1. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
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    Edit: Sorry this should be in the 'General' forum

    Does anyone agree with these statements? I don't think I fully agree:

    Currents don't have Voltage

    Voltage is not a characteristic of electric current. It's a common mistake to believe that a current "has a voltage" (and this mistake is probably associated with the 'current electricity' misconception, where people believe that 'current' is a kind of substance that flows). Voltage and current are two independent things. It is easy to create a current which lacks a voltage: just short out an electromagnet coil. It is also easy to create a voltage without a current: flashlight batteries maintain their voltage even when they are sitting on the shelf in the store. Water analogy: Think of water pressure without a flow. That's like voltage alone. Now think of water that's coasting along; a water flow without a pressure. That's like electric current alone.
     
  2. Wendy

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    Mar 24, 2008
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    I don't agree, without voltage there can be no current. The reverse is not true, there can be voltage and no current, but without EMF current can not flow.

    There are circumstances current can exist without voltage, but it is rare. For example, if you induce a current in a superconducting loop, and close that loop, the current will continue to flow with no voltage present (and there will be a magnetic field as a result). The magnetic field reflects the stored energy. However, there must be a voltage to start the current flowing to begin with.

    The water analogy is also false. Without a pressure differential water won't flow either, they are trying to describe inertia. However, something started the movement to begin with, else 3 laws of motion rule (an object at rest tends to remain at rest).

    Where did you see these statements?
     
  3. Jony130

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    Feb 17, 2009
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    But in theory it is possible that there is current without the voltage.
    The whole concept of current source.
    And some time is is better to look that the voltage is the cause of the current flow.
    And another time is is better to look that the current is the cause of the voltage.
    Some kind of dualism.
     
  4. Wendy

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    Show me an example.

    A current source still has to have a voltage source in back of it, you can not separate the two. Voltage induces current, you can not have current without a potential difference.

    So I ask you to show an example.

    The superconducting ring still needed something to start it, but not to maintain it. It qualifies, with additional qualifications.
     
  5. beenthere

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    You might be throwing out a baby with that bath water. Current is the actual movement of electrons in a wire (we will confine the example to wires). The measure of current is defined as a quantity of electrons passing a point in one second.

    With no difference in potential - read voltage difference - there is also no mechanism to compel those electrons to move. No voltage, no current in any case. No current means no voltage, provided there is an unbroken conductor between points.
     
  6. loosewire

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    Apr 25, 2008
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    There ort to be a law,that small battery operated radio's
    should be fused for current. Maybe there is one.
     
  7. studiot

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    Nov 9, 2007
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    In general Bill's post#2 gives a very good answer to your questions succinctly.

    However if you want to delve a little deeper and find a better mechanical analogy than the hydraulic one consider this.

    If we ignore thermal considerations and confine our inquiries to potential and motion energy we can represent the total energy of a system by the sum of the PE and the KE.
    Now Bernouilli's theorem for fluids descrinbes the relationship between these for your flowing water example and you can calculate the apportionment of the total energy between KE and PE from it. For any fluid the faster it flows ( a measure of KE) the lower the internal pressure (a measure of PE) the total being constant in your example.

    Electrical systems work the same, the system energy being distributed between the PE (voltage) and the KE (current).

    Now to find a zero current (all PE) or zero voltage system (all KE) mechanical analog consider a swinging pendulum.

    At the top of its travel the pendulum is momentarily stopped and all its energy is PE.

    At the bottom of its swing it is travelling at max velocity, has zero PE and all its energy is KE.

    Its cycle is a constant interchange between these forms of energy.

    In the same way we can create oscillatory circuits where the energy is interchanged between current and voltage, with momentary times when the energy is all current or all voltage.
     
    Last edited: Jan 10, 2010
  8. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
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    Bill,

    I agree with your statements which is why that paragraph struck me as puzzlingly wrong. Is a good example of a voltage without a current using a diode?

    studiot,

    I see your KE/PE analogy, but I don't understand your statement:

    'Now to find a zero current (all KE) or zero voltage system (all PE) mechanical analog consider a swinging pendulum.'

    Wouldn't a zero current be a zero (not max) KE? and wouldn't a zero voltage be a zero (not max) PE? i.e. if you have no charge flow, you have no kinetic energy and no current... and if you don't have a potential different or voltage, you have no potential energy analogy?


    By the way, that paragraph was from me reading this from browsing earlier:

    http://www.eskimo.com/~billb/miscon/voltage.html
     
  9. Wendy

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    Yeah, Mr. Beaty has a reputation, and from my point of view not a good one.
     
  10. studiot

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    Yes well spotted I got the PE and KE the wrong way round in the pendulum statement.

    But it shows you read my thing and it made you think.

    Sorry now corrected.
     
  11. Tesla23

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    May 10, 2009
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    What if your pendulum was charged?
     
  12. studiot

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    What if?

    Go on then, develop this train of thought. The pendulum can either carry an isolated charge or it can carry a charge under the influence of a potential field from another charge.
     
  13. JDT

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    Feb 12, 2009
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    A metal ring on a transformer core has current flowing - it gets hot, it generates a magnetic field.

    Where is the voltage?

    Well, it is there on a molecular scale.

    There must be a voltage in order to move the electrons.

    Best to think of it as an almost infinite number of voltage sources and resistors connected alternately all round the ring. Each resistor has a voltage drop that exactly matches its voltage source.
     
  14. Wendy

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    Even with an antenna or LC circuit when current is at it's max there is still some voltage, unless you have a superconductor in your hip pocket. Any resistance at all combined with current is voltage, basic Ohm's Law.

    You can have a capacitor hold a charge for a long time, but a coil (unless it is the superconducting variety I illustrated earlier) does not cooperate so well. It is in an unstable condition.
     
    Last edited: Jan 10, 2010
  15. BillB3857

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    Feb 28, 2009
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  16. studiot

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    When I went to school both the current and voltage passed through zero in an alternating waveform, although usually not at the same time.
    How is it now possible for this to be otherwise?
     
  17. BillB3857

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    In an AC circuit when the reactive voltage equals the opposite polarity of the applied voltage, current is zero. There are two voltages involved.
     
  18. studiot

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    So ?

    Voltage without current has already been demonstrated,
    My discussion was aimed at providing an example, albeit momentary, of current without voltage. This occurs whenever the alternating voltage passes through zero and the current is not zero.
     
  19. Blackbull

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    Switching on the radio: “.....60000 Amps, there is no resistance it is a superconductor at almost absolute zero.” Apparently it was referring to the CERN project in Switzerland. I = V/0; division by zero does not exist in mathematics!
     
  20. ke4pjw

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    Jan 10, 2010
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    It has always been my understanding that current is the number of electrons flowing over a period of time (coulombs per second) through a conductor. Voltage was merely the presence of electromotive force (charge) between two points. The only way I can imagine a current that has no "voltage" between two points on the conductor is if there is no opposition to the current flow (resistance). A perfect superconductor could have current without a voltage present across two points of the conductor. I don't think such a material exists, but there are some that are close.

    That's my thoughts on the matter, but I could be wrong.
     
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