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Static Electricity vs Current in a Closed Circuit

  1. Mar 15, 2014 #1
    Hi,

    I'm an automotive technician. I have trouble understanding a couple basic electrical concepts.

    The problem is that I am more or less taught that current flows through CIRCUITs.

    When analyzing electrical problems, I think current will only flow if there is a voltage (potential difference) and a conductor between A & B.

    But we normally in my work have circular circuits, so we have to have a path to a load, and from a load, and the current must flow in a circle. Everything that flows out returns back, in theory.

    I cannot power a load from the + and - posts of two batteries without connecting the other two posts to make a closed circuit, in theory. Or can I?

    When I get out of the car and touch it, sometimes I get a small shock. In this case, didn't electricity flow in a straight path without a circle?

    Isn't that *static* electricity, where items can be at different potential voltages, and equalize when linked by a suitable conductor?

    In my mind I am confused slightly, and do not know whether current will flow from points of different voltage without some kind of circular path... It is important as there are Many many Many different voltage levels at different points on an automotible. It is like a gradient of electric potential, not a strict set. And it gets much more complicated because of resistive losses across the frame, and transformers and inductive spikes.

    The other thing that confuses me quite a bit is when induction creates a voltage in a conductor, but that voltage is not sufficient to overcome the resistive path to create a discharge.

    The classic example is an open circuited ignition coil or other transformer.

    What happens to the induced energy?

    In my mind it is like if I took an oar and swept it through a fish tank. The waves that result may not be high enough to clear the walls of the tank, so instead they slosh back and forth...

    Maybe they damage the walls structurally, or break them completely? (Dielectric breakdown?).

    Help!

    Please help. These concepts are well beyond even advanced automotive technical education.

    But they are important to understand in my work on some occasions.

    Thanks
     
  2. jcsd
  3. Mar 15, 2014 #2

    Baluncore

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    On the subject of automotive wiring circuits, they are circular, or should that be circuitous. You must complete the circuit, often through the chassis, in order for current to flow.

    Static electricity is a buildup of charge with no escape path. The potentials involved are measured in kilovolts. When a path is provided, or the insulation breaks down, the excess charge bleeds away.

    An inductor produces a voltage spike when the primary current is stopped because the magnetic field, (that had been built up), collapses and must dump it's energy somewhere. If no spark plug is connected there will be a breakdown through or over the insulation.
     
  4. Mar 15, 2014 #3

    meBigGuy

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    The car is a capacitor to ground that has a static charge. You are a capacitor to ground (or a resistor). When you touch the car the circle is through you, to ground, and back to the car.

    A collapsing field in an inductor with no obvious discharge path will dissipate its energy through radiation, capacitive coupling, and/or insulation breakdown. It will find a path.
     
  5. Mar 15, 2014 #4
    meBigGuy:

    the car's tires insulate it and prevent it from discharging to the ground on its own right? The way they rub across the asphalt may be the source of the accumulated electric charge perhaps?

    If the accumulated charge of the vehicle reaches the threshold necessary to discharge across the tires, it will increase no further as it will discharge, is that correct?

    When I touch the car, even with my shoes on, I present a less resistive path that may be overcome by the voltage of the static electricity stored on the car.

    If the current does flow to ground in the literal sense, and then back to the car, how come it does not discharge itself to the ground without my help to begin with?

    I know there are some commercial vehicles such as those that transport explosive gases that either have special tires or have a dragging chain that actively grounds the vehicle to prevent static discharges.

    To begin with, why do we refer to the low potential in electrical theory as "Ground" anyway?

    As for collapsing fields and inductors, I understand insulation breakdown, and somewhat understand capacitive coupling, but do not understand radiation energy at all. I know nothing of antennas and radio.

    In my example of a fish tank, if the walls were high enough and strong enough, the water would just slosh back and forth until it settled, and the energy would dissipate that way.

    Is there no corollary for that in electrical theory?

    Considering it a bit differently, a collapsing magnetic field induces a voltage, but whether any current flows, or rather how much flows, isn't that dependent on the effective resistance as per Ohm's law?

    A heavy duty dielectric will be subject to the induction of a time or spatially variant magnetic field but will not have current flow, like a conductor would, would it?

    Scratching my head, I dunno.

    Thanks very much for responding!
     
  6. Mar 15, 2014 #5
    Hi Baluncore:

    >On the subject of automotive wiring circuits, they are circular, or should that be circuitous. You must complete the circuit, often through the chassis, in order for current to flow.

    That's what I'm inclined to think. But I have been shocked by ignition systems before and I was not touching the vehicle nor the battery, IIRC, just the cement floor with my shoes on.

    Is that possible / does my memory deceive me, or was my body a non circuitous path of current flow?

    >Static electricity is a buildup of charge with no escape path. The potentials involved are measured in kilovolts. When a path is provided, or the insulation breaks down, the excess charge bleeds away.

    Sounds like I might want to be careful about how I hook my voltmeter up. By the SOUND of it a static electric discharge through the meter could fry it. That may go for the oscilloscope I just received, even if I have the 20:1 attenuators on it that I intend to buy shortly.

    While I don't intend to expose either tool to ignition voltage, it may happen by accident due to the dielectric breakdown you mentioned, of vehicle components. Some kind of failure of ignition coils does certainly burn their controlling modules up, the exact mechanism of that failure is beyond me at this time.

    Static electric shocks are a potential hazard as well, perhaps, to my equipment. I'm reminded of the symbols on modules in schematics, warning of electrostatic discharge damage hazards.

    >An inductor produces a voltage spike when the primary current is stopped because the magnetic field, (that had been built up), collapses and must dump it's energy somewhere. If no spark plug is connected there will be a breakdown through or over the insulation.

