Negative voltage

Discussion in 'Homework Help' started by terrakota, Dec 9, 2005.

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  1. terrakota

    terrakota Thread Starter Well-Known Member

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    Hi,
    can somebody explain to me what is negative voltage?, how a negative voltage is produced from a typical battery?, when and why you must use a negative voltage?.

    can someboady drives me to a site with some usefull tutorial, guide, etc. explaining this?

    thanks for an help
    and please excuse my poor english
  2. Ron H

    Ron H E-book Developer

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    When we measure voltages in a circuit, they are usually referenced to a common node in the circuit called "ground". Ground does not have to be connected to earth. It is just a term that indicates the common node in the circuit.
    The schematic below should help explain this concept. As you can see, whether a voltage is positive or negative simply depends on which end of the battery (or other power supply) is connected to ground.
  3. terrakota

    terrakota Thread Starter Well-Known Member

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    ok i understand but,

    with a diode if you set the polarity of the battery to forward bias the diode you are applying negative voltage no? becasue the electros leave by the negative side.
    if reverse polarity you are applying positive voltage to the reversed bias diode, so so, whats the difference?

    thasnk for reply
  4. Brandon

    Brandon Senior Member

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    Forget about moving electrons because they teach you incorrectly to make it easier in the beginning. Electrons don't move, well, they do, but they move like 1 meter every 20 seconds in copper so its obviously not the electrons delivering the energy. You're looking to deep into this, its very basic.

    BY CONVENTION, you assume current flows from + to - for all calculations. You also assume that current goes in the direction of the voltage drop. I.e. if point A is 3v and point B is 4v, since A is lower that B, the current would flow from B to A, the direction of the voltage drop. This also works with negative voltages. A is -4 B is -5. It would flow from A to B since B is lower than A.

    Diodes (+) --[>|--- (-)

    The polarity on diodes is used to tell you which way current will only typically flow in the diode in normal operation. It flows from the + to the - if you apply enough voltage to the + side such that the difference between the + and the - is atleast the diodes forward on voltage. Typically the ones u use in school n what not are 0.7 volts.

    Remember, voltage is just a difference. It is that difference which forces current to flow and the current flows from the higher voltage to the lower voltage.
  5. JoeJester

    JoeJester AAC Fanatic!

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    terrakota,

    Look at the attached circuit. All the voltages are negative with respect to ground. Is the diode forward biased?
  6. n9352527

    n9352527 AAC Fanatic!

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    ???
  7. n9xv

    n9xv Senior Member

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    Ok - I'am gonna have some fun with this just to warn ya!

    FOR ILLUSTRATON ONLY!!! - KIDS DONT TRY THIS AT HOME!!!

    Brandon, if electrons dont move - or - they only move at the incredibly slow pace of 1-meter / 20-seconds, then you should be able to plug an AC line cord into your 120-VAC outlet with the ends stripped and hold one in each of your (wet) hands. If what your saying holds, then you have some 20-seconds before you get "bit" by that all mighty force behind the fuse pannel!

    Now, I have a feeling that you wont actually try this but - - - what might be stopping you?

    M-M-M-M-m-m-m-m-m-m-m!
  8. n9xv

    n9xv Senior Member

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    "can somebody explain to me what is negative voltage?"

    Any voltage can be negative with respect to a more positive point.

    Example:

    Take two 12-volt batteries, connect in series - negative terminal of one battery to the positive terminal of the other battery. Now, for purposes of reference and this explaination, drive a ground rod into the ground and connect the "tied end" of the batteries (negative-to-positive connection you made earlier) to the ground rod. The ground rod is the reference or common connection for our 12-volt battery arrangement.

    So, if you measure between the ground rod and negative terminal of one battery (black lead to ground rod & red lead to negative terminal) you will read "- 12 volts". The voltage at this point is more negative with respect to ground. Conversely, you could say that ground is more positive with respect to that batteries negative terminal.

    Electrons always flow from negative to positive. The positive-most point in any circuit represents a depletion of electrons and the negative-most point represents a surplus of electrons. The surplus (-) condition is always trying to satisfy or equilize the depleted (+) condition. Thus, electron current flow.

    "Conventional current flow" depicts current flowing from positive to negative as a matter of conveinience. All is good and well as long as you realize that it is actually the movement of electrons doing the work.

    "when and why you must use a negative voltage?."

    The reasons are as varied as the circuits themselves. One common application of a negative voltage is for biasing circuits. A negative voltage may be used to turn a device on whereas a positive voltage may be used to turn it off and visc-versa.

