Ground and Other References: Video Lecture Q

Discussion in 'Electronics Resources' started by foolios, Nov 9, 2009.

  1. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Why is it that a ground can exist on that circuit on the left?
    Why doesn't the current travel to ground instead of going through to the rest of the circuit.
    In a house, I would think this would be a direct short. Is there a difference with ground when it comes to DC vs AC?

    I can kinda see why it would work on the circuit on the right because current has no other way to complete the circuit. It has to travel through the earth or grounded frame of metal to get to positive.


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    Last edited: Jan 23, 2010
  2. ELECTRONERD

    Senior Member

    May 26, 2009
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    When you see the ground nodes on the way bottom of each wire that means those wires connect to ground. So on the circuit to the right they would both connect together.
     
  3. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    For the circuit on the right:
    Seems right. I can imagine that current will find a path along that ground, whether it's two seperate points on a conductive material or the same point tied together.

    Now on the left on the other hand, it would appear that we have two connections that could take current. One leading throughout the rest of the circuit. But the other, a ground, is another complete path that should take away current, no? Grounds are non-resistive right? Creating a wonderful path for current to take, usually a path that it wants to take when there's a problem. No problem needed in the circuit on the left, we just gave the current the least resistive path to take, one that's not through the resistors. And it's not connected back again to any other part of the circuit like the one on the right is.

    Why is this so?
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Think of it as a circuit that lights up an LED in a car. Ground is the vehicle's metal chassis, and the circuit is still valid, whether the "return" wire uses the chassis to get back to the negative terminal, or connects directly to the negative terminal, which connects to the chassis (hence the ground symbol).
     
  5. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Where you write returning back to the negative.
    Is this strictly an AC circuit where the current is reversing? Is that what you mean?

    If this is a DC circuit or if it can be both, I am very confused. I thought that the current travels from negative to positive for DC.

    Is it safe to say that ground doesn't draw current like a positive terminal? Can we add grounds to any circuit?
     
  6. wr8y

    Active Member

    Sep 16, 2008
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    Wrong.

    No.

    Wrong.

    I underlined your answer:

    The circuit on the left has a tie to ground, but it does not matter! NO current flows to ground on the left because there is no complete path thru ground and back to the other side of the battery.

    Grounds are not magical. Grounds are not "non-resistive" (to use your term). Current only flows if you can find a path from the source, thru a circuit, and back to the other side of the source. PERIOD. (Ignoring the world of RF, of course.)
     
  7. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Ok, that makes a lot of sense. That's the big difference between the two. The left one doesn't have a way for the current to get back on track.

    But then what is the ground for? I think that's where I am really confused. Why are we grounding that circuit that way if it's not going to take current away?

    Now there would be a problem though if those two circuits were grounded onto the same frame tho right?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    As there is an explicit electrical connection between the battery and R3 to complete the circuit, neither representation really matters. The circuit would work perfectly well without being tied to ground.

    The ground symbol in the left circuit gives a critical bit of information, though. It defines the point to which all voltage measurement must be referred to, so anybody can take a voltage reading that will be consistent with any and all reading taken on that circuit. The one in the example is trivial, with only three significant voltage points. In a circuit with dozens of points, a common reference becomes quite important.

    In the right circuit - equivalent to the left - using the two ground symbols only incidentally includes the ground conductor in the current path. It is really an illustration of how one can avoid an explicit connecting line in drawing schematics. The two ground symbols show that those components are connected, without having to draw the line between them. Again, the example is trivial. But in a schematic involving hundreds of components, the elimination of many lines without confusion as to their connections is critical to making the drawing readable.
     
  9. wr8y

    Active Member

    Sep 16, 2008
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    Nope! ;)
     
  10. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    ah ok, because they have the same voltages. It would be like sticking both circuits in series along with their sources. Still run the same then yes? I guess I can just answer my question here by doing the math and seeing that it all adds up just the same.
     
  11. wr8y

    Active Member

    Sep 16, 2008
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    NO.... no no no.

    Voltage does not matter. I have equipment here on my bench that uses power sources of 5 volts, 12 volts and (in one case) 2500 volts. All three supplies are referenced to the same ground.

    Again, follow the path from the source, thru the circuit, then back to the ohter side of the source. IF the other circuits only share a ground, then no "complete" circuit is formed between them - hence no interaction.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Why is it that teachers must make every simple thing complicated?

    A ground has one defining characteristic that works for all purposes, circuit theory, RF, protections and safety etc etc.

    A ground is a point or object that does not change potential no matter how much current flows into or out of it.
     
  13. wr8y

    Active Member

    Sep 16, 2008
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    You answer you own question in your sigline! :)
     
  14. studiot

    AAC Fanatic!

    Nov 9, 2007
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    But I'm not, and never have been, any sort of teacher.
     
  15. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Is this simply because that circuit's current flow is finding the shortest path?

    Ok, lets say that the circuit on the left doesn't have a source, it's just a complete circuit without a source; and that ground is connected to the same ground that the circuit on the right is connected to.
    Now if that non-sourced circuit has less resistance, won't the powered circuit try to charge the un-powered?
    Now if all that is so, what is it about the sourced versions of these circuits that prevent this from happening?
     
  16. wr8y

    Active Member

    Sep 16, 2008
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    Yes.

    Stop thinking about "charging" things. Think of electricity as ONLY current flow. If you have a direct path from one terminal of the source (battery, etc.) to the other terminal of the source, current will flow.
     
  17. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I think that you are confusing the ground in the circuits with "Earth Ground".

    In a schematic like you posted the ground sign really means "0" volts.

    Some people when they draw a schematic use "ground" symbol (three horizontal lines making a upside down triangle shape), WHEN it should be a up side down triangle outline.

    The ground symbol(three horizontal lines in a triangle up side down triangle shape) is really "EARTH GROUND"

    Cary
     
  18. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    For sure, earth ground(ac) and ground(dc) had me confused. I was mistakenly thinking that dc circuits were using ground as a safety mechanism like it is used for in ac house wiring.

    But that's not the case with this circuit. It's not saying that there is a ground that can carry away power from the circuit. I am still not exactly sure what the ground is saying in that one circuit on the left since there is already a path, but I can surely see that the circuit on the right is using the ground as it's path. I am guessing from all the reading I have been doing through these posts to understand that particular ground's function is to simply supply a point for testing purposes. A point in the circuit that is at zero. Simply to be used as a way to find out what potential differences exist in the circuit. Anything else?

    I can understand how a car battery uses the ground attached to the car's frame to give a rear lamp a return path to the battery without the extra wiring.

    I can understand how house wiring uses the ground to divert power away from devices so that people won't get harmed. It provides a path of least resistance for current to travel away from a metal frame of an item that would zap you when touching it. But the ground helps divert these kinds of hazards.

    I think that last part is what made it so difficult to understand how ground is used in dc.

    Am I starting to get anywhere in this understanding or do I need to have a cattle prod stuck in me?

    Thanks for being patient. I want to learn and understand.

    Thanks again for all your efforts.

    EDIT: Also forgot to mention about learning that the neutral in house wiring is also referred to as the ground, kinda like the return in the dc respect someone mentioned earlier.
    The use of ground for both earth ground and return in regards to house wiring can be a little confusing too.
    sheesh!
     
  19. foolios

    Thread Starter Active Member

    Feb 4, 2009
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  20. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Just to repeat my message.

     
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