it is like mass flow rate times pressure.

    The electrons or holes thing is completely arbitrary, afaik.
     
  4. Sep 23, 2009 #3
    Scenario: A charge q is moving in an arbitrarily chosen path from point A to point B in a region of an electric field E. The field will exert a force on the charge at every point along the path, and hence does a certain amount of work in moving the charge. The total work done by the E-field in moving q from A to B is

    [tex]W_{AB}=q\int_{A}^{B}\vec{E}\cdot d\vec{l}[/tex]

    Dividing both sides by q reveals that the line integral of [tex]\vec{E}[/tex] from A to B has the physical meaning of work per unit charge done by the field in moving the charge from A to B. This quantity is known as the voltage between A and B along the specified path.

    [tex]V_{AB}=\int_{A}^{B}\vec{E}\cdot d\vec{l}[/tex]


    Watt is the unit of power. It's the power needed to produce energy at a rate of 1 joule per second.

    There is no "picture" of how electricity works. There is, however, a mathematical model of how it works. Unless you're Yoda, you need to understand the math.

    It's dependent on what impurities are added (see doping) to the semiconductor.

    N-type materials are doped (e.g., with antimony, arsenic or phosphorus) in such a way as to have a fifth valence electron that is unassociated with any covalent bond, and thus free to move within the material.

    P-type materials are doped (with e.g., boron, gallium or indium) in such a way as to have three valence electrons, causing vacancy -- known as holes -- in the crystal structure.
     
  5. Sep 23, 2009 #4
    Want to have a picture of electricity without the math. I hear you.

    Being that no one actually sees electricity, per se, entirely anyway, people in different fields have different ideas about it.

    For example, an Electrician works with it every day, and has more intimate contact with it than any PhD does, however, they're pictures are very different, but do share some similarities.

    The electrician is going to tell you that all the voltage wants to flow back into the ground.
    A physicist will tell you, that like charges repel, and unlike charges attract, and it's the interactions of these charges, as mathematically analyzed with "fields" that we can understand this.


    See, physics really starts out with fields to analyze the static and motion properties of charge. Electricity, for all practical purposes, is charge moving through a conductor. It's very simple to analyze what electricians deal with.

    Then there's what electrical engineers do.

    To really understand it though, especially things like impedance and reactance, and AC, you really need the math, because without it, the picture is extremely limited.

    I have the math, and I'm still working on my picture.
     
  6. Sep 23, 2009 #5
    Thanks you two. :)

    Okay, so voltage is a property of the electric field generated by charges, and it is this field that causes electrons to seek equilibrium in an area where they are deficient. Would it be accurate to say that electromagnetic fields are always actively fluctuating in electrically neutral objects, and it is a surplus of electrons specifically that offsets the balance between positive and negative fields, thus creating electromotive force that manifests as voltage? Again, I am very green about this and I am trying to teach myself.

    About the watt, the "watt" itself will just be an output of work in the form of a constant that is dependent on voltage and amperage, and isn't a property of electricity so much as it is an expectation of what electricity will do when converted to energy?

    I didn't mean to downplay the math involved in electricity. :) If I may make an analogy, it's akin to "understanding" gravity - I do not know the mathematical particulars of gravity, but I have a "grasp" of it and the way it causes things to interact. I don't have that understanding of electricity just yet.

    EDIT: Whoops, missed a third reply, reading now!
     
  7. Sep 23, 2009 #6
    Yes Nick, I'm trying to familiarize myself with the behavior and nature of electricity before I acquaint myself with the hard math behind it, in which I hope to avoid overloading and confusing myself behind help. :(
     
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