    BTW, JoeJester provides a good example for thinking "outside the box" - if you will.
  9. terrakota

    terrakota Thread Starter Well-Known Member

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    yes its forward bias, well i think :)

    thanks to all for your help now i have a clearer image
  10. JoeJester

    JoeJester AAC Fanatic!

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    One prime example of negative voltages is RADAR.

    If you have a radar aboard a ship that uses a waveguide connected to the anode of the magnatron ... that waveguide is typically metal. The waveguide is attached to the bulkheads and overheads working it's way up the mast to the rotating antenna. Now, last time I checked, large ships were METAL so anything attached to the bulkheads, overheads, and masts were at the same potential as the ship itself [ground].

    In order to have the anode of the magnatron at 20,000 volts, you must make the cathode negative 20,000 volts.

    Another example is in your kitchen. A microwave oven. Again, the magnatron anode is at ground potential. You need to apply a large negative voltage to the cathode to get that magnatron to be forward biased.

    There are times you want and need negative voltages. The a$$ you save maybe your own.
  11. JoeJester

    JoeJester AAC Fanatic!

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    terrakota,

    The potential across the diode is 0.9 volts [subtracting the anode voltage from the cathode voltage], so it is above the expected 0.7 volts to forward bias the diode.

    Had I placed a meter across the diode with the negative lead on the cathode and the positive lead on the anode, you would have seen quickly that it was forward biased, but, that wasn't the point of the circuit.
  12. Brandon

    Brandon Senior Member

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    Actually, its more along the lines of millimeters a second. Charge is what has the energy, not the electrons. The electrons make it possible to conduct the charge in the valence bands of a conductors electron orbits.

    http://hyperphysics.phy-astr.gsu.edu/hbase...ric/ohmmic.html
    http://www.glenbrook.k12.il.us/gbssci/phys...uits/u9l2d.html
    http://amasci.com/elect/elefaq1.html#ae
    http://www.electricityforum.com/electricit...-magnetism.html
    http://www.britannica.com/eb/article-9032271

    There is no need to be a smart ass or propose stupid experiments. You have to admit, it was stupid. Where is your control? You haven't accounted for nor elimitated any positive charge which could be in the line and foul the experiment. For all we know, the power companies could be sending + charges down the line and then this would be a moot point.

    I know some of you remember Physics II when we learned about + charges and - charges and you put a test charge out here and used the EM version of the gravity equasion Q*q1*q1/d^2 to calculate force... That force is the beginnings of a electric potential which will cause the charge to move and that movement of charge is current which leads to power and me grabbing the leads and being knocked on my ass by the charge in the line and then filing a suit for proposing such a stupid experiment.

    ;)

    Tacos anyone?
  13. n9352527

    n9352527 AAC Fanatic!

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    ???

    <!--QuoteBegin-Brandon
    @Dec 13 2005, 04:15 AM
    For all we know, the power companies could be sending + charges down the line and then this would be a moot point.
    [post=12419]Quoted post[/post]​
    [/quote]

    ??????

    I am not intending to propose a stupid experiment or start a flame war, but I just need to point out that maybe you could kindly clarify these points, preferably with a few valid references?
  14. n9xv

    n9xv Senior Member

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    The particles which carry charge through wires in a circuit are mobile electrons.

    In terms of electron movement:

    6.25 X 10^18 electrons moving past a point for a period of 1-second is the deffinition of an ampere of electric current.

    The electrons "possess" the charge - the electrons are indeed moving in the wire.

    It appears that you were reffering to the drift speed of an electron which is "WAY" different than the speed of an electric "current", which is 300-million meters/second. When one speaks of the flow of current, it is understood to be 300-million meters/second and not the drift speed of electrons. You dont make that distinction in your post. You made it sound as if it would take 20-seconds for the current to flow in an otherwise small electronic circuit.

    As far as the concept of charges go, electrons go one way and "holes" (the space where an electron used to be) go the other way.

    The electric field direction within a circuit is by definition the direction which positive test charges are pushed. Thus, these negatively-charged electrons move in the direction opposite the electric field. But while electrons are the charge carriers in metal wires, the charge carriers in other circuits can be positive charges, negative charges or both. In fact, the charge carriers in semiconductors, street lamps and fluorescent lamps are simultaneously both positive and negative charges traveling in opposite directions.

    Electrons are actual physical "things" that have mass & weight and possess an electric charge. They are what moves and constitutes an electric current.

