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Author Topic: basic electrica theory question. . .  (Read 2073 times)
KB8UAQ
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Posts: 24




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« on: September 30, 2010, 10:20:04 PM »

Hi, I just want to double check my understanding of one of the basic bits of physics about how electrical circuits work, and particularly the concept of 'ground'.

So, first, I believe, when you've got a source of electrical potential, that potential is created by creating an imbalance of electrons in the system - that is, you strip electrons from some atoms (in the positive terminal), and force those electrons to other atoms - this can be done with magnetic fields and mechanical work on those magnetic fields, as in a generator/alternator, which moves electrons down the length of the conductor inside the magnetic field, so that those electrons accumulate on one end of the conductor, and you have a bit of a deficit on the other end, or it can be done with chemical processes, as in the manufacture of a battery, or in the case of a rechargeable battery, applying a reveresed electrical potential to the terminals, to pull electrons off the positive terminal and deposit them in the negative terminal?

Am I about right so far?

Then, when you close an electrical circuit (that is, turn it on by completing the 'open' circuit, usually by closing a switch which connects two otherwise unconnected segments in the circuit, forming a complete loop), the electrons then flow from the negative terminal, to the positive terminal?

Now, as far as the concept of 'grounding' goes, let me see if my understanding is correct - the positive terminal doesn't really care where the electrons come from, as long as it can get them from somewhere, and that's where the concept of 'ground' comes in - the Earth has an enormous supply of 'loosely' bound electrons which will easily flow up into the circuit and to the positive terminal, completing the circuit?

People often discuss ground as if it were a 'dump' for electricity in a pinch, but since, I think, electrons flow from ground to the positive terminal, in reality, instead of the other way 'round, it's really the pos. terminal which is the 'dump', and the ground is just an infinite supply? But, for some reason, do people find it easier to think of it the other way around, or something? Or am I confused and do the electrons actually flow to ground from the negative terminal, with the earth acting as a large positive terminal?
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KJ6EAD
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« Reply #1 on: October 01, 2010, 01:46:13 AM »

I'm confused. What was the question? Ground is just a reference voltage. Since it's relative, not absolute, it can be more positive or more negative than some other node in a circuit.

Take a look at the note for question 4 in this workbook: http://www.allaboutcircuits.com/worksheets/volt_m.html

This video covers some theoretical and practical ground concepts. http://www.allaboutcircuits.com/videos/23.html

If you're coming from a physics background, the following discussion of the conventional and electron flow conventions may help:
http://www.allaboutcircuits.com/vol_1/chpt_1/7.html

Some additional discussion of the concept of ground especially from a safety perspective is here: http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

One of the fun things to do in a lab full of first year electronics students is to watch their eyes bug out when you connect the red terminal to the black terminal on two 20 volt adjustable supplies to use them as a split supply for an op amp circuit. The positive terminal from one supply and the negative terminal from the other are linked together to form the ground reference for both.
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AA4PB
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« Reply #2 on: October 01, 2010, 06:23:59 AM »

Ground is just a reference. If you connect a flashlight bulb between an 8-foot ground rod and the + terminal of a battery does current flow in the bulb? No because there is no complete circuit. If you now connect the negative terminal of the battery to another nearby ground rod does current flow in the bulb? Yes because you now have a complete circuit from the - battery terminal to a ground rod, through the soil resistance to the other ground rod, thru the bulb back to the + battery terminal. Depending on the ground resistance the current may or may not be enough to actually light the bulb.

Here's why I think people get confused about grounding to the Earth. People have learned from experience that if they stand bare-foot on the Earth and stick their finger into the hot side of a light socket they get a shock. They don't realize that this is because the power company has attached the reference of their home's electric service (actually the center tap of the pole transformer) to a ground reference on the pole. They therefore incorrectly assume that "ground" has some magical property that causes current to flow into any source.

Any grounding issue, so called "ground loop", etc can be analyzed by looking for the current paths. If you can identify the current paths you can always find a solution for grounding problems. If you've got a high enough frequency and high enough voltage some of the current paths can be through capacitance, but the path is always there somewhere.

