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Discussion in 'Electronic Basics' started by Music Man, May 20, 2005.

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  1. Music Man

    Music Man Guest

    How is a volt defined.
    If a 9 volt battery can start a car,creating a very large current how does
    it differ from a 9 volt battery?
    Is it due to the amount of ionised material available?
    Is it the amount of P.D between both terminals that creates the amount of
    voltage(pressure) available?
    If an object has a large amount of free electrons or extra ones does that
    mean the voltage potential would be higher?
    As for static electricity,is this a state of an object being ionised?

  2. Nog

    Nog Guest

    100,000 lines of force, cut once each second by one conductor, will generate
    a potential of one volt.
  3. Originally from comparison to a standard chemical cell. Now, in the
    metric system as a relationship between the fundamental units of
    charge and energy (a given charge acquires a given energy when it
    falls through a volt).
    Their internal resistance varies. As current passes through the
    battery, its internal resistance uses up some of the battery voltage
    by ohm's law. 9 volt alkaline batteries have an internal resistance
    on the order of ohms, so can dump only an ampere or so before their
    internal resistance uses up all of the 9 volts. A big lead acid
    battery has internal resistance on the order of milliohms, so it takes
    hundreds of amperes to drop all of its voltage.
    The chemistry produces volts, the area of the plates determines the
    internal resistance.
  4. Dwayne

    Dwayne Guest

    Volt: A unit of electromotive force. It is the amount of force required to
    drive a steady current of one ampere through a resistance of one ohm.

    All baterries have a voltage rating and a energy rating (typically measured
    in Amp-Hours or milliAmp-Hours). The energy rating will determine whether a
    6V battery power a torch/flashlight or a motorcyle starter.

    Current and power are needed to determine if a particular voltage is
    dangerous. For example, compare the jolt from a 3KVA transformer with 230V
    on the primary side (source) and 2300V secondary (load) to a charged up
    person at 23,000V (walking on carpet in winter) touching a door knob. You'll
    feel a shock if you touch the door knob (23,000V), but will survive the
    experience. If however you grab onto the 2300V terminals of the transformer,
    you would be medium-well in a matter of seconds.

    Dwayne D. Chrusch
  5. It's one Joule per Coulomb. Imagine placing two plates out in the
    vacuum of space and providing a potential of one volt between them.
    Now take a cup with one Coulomb of electrons and gently empty it out
    very close to the negative plate. The electrons will accelerate away
    from the negative plate towards the positive one. When they are just
    about to hit the positive plate, their total potential energy
    (relative to the fixed plate) will be one Joule. Works the same if
    you used protons (much more massive) released at the positive plate,
    which will accelerate more slowly towards the opposite plate.
    Distance between the plates isn't important.

    Given this, we have:

    Volt = Joule/Coulomb
    Amp = Coulomb/Second
    Ohm = Joule-second/Coulomb^2 (a Joule-second is angular momentum)
    Henry= Ohm-second = Joule/Amp^2 = Joule-second^2/Coulomb^2
    Farad= Coulomb/Volt = Coulomb^2/Joule

    A nice result is that:

    R*C = (Joule-second/Coulomb^2) * (Coulomb^2/Joule)
    = seconds

    L*C = (Joule-second^2/Coulomb^2) * (Coulomb^2/Joule)
    = seconds^2

    Which is why you look at the timing constants of RC as proportional to
    just that and the timing constant of an LC as proportional to

    It's nice when the units work out.

  6. PeteS

    PeteS Guest

    Strictly speaking, the Ampere is the primary *defined* SI unit of
    electricity. All other units follow from there.

