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Voltage

Discussion in 'Electronic Theory' started by ElectroMaster, Jul 19, 2002.

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  1. ElectroMaster

    ElectroMaster Administrator

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    The electric pressure that must be applied to cause electron movement is called voltage. When such a voltage is applied to a conducting medium, free electrons move progressively from atom to atom and constitute what is known as current flow. In electronic and electric practices, a continuous path for current flow is formed by interconnecting wires, controls, relays, transformers, transistors, resistors and many other components to form what are known as circuits. Such circuits serve many purposes, but they all control, regulate, and harness the flow of electrons or which is electricity.

    The required voltage neccesary to force current through an electric circuit may be obtained from many sources. Some of these sources may be a battery or a generator. There are many types of both, a flashlight battery has only one cell which supplies 1.5 volts. If you place more than one end to end the voltage is multiplied by the number of batteries. So if you have a two battery flashlight, the voltage is 3 volts, a 3 cell the voltage is 4.5 volts and so on. Some batteries have more than one cell to the battery, your car battery for instance is probably a 12 volt battery, meaning it has 8 cells of 1.5 volts each.

    There are many types of generators also, from the thermocouple which is two dissimilar pieces of metal connected together, to the power plant size generator that supplies voltage to your home and office.

    Units of voltage
    Electrical pressure is needed to move electrons through a conductor to cause current to flow. Such pressure is known as electromotive force and is abbreviatedemf. The unit of emf is the volt named after Alessandro Volta (1745 - 1827) the italian researcher who first built a cell which provided electromotive force and which was the forerunner of our modern battery. Either V or E designates voltage (electric pressure), and sometimes the word Potential is also used. All these terms have the same meaning. A volt is the quantity of electromotive force that will cause one ampere of current to flow through one ohm of resistance.

    The source of electrons, from a battery or othe electric generating devicec, is referred to as the negative terminal or negative section of the unit. The terminal toward which the electrons flow is designated as the positive terminal. Instead of referring to the two different types of potentials as charges, the term Polarity is used. Thus, a flashlight cell may have a potential (emf) of 1.5 volts, with one terminal having a negative polarity and the other terminal a positive polarity.

    In practicle electrical and electronic applications voltages of a fractional value will frequently be encountered, as well as voltages having values up in the thousands, depending on the amount of electric pressure necessary to force current flow through the resistances encountered in the various circuits and devices. Fractional unit values of voltage are often in terms of a millivolt (one thousanth of a volt) or microvolt (one millionth of a volt). Thus 0.001-V can be expressed as 1 millivolt (1-mV) while 0.00003-V can be expressed as 30 microvolt. High voltages are often designated as kilivolts(kV),to indicate thousands of volts. Thus 10 kV indicates 10 thousand volts.

    Current and Conductivity
    A current flow path is provided by wires or other metals and thus form conductors of electricity. The ability of a substance to conduct electric current is termed conductivity. Current can also flow in substances such as liquids, gases, or materials whose composition offers opposition to the flow and limits the amount of current to definite quantities. The unit of electric current is known as the Ampere, named after Andre' Ampere (1775 - 1836) the famous French expermenter and scientist. One ampere of current represents the exact quantity of electrons that flows past a given point in one second and is equal to one Coulomb. The symbol for current is the capitol letter I for intensity or the symbol A for ampere.

    All substances do not provide the same degree of conductivity since the number of free electrons present depends on the atomic structure of the substance. Various metals offer different opposition to current flow, with some providing good conductivity and others opposing current flow to a considerable degree. The opposition of a substance to the flow of current is known as Resistance and the unit of measurement for such resistance is termed an OHM, in honor of George Ohm (1787 - 1854) the German professor who formulated the basic law relating to current flow and resistance known as Ohm's Law. The symbol for resistance is the capitol letter R and the symbol for ohms is the greek Omega symbol.

    The measure of how well a substance will permit current flow is known as Conductance. Because conductance is functionally opposite to resistance, it is the reciprocal of resistance and is therfore equal to the numeral one divided by the value of resistance, as expressed by the formula I/R. Thus, if a particular resistance is 1000 ohms the conductance is one-onethousanth or 0.001. Because conductance is the opposite of resistance, the unit for conductance is expressed as the word ohm spelled backwards which is MHO. The conductance of the example is 0.001 Mho. A fractional measurement of mho is the micromho. This is one-millionth of a mho. The symbol for mho is or conductance is G.
     
  2. fersarr

    fersarr New Member

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    TY

    Ty... very useful, im learning :D
     
  3. Roff

    Roff Well-Known Member

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    Good basic info.
    Just a nit-pick...
    Actually, lead-acid car batteries are made up of 6 cells of approximately 2V each.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You mean a car battery isn't just 8 "heavy-duty" AA cells in series? (smiley)
     
  6. srimannarayanakarthik

    srimannarayanakarthik New Member

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    AC

    Can we use this definition of flow of electons as current for alternating current also. i think v have to make a lot of changes in the definition of current for AC and DC
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No, it's not really any different, just with AC the current flow changes direction - or at least it's level of flow.
     
  8. srimannarayanakarthik

    srimannarayanakarthik New Member

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    but how can u define the negative phase of current, can u say that the electrons are flowing backward??? how can u exactly define the flow of electrons in AC.i think the very defintion of flow of electons cant be directly used for AC
     
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Makes no difference, electrons can flow either way.
     