    I understand. Repeated breakdown events may create a semi-conductive path over that insulation eventually? I am not sure how to test a coil's high voltage insulation except for an output test. Other problems like high resistance will cause low output.

    I believe most ignition coils today are "potted" in a high temperature resistant epoxy. Whereas the old ones were oil filled "autotransformers" 3 taps, instead of four, not isolated primary and secondary windings.

    I have a box at home with three or four samples of ignition system components representing four decades or so of technology. Mostly gathered from the junkyard, a few items are known bad parts from work from late model vehicles.

    Actually, come to think of it, there is one other way that energy from a field collapse is absorbed. It creates an inductive "kick" that rings back to the thing that drove it.

    I forget whose law it was (lenz? faraday?) where they said that a change in current or a magnetic field over time produces a change in voltage that opposes it.

    I *THINK* this inductive voltage spike on the primary side is used in some applications to detect the correct firing of the ignition system.

    Which is VERY important for emissions controls.

    Thank you for your reply.
     
  7. Mar 15, 2014 #6

    Baluncore

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    The current flowing when the switch is opened must continue to flow. The voltage will adjust itself according to ohms law, based on the resistance of the breakdown paths. As the magnetic energy is dissipated in the breakdown path resistance, the current gradually reduces until it reaches zero.
    The voltage, V = inductance * di/dt, so the current changes at a rate of di/dt = Voltage / Inductance.
     
  8. Mar 15, 2014 #7

    Baluncore

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    thender.
    Your questions are too mixed and involved to answer quickly. Ask fewer questions in a post. Then ask for more details if you need to. Restrict a topic to one issue.

    You can quote extracts of other posts by using BBcode. Use the word quote in square brackets at the start and the word /quote in square brackets at the end to put the text in a quote box.

    See; http://en.wikipedia.org/wiki/BBCode
     
  9. Mar 15, 2014 #8
    I see. Thank you.

    I am still a little confused though. While the switch is closed and the coil is saturated via current through the primary winding, the secondary winding is still an open circuit. Does current flow through the secondary winding leading up to the point where the switch opens the primary circuit?

    Logically, a magnetic field is moving across the secondary winding, so current should be induced. But, on the other hand, the voltage is small at first, and the resistance is extremely high.

    Can you see why these concepts have challenged me? Maybe after using my scope that I received as a gift soon, it will be easier to understand.



    I found the source that made that claim about ignition system monitoring via primary voltage:

    http://www.underhoodservice.com/Article/89493/coil_on_plug_ignition_the_wired_differences.aspx

    "A zener diode placed in parallel to the PCM internal switching electronics is rated at a specific zener voltage.

    See Figure 1.

    When the transistor interrupts current flow, the resulting inductive voltage crosses the zener to provide a confirmation pulse of coil firing to the microprocessor. An absence of kick, say from an open or an unplugged coil, results in a P035x coil control circuit code. "

    That's what I was talking about.
     
  10. Mar 15, 2014 #9

    meBigGuy

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    Regarding static:

    The car is a capacitor to ground. It holds a static charge. Otherwise, as you said, no charge could accumulate. When your body completes the circuit, current flows. Your body could be seen as capacitor or resistor or both.

    BTW, it is more likely YOU are the source of the static charge, not the car, but the same principle applies.

    Regarding Inductor:
    Ignore radiation and capacitive effects for now. When the primary charges, the increasing field induces a voltage on the secondary, but if the secondary is an open circuit, no current flows. The voltage on the secondary is relatively small and is related to the turns ratio. When the primary opens, the field will collapse and must find a path to dissipate the energy.

    Regarding the zener, it conducts a small current (limited by the resistor) when the coil field collapses and the voltage across it increases. If the coil is not connected at all, is open, or the transistor has failed, the zener will never fire.
     
  11. Mar 15, 2014 #10
    Ahhh, a very insightful observation, I hadn't considered that I may be the source of the discharge.

    Correct me if I am wrong, but I had thought with static electricity like the kind that shocks me once in awhile, the voltage was the result of ionization. That my body can act as a capacitor because has a very limited capacity for storage and release of electrons.

    So I thought that with this type of voltage, a circular path is not necessarily needed, and the two things (my body, and the car) start at different voltage potentials with regard to something else (the ground, for example), and their potentials are equalized when they touch.

    In other words, I thought electric charge flows from one to the other, I didn't consider it as a closed circuit that involves the ground.


    I notice one other thing I neglected previously. The open in the primary circuit is merely a point of "high resistance" in another way of thinking about it. If the magnetic field collapse is not relieved on the secondary side, it may well collapse across the "switch" on the primary side...

    I have to say it's a little dizzying to picture, but I imagine when the primary side closes electrical energy in the form of current flow creates a magnetic field that is strengthened by the laminar iron core.

    Now we have energy in the form of a magnetic field supported by primary current flow. When primary current flow is interrupted, the magnetic energy will collapse backward against the direction of primary current flow...

    At the same time, it will collapse over the secondary windings, inducing voltage into the secondary circuit, which, if a spark plug gap for instance is small enough, will cause current flow.

    To tell the truth, a voltage on the secondary side is probably created well before the point of the gap being ionized, during the collapse of the magnetic field. I imagine it intensifies over the course of that event.

    If I remember right, sometimes avalanche diodes are used to prevent the plug from firing until a desired voltage is achieved. This way, the timing of the firing event can be controlled more precisely.

    I have heard of "capacitive discharge" ignition systems, and know just a tiny bit about earlier designs that used "condensers" (capacitors), but these are antiquated technologies I do not know much about.

    Thank you for your help in understanding the matter.
     
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