    Although 1 Amp = 1 Coulomb / sec, that's a statement of equivalency to
    charge carriers. The older standard definition is (although I believe
    it's getting updated)

    1 Amp is that current, when flowing in two rectilinear conductors of
    negligible cross section, 1 metre apart in vacuuo, will produce a force
    (due to magnetic fields, my comment) of 2 x 10^-7 Newtons between them.
  7. tlbs

    tlbs Guest

    That's one thing I never understood -- why the SI comittees never chose
    the Coulomb as the "fundemental" unit, but instead chose the Ampere.
    The Amp is a derived unit equivalent to the rate of charge, while
    charge itself is the 'more' fundemental unit.

    They need to define a new unit of charge that is a mole of electrons
    (or charge), also -- but since all other common useage units (Farad,
    Henry, Ohm, etc.) are already in use, this new unit would only be

  8. I know. I've read the CRC handbook where it describes this. However,
    we humans do like to think in countable things and Coulombs are

    In any case, I wasn't trying to stay in SI units as should have been
    obvious from the various ways I wrote what I did. I was merely trying
    to get the point across that one doesn't have to accept that the units
    in electronics are defined only in terms of the other units, in some
    kind of circular definition. I didn't like the idea of defining volts
    in terms of amps and ohms, where ohms is defined in terms of volts and
    amps. Neither ohms nor volts gets any clearer, that way.

    The electric units really do have physical meaning.
    I remember reading something like that. Good to see it written.
    Thanks. So we'll call a volt, Joule/Amp-second, and when trying to
    determine volts as Joules/Coulomb, put our cup at one end of those
    wires and let the electrons drain into it for one second.

    I wonder if the more "natural" idea of counting electrons as the
    primary SI unit (or protons) as being a Coulomb fell away to Amps
    because electrostatic methods of establishing the value depend too
    much on hard to control shapes and intervening materials.

    In any case, thanks.

  9. Fred Abse

    Fred Abse Guest

    At the time, it was much easier to measure, and create repeatable
    standards for.

    Probably still is.

    It used to be defined electrolytically in terms of mass of silver
    deposited under defined conditions in a given time.
  10. Jamie

    Jamie Guest

    1 Volt is the results of 1 amp of current in a 1 ohm Resistor.

    i also have explain this in different terms when educating youths

    Voltage is like water moving at a set rate of speed.
    current is like the amount of water moving.

    water coming out of a small pipe at high pressure is like having
    high voltage but low current! you can stop the water with out to much
    problem because the cubic feet per second isn't that great ! thus the
    weight of the water is not that heavy.
    now change the pipe to a large size, you now have a large volume of
    water! even at a slow moving rate (low voltage), it can knock you over
    due to the cubic feet of water hitting you! thus the weight factor.
    so this all comes down to this.
    given enough current (volume of the water) it can cause harm even
    at slow moving speeds (voltage).

    the speed (Volts) and cubic feet of water(current) is the
    Electromotive force.

    i have used this to explain to young one's about electricity and many
    to this day have learned well on this theory the difference between
    VOLTS and AMPS!

    hope that helps you also.
  11. Rich Grise

    Rich Grise Guest


    ** ** ******** ***
    *** ** ********** *****
    **** ** ** ** *****
    ** ** ** ** ** *****
    ** ** ** ** ** ***
    ** ** ** ** ** *
    ** **** ** **
    ** *** ********** ***
    ** ** ******** ***

    NO!!! NO!!! NO!!! NO!!! NO!!! NO!!! NO NO NO NO NO!!!!!!!!!!!!

    Voltage is _PRESSURE_ - it has nothing to do with "speed" (other than
    that more pressure can force more flow through a given resistance. See
    "Ohm's Law".)

    Now, if you have a PSI gauge at the faucet, its pressure reading might
    decrease when you open the valve - that's because of the PRESSURE DROP

    Another thing - a water pressure gauge measures PSIG (pounds per square
    inch gauge) or PSIA (pounds per square inch absolute) and its reference
    is intrinsic, or built-in - it's either atmospheric pressure, 15 PSIA,
    or zero (vacuum). With voltage, the reference is usually Earth ground
    or circuit ground or circuit common - a voltmeter measures the voltage
    between two points (AKA potential, or electromotive force (EMF)). Period.
    A water pressure measures the difference in pressure between the water in
    the pipe, and the air, which doesn't seem obvious at first, because we're
    embedded in it. The "other lead" of a water meter is the outside of the
    diaphragm, which is exposed to air. This is where the water pipe model of
    electricity kind of breaks down.
    Yes, Now we're getting a little closer.