    Show me text that suggest & proves otherwise!!!

    Dont take yourself too seriously Brandon. What you should take seriously are the facts.
  15. Brandon

    Brandon Senior Member

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    n9, I responded in kind, no more no less. I don't take myself too seriously and you shouldn't suggest a potentially lethal and idiotic test. I hate ee. Hate that I know it., Hate that I am basically forced to get a masters in it as well at this point, but anyways, more text as requested.

    Speed of electrons as related to their conductor.
    Current I = nAvQ
    n=number of charges particles per unit volume
    A = cross sectional area of the conductor
    v = electron drift velocity
    Q = charge on each particle

    When you solve this, you find electrons move slow which is counter intuitive, so something else might be going on. Here's where Maxwell's Equasions come into play.

    http://www.ece.rutgers.edu/~orfanidi/ewa/ch01.pdf

    Speed of propagation
    Please refer to equasion 1.3.8 dealing with propagation speed, don't feel like putting in all the square roots n what not. But basically it comes down to that the signal propagates through a conductor based upon the difference in the conductors permativity and permeability in reference to a vacuum where signals travel at the speed of light.

    This propagation is seperate and distinct from the electron flow. The signal is carried by the eletrons which moves much faster than the eletrons themselves.

    Anyways, save that pdf, its a great EM reference.


    Now, i've given 5 points to support that current is not the flow fo electrons but the flow of charge. I have yet to see one from you.
  16. n9xv

    n9xv Senior Member

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    "Speed of electrons as related to their conductor.
    Current I = nAvQ
    n=number of charges particles per unit volume
    A = cross sectional area of the conductor
    v = electron drift velocity
    Q = charge on each particle"

    Your above equation concerns semiconductor physics, not simple current flow in a copper wire conductor. The equation is a small part of a much bigger picture in the study of semiconductor physics.

    "Speed of propagation"

    You getting into propogation delay & velocity factor which does not apply to simple power transmission. The speed of current through a copper wire is generally taken as .95 that of the speed of light. A 5-% reduction due to the medium in which it flows. However, to simply get power from point A to point B this is of no significance.

    "I hate ee. Hate that I know it., Hate that I am basically forced to get a masters in it as well at this point"

    You hate electrical engineering (ee). . . You hate that you are "forced" to get a masters in it. . .

    Perhaps this explains your lack of knowledge and where to apply it.

    Maybe you should only post in forums that deal with subjects you like, not hate!

    I guess the reference to me as a smart ass makes you the high preist of "RECTAL KNOWLEDGE"!!!
  17. Brandon

    Brandon Senior Member

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    n9.. n9.. n9... I feel like columbus trying to tell the populace that the world is round and being spit on and laughed at in the process.. reminds me of middle school.. damn my mom for dressing me like a nerd.

    Here are a few more proofs and examples.

    Charge and electrons are different. electrons posses charge just like the other particles, negatrons, positrons, protons, etc.

    If electrons did flow at the rate you suggest then a simple calculation you can do on your own at any time should be able to prove or disprove this point very quick with a common everyday battery. Mass it. Cut out about 10% of it for the housing and terminals. Then 1/2 this mass since you need an electron rich paste as well as an electron deficient paste for the internal redox reaction of the battery.

    From this mass and the chemical composition of the alkali paste, you should be able to get the molar mass of the paste. As well, you should be able to get the # of electrons each compound would contribute during its reduction. You have the total # of free electons in a battery.. but there is a problem with that. You cant use all the electrons. You notice how a battery still has a significant voltage even when dead? Its because there is still a large difference in the number of electrons in both pastes.

    Now, if you go to the website for the manufacturer of the battery you will get its operating life given in Ampere hours. i.e. how many hours the battery can run while giving out one amp. Basically, if your saying that 1 Amp is 1 C/s where 1 C = 6.02 x 10^23 eletrcons (1 mol) then every second of operation, the battery must be able to deliver this amount of electrons.

    If you notice, this means that a .5Ah battery has 30 mins * 60 seconds = 1800 mols of electrons. If 1 mol of your paste would to say give 3 moles of electrons then you would need 600 mols of alkaline paste for the eletron rich side. Lithium, a very light alkaloid has an atomic mass of 7 on its own. 600 mols of Lithium would weigh 4.2 kilos. Any we haven't even made a reducable compound yet.