  10. srimannarayanakarthik

    srimannarayanakarthik New Member

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    if they are flowin in either way then how the effective flow of current is specific to one direction
     
  11. Roff

    Roff Well-Known Member

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    With AC, the current changes direction periodically. That's why it is called Alternating Current. It's direction alternates. It doesn't flow both directions at the same time.
     
  12. Optikon

    Optikon New Member

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    <pickiness>

    Not necessarily periodically. General = time-varying.

    </pickiness>

    :D
     
  13. Roff

    Roff Well-Known Member

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    <Snort> Yeah, I thought about that, but I was afraid that if I mentioned it, it would lead to yet another strange discussion. If it does, you explain it. :)
    How did you get the grinning idiot icon into your message? I only know how to do :) and :(.
     
  14. srimannarayanakarthik

    srimannarayanakarthik New Member

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    my idea


    well i have my idea for this. but i am just novice to electronics and dont know whether my idea i correct or not. can u please jutify me.???
    i want to take a analogou situation and present my idea..........
    let us uppose that 3 car are travellin on the road. 1st one with velocity "V-a" 2nd onw with "V" and the 3rd with "V+a". now to the person who i drivin the car 2 feel that car 1 is going with a velocity a and crosses him and 3rd car will go with a velocity "a" backwards.
    now let compare this with the flow of electrons. by using the drift velocity equation of current,the velocity of flow of electrons i gonna decide the magnitude of current. so now car2 is the zero velocity state of electrons. car1 is the maimum value of curent and car 3 is the minimum valve.
    this means that all the electron are flowin the forward direction but with different velocities.the energy is tranfered more rapidly in the form of waves rathter than the actual flow of electrons.
    this can alo be jutified by the drift velocity equation.because the drift velocity in a copper wire i very less and it will take months to travel the distance to a generating station to your home.
    this is the rough idea what i felt.i tried my level best to give the complete idea fo what i feel. but if i was unable to make u understand or was wrong in some concept please correct me.
    karthik, srimannarayanakarthik
     
  15. Hayato

    Hayato Member

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    Well, keep in mind that the important thing is the "movement" of electrons creates the current.
    DC = Electrons are running to get to the job in time. (You have movement)
    AC = Electrons are dancing in a disco. (You have movement)
    :D
     
  16. Optikon

    Optikon New Member

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    OK, here's the strange discussion! (and I can't explain it!)
    The person who is driving car 2 does see forward and backward moving cars 1 & 3 from THAT perspective. However, an outside observer sees all three cars moving in the SAME direction.

    Now, are you an outside observer (with ammeter in hand!) or are you riding on an electron trying to measure/observe the current?

    An electromotive force can "push" an electron. Electrons can push each other due to same charge even though they don't need to "move" very far.
    So if it makes sense to you how they can "flow" in one direction in a conductor, then they can also flow the other way.

    Here is the collection of smilies apparently available:
    :eek: :( :) :confused: :mad: :p ;) :D :rolleyes: :cool: :eek:
     
    Last edited: Jul 18, 2006
  17. srimannarayanakarthik

    srimannarayanakarthik New Member

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    but


    but if they are movin in either direction then how can say 1 wire as phase and the other as neutral.
    and about the ammeter, it too contains coils. so what v measure is just a reference quantity. but not the exact value. so the electrons in the ammeter either vibrate with the same frequency of car2 so that v would be able to meaure the accurate value
     
  18. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Phase and neutral don't mean anything, they are purely arbitary terms, it's ONLY how the neutral is connected that makes it neutral, and the other phase (or live).
     
  19. dknguyen

    dknguyen Well-Known Member

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    The voltage difference between the two leads just happens to be the AC sinusoid. In reality, either lead could be whatever voltage you want relative to the surroundings/ground, as long as the voltage difference between the leads always makes a sinusoidal wave.

    The neutral just means that one of the leads just happens to be tied to the ground so that it doesn't float around too much. But it doesn't have to be for AC to work.

    IN the general case it is not necessary to have a "fixed constant" voltage reference in AC. It's just the difference between the two. You can think of it one of the wires (it doesn not matter which one,) as a reference that changes. Sometimes one is tied to ground which fixes the reference, but again, it's not necessary.
     
    Last edited: Jul 20, 2006
  20. dreadengineer

    dreadengineer New Member

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    I think we should be clear that in AC current, the electrons do actually turn around and flow back the other way. If it doesn't make sense that you can get power out of that, think of it like water in a pipe. If the water flows back and forth, you can still put a propeller in the water, and it will be turned alternately forward and backward, and you can get power from that. The source of the power comes from whoever is pushing the water forward and pulling it back, which is the power company. You can get power from AC current the same way. For example, an AC motor makes the electrons do work as they move forward and again as they move back.
    It's probably less confusing to just think of the voltage: the voltage between the two plugs on a power outlet alternates -- first a higher voltage on the right one, then a higher voltage on the left one, and you take that voltage and do something with it.
     
  21. dreadengineer

    dreadengineer New Member

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    Also, about voltage, like the post above was saying, voltage only has meaning as the RELATIVE voltage between two things. That's how we can say that one wire of the power outlet is neutral: it's held at the same voltage as the ground. So if you touched that wire and touched something that was grounded, like a water pipe, you shouldn't get shocked. But the other wire is going between +120V and -120V relative to ground. So if you touch it and the ground, you will get shocked, because there's a relative voltage. Whenever someone says something is at a voltage, like, "this wire is at 5V" it is implicitly relative to a reference. For example, in a portable device, it's relative to the - terminal of the battery. Saying that the wire is at 5V means it's 5V higher than the - terminal of the battery.
     
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