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STAYVERTICAL
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« Reply #3 on: October 02, 2010, 02:25:20 AM »

Current is just the movement of electrons from one place to another.
The number of electrons passing a point in a second is a measure of the current flow.
A Coulomb is just a particular number of electrons all hunkered together ready to move.
If one Coulomb of electrons passes a point in one second it indicates a current flow of 1 Amp.

Flow of electrons is from the negative to the positive terminal of a battery, but in the early days of electricity,
the engineers got it wrong, saying it flowed from positive to negative (intuitive but wrong).
So when they got it right, they relabelled all those equations and books to indicate the original (wrong) assumption was
now called "conventional current flow". The new correct direction ( negative to positive ) was called electron flow.

Any battery, generator, van-de-graf machine or whatever else is used, simply produces an EMF (Electro-Motive-Force)
which like the potential energy of holding a rock up, can do work if it is allowed.
The EMF or Voltage is even called a potential difference.
The voltage does not do any work, it simply has the potential to do work if a circuit is completed and current allowed to flow.

The Earth, as denoted by the planet, is not a source of electrons but acts like any other terminal of a voltage generator
to provide a second point for the Voltage to be referenced.
In the case of lightning for example, the charged air is high above the earth but eventually the potential difference (Voltage)
becomes great enough for a spark to bridge the gap between cloud and earth.

As an aside, some people think the space between the earths surface and the ionosphere which is being excited by
thunderstorms all over the world is more like a waveguide at a frequency in the 7 or 8 hertz region.
Nikola Tesla, the inventor of radio, A.C. motors, A.C. power transmission, flourescent lights and even the car speedometer,
tried to transmit power around 1900 by setting up standing waves in this waveguide by using resonance at the Earths natural frequency with a gigantic Tesla coil.
In that case he was able to pluck electrons from the air and earth, but that was a very special circumstance.
But that is digressing, good luck and thanks for the question.

73s.

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KB8UAQ
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« Reply #4 on: October 02, 2010, 08:00:06 PM »

Current is just the movement of electrons from one place to another.
. . .

Any battery, generator, van-de-graf machine or whatever else is used, simply produces an EMF (Electro-Motive-Force)
which like the potential energy of holding a rock up, can do work if it is allowed.

But what is actually *causing* the EMP/Voltage? My understanding is it's the imbalance of electrons? What I mean is, if I understand correctly, any molecule that has more electrons than protons, will be in a situation where the negative charge on all the electrons is repelling the electrons away from each other with greater force than the charge of the protons is able to attract the additional electrons. But, since the electrons have no where to go (when a circuit is opened), they just stick around, until the circuit is closed, then that repulsive force starts moving those extra electrons away, right? Meanwhile, the positive terminal has atoms with 'too few' electrons, so they want to absorb electrons (because of more protons having a stronger attractive force than the number of electrons currently on those atoms exert repelling force), and the negative terminal has atoms with an excess of electrons, the combination of repulsive force on one side, and attractive force on the other, powefully moves the electrons?

Quote
The Earth, as denoted by the planet, is not a source of electrons but acts like any other terminal of a voltage generator

So does that mean the ground (in this case, literal ground), is acting somewhat like a positive terminal? That is, the atoms of the soil can absorb excess excess electrons and bind them to atoms in the soil?

Here's a followup question - if EMF/Voltage is caused by the mutually repelling force of electrons. . . why don't the extra electrons just shoot off into space? Why do they stick around until they can find a path to a receptive atom?
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STAYVERTICAL
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« Reply #5 on: October 02, 2010, 09:04:09 PM »

I think I may be out of my depth here as to the actual physics of electrons.
Perhaps the physicists amongst the ham community can help here, but my basic understanding is that the electrons are generated through some process, for example a chemical reaction in a battery.
If you look at the batteries chemical reaction formula, you will see that electrons are produced as a product of the chemical reaction and accumulate around the negative terminal since that is the where that particular part of the reaction occurs.
The positive terminal may be positive simply relative to the negative terminal.
A (probably poor) metaphor is if I was to pick up a rock which was laying on the ground and suspend it.
The rock has potential energy which will be converted into kinetic energy but only if I release the rock.
When you separate electrons from an atom the atom will have a net positive charge and the loose electrons will be free to travel if the surrounding matrix of atoms is conducive, such as metal.
Another example of charges travelling is a semiconductor material where the movement of electrons in one direction can be equated to the movement of "holes" in the opposite direction. It is all a matter of point of view, as to whether you consider electrons or holes moving.
In semiconductors for instance, P type materials have holes causing the current flow! but this is really just a matter of electrons going the other way.