    Just for reference, if you notice a 1.5AA has 2.8-3.1 Ah which would put the paste requirement up to 24 kilos. Just do the math. You will see that it does not add up. The amount of availbile current does not match the number of availible electrons.

    http://www.climber.org/gear/batteries.html
    Second Example, Super Conductors

    You've all heard of them. I'm sure you know what they do. Just in case, its a material which does not disipate any power when an electrical current ispassed through it. It is typically accomplished by using cryo temperatures

    Funny thing with temperature.. back to physics. The addition of heat agitates matter and causes it to vibrate/move more violently. This occurs in conductors as well. Typically, if you looked at some of my links, you will have seen the pic of a zoomed in wire showing random motion of eletrons in the wire. This random motion always occurs until a potential is connected across the wire whereas the eletrons will begin to move, but very very slowly.

    When a wire get hot the resistance goes up. Anyone who has messed with a thermocouple or thermistors knows this. When a wire gets cold its resistance decreases... now the question is why? The only thing moving in the wire are electrons.

    Assumiung that eletrons DO flow at the speed suggested by my nemisis n9, then when the wire got hotter it should flow faster and when the wire get cooler, it should flow slower, but we all know such is not the case. Therefore there must be something else in the wire which is traveling which is not effected by temperature, i.e. charge.

    When you bring a conductor down to cryo temperature, the eletrons stop their gaussian random .. maybe it poisson, whatever, motion and they line up into distinctive channels. When this occurs charge finds there is no resistance and can freely flow in the wire. But if the wire is heated and the resistance goes up at the same time the electrons begin to randomly move more, its a good sign that the motion of the eletrons impedes the flow of current.

    Real world example

    A long hallway filled with people. Everyone MUST continuly walk around randomly, may not line up and they must also try to file out of a single door at the other end. Given that they can't line up, going to be a pain in the ass. Now, here comes a charge. The charge is a tennis ball given to one person at one end of the hall. They charge wants to get to the other end to person after person hands it down the hallway until it gets there. But since everyone must walk around randomly while doing this, there is bit of resistance to get the tennis ball to the other side.

    When the hallway reaches a 'super conductor' arrangement, all the people line up single file, shoulder to shoulder. Tennis ball come in and its very easy to pass it to the person in front of you while everyone is slowly shuffling out the door. They tennis ball get their first where as the people move much slower.

    The people are the eletrons, the tennis ball the charge. This is the simplest explanation we were able to come up with which made sense.


    n9, if you want to dispute this, go ahead. As I said, you started all this stupid crap by being the initial smart ass with your 'experiment' while I was trying to explain an often misunderstood concept about electricity.

    You know the subject I like best? Teaching.
    Want to prove me wrong? Take what I posted to counter it.
    Use Maxwell equasions and show me I am wrong. Take everyday life and and use it.
    I don't mind being wrong. I'd rather you prove to everyone and yourself that I am full of it then to let you try to end this thread with a simple insult.

    That is my challenge to you.
  18. cookevillain

    cookevillain New Member

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    I do not see why such a heated discussion would suddenly arise from a mundane question of what constitutes signal propagation. Both Brandon and n9 are right. The energy is carried by the EM radiation which does propagate at 300 000 km/sec (a bit slower in a conductor). Any good EM book (thanks for the link, Brandon!) would have a discussion about how the electrons start moving almost in unison along the length of the conductor even thought each individual electron is moving very slowly, constantly bumping into the atoms in the lattice (I know, quantum mechanics says otherwise). So there is no contradiction! As far as dangerous experiments go---think about how a microwave oven works: there is NO flow of charge whatsoever (between the microwave oven and your bowl of soup) but your food gets warm nevertheless! The reason? EM radiation. An even simpler example---a transformer: the coils are isolated from each other but the energy still flows: the magnetic field in the core transfers it, charge has nothing to do with it.

    C-villain
  19. n9xv

    n9xv Senior Member

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    All you've proven Brandon is that you know how to cut & paste technical data from a battery manufacturer. You have'nt altered the reality of the priciples of current flow in a simple conductor. I like how you dodged the prompt as to why you are wasting your time in a subject (EE) in which you hate. That must be a rough way to go. Some people hate to get up in the morning and go to work... some people hate to go to the gym and workout etc. I cant imagine spending all the money & effort it takes to get a masters degree in a subject you hate! Back in my origional post I prefaced it by saying I would have a little "fun" with it. Your obviously too immature to have realized that. Should'nt you be helping mama bake christmas cookies or something?
  20. terrakota

    terrakota Thread Starter Well-Known Member

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    thanks to all for your great help
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