    Yes. Voltage is pressure. Rate of flow is current, like the current in
    a river.

    Speed is just another aspect of flow rate. VOLTAGE IS PRESSURE!!!
    Well, you've been wrong.
    Well, STOP!
    Don't worry, now that you've been corrected, it might.

  13. JeffM

    JeffM Guest

    Voltage is like water moving at a set rate of speed.
    Electromotive FORCE, actually.

    ----- = pressure

    Electricity has no analogous equivalent for pressure.
  14. Jamie

    Jamie Guest

    Rich, i bet you call you DICK in for short.

    gets to the point very fast.
  15. Bob Monsen

    Bob Monsen Guest

    Voltage is a 'potential function'. It is a scalar function of position.
    Force, on the other hand, is a vector function of position.

    If you think of a wire, and a voltage difference along the wire, you can
    see that the electric field is really being directed along the wire. So,
    if you take a charge, and move it along the wire from one point to
    another point, you'll do work on that charge because of the force
    imparted on the charge by the field. The amount of work you do will be
    the energy required. However, this amount of energy depends on the
    amount of charge you move. More charge = more force. Thus, by dividing
    out the amount of charge you used, you get a value which is independent
    of the amount of charge. This is the voltage.

    It's formally defined as the negative of the path integral of the
    electric field between any two points. Since the electric field is
    simply the vector equal to force on a charge divided by the charge, it's
    easy to see that voltage is really just the difference in potential
    energy, divided by the charge used to measure the energy. It's units are
    joules/coulomb, which makes sense.

    As far as analogies go, you think of the electric field as being like
    the acceleration of gravity. Gravity is a vector field, pointing
    downwards (the gravitational field). The resultant force on an object is
    the field at that point, times the mass of that object. The 'voltage'
    between two points, in this domain, would be the path integral of the
    field (ie, acceleration of gravity) along the path between those two
    points. In the gravitational realm, voltage is thus simply the
    difference in height times a constant (assuming a constant gravitational
    field). Given the right units, it would just be difference in height.
    The potential energy difference between these two heights is the mass of
    the object times this potential difference. Letting an object fall
    through this height will give you a corresponding amount of kinetic energy.

    Thus, voltage is neither pressure nor force. It's a way to predict the
    amount of work you can get from an electric field.

    The water analogy is that the electric field is like gravity, charge is
    like mass, the rate of change of mass along some path is current, and
    voltage is the difference in height between different pools. Voltage
    differences don't make the mass want to move; it's paths along the
    gravity vector that makes water move. Resistance is flow restriction of
    the path. Power obtainable is head (which is height) * flow rate. etc, etc.
  16. Don Kelly

    Don Kelly Guest

    If you had equated pressure to voltage, your analogy would be quite a lot
    better. Not great, not true, but commonly used.
    You can have a voltage with no flow. This can be high or low. If you have no
    water flow- there is no "speed". Your analogy breaks down right away. Note
    that you are also involving a speed when you talk about cu ft/sec which
    comes down to mass*velocity at a given measurement point. You can also have
    high or low water current at a given high or low voltage.
    Your analogy breaks down immediately ,and, if the student has to learn
    more, he or she will have to unlearn a lot.
    The old, commonly used analogy is better than what you are presenting. It
    has its faults but all analogies do.

    While Bob Monsen is right it is not absolutely necessary to start with
    field concepts, and basic circuit theory doesn't actually require
    these concepts so the pressure<>voltage , current<> flow analogy goes a lot
    further conceptually than what you are using.
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