The concept of positive or negative is not really absolute but relative.
So whether the earth acts like a positive or negative terminal depends on what you compare it against.
Since the Earth (planet) is one pole in space it is not really positive or negative relative to anything we can do, unless we actually
plunge a rod into it and use it with a separated charge somewhere else.
Perhaps, relative to the Sun and its solar wind or something the earth may seem as one polarity or other but that is beyond my expertise to comment apon.

As to why electrons don't fly off into space, in most EMF generators there is a limit to the number of electrons which they produce before equilibrium is reached, for example in a chemical reaction (battery) the reaction stops after a while and will only recommence when the a current is allowed to flow between the positive and negative terminals.
This charge (quantity of electrons) is generally not enough to produce a strong enough electric field to rip off electrons from air molecules, so they stay put.
If the electrons were allowed to accumulate, such as in a van de graf generator, the point would be reached when the potential difference caused by all the electrons would produce such a strong electric field that electrons would be ripped off air molecules and a current would flow, causing a spark.
Lightning works the same way.

I am sorry I could not answer more of the esoteric questions, I am but a humble ham, and you probably need a physicist to properly explain some of your questions. Electrons, for example, don't really exist but are actually probabilistic entities which come into existence when measured! So there is much more to them than meets the eye, particularly in the realm of quantum mechanics.

In any case I hope I provided a glimmer of light, imperfect as it may be.

73s.

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N3OX
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« Reply #6 on: October 03, 2010, 12:50:09 AM »

Quote
That is, the atoms of the soil can absorb excess excess electrons and bind them to atoms in the soil?

<snip>

 why don't the extra electrons just shoot off into space? Why do they stick around until they can find a path to a receptive atom?

The electrons in electrical conductors (even weak ones like soil) aren't really bound to any particular atom.  In conductors, a better picture is a sea (or gas) of electrons bound to a whole bunch of nuclei.  So they're free to move about in the conductor, but the "electron collective" is bound to the big chunk of positive stuff.   If you actually remove electrons from a wire, it will become positively charged and if you bring it into contact with something closer to neutral than it is, it will equalize voltage with that thing and suck up some electrons to replace the ones you took away.

But electrons can "slosh" around in a chunk of metal without any leaving it.  The RF current on a parasitic yagi element is a good example.  No charge enters or leaves the antenna, but currents still flow as the electrons move from end to end.

So you can move electrons around without needing to move them off of one conductor an onto another.  Now, if you do bring something that's charged up with respect to something else, and touch them together, or like STAYVERTICAL says, build up so much charge that there's a spark, you do actually transfer electrons from the more negatively charged thing to the more positively charged thing.

But when you make some types of electric circuit, let's say by using a DC permanent magnet generator and a wire, you're just circulating the electrons that are already in the circuit and the generator windings by pushing on them, just like when you have a closed loop piping system and you turn on a pump.

A battery is sourcing or sinking electrons in chemical reactions, but it is also an object that is, when looked at as a whole, not charged up positive or negative.  It just keeps all of its electrons piled up at the negative end.  I like water analogies, though this one is a little weird.   A battery is kind of like a pair of pressure tanks, one that has a lot of pressure built up and the other which has a vacuum in it.  Cheesy

Quote
So does that mean the ground (in this case, literal ground), is acting somewhat like a positive terminal?

When I first learned about grounds I think someone told me that the earth was like an infinite, neutral reservoir so you could add or subtract charge from it without changing its voltage.  That's sorta-kinda-true in terms of human-scale electrical circuits.  But it's not a very useful concept, because the point of connecting electrical equipment to ground doesn't have much to do with the ground absorbing or emitting electrons easily in large quantities.  If you ask yourself where you would ultimately find some some extra electrons to put into the earth, you'll realize that you pretty much have to take them out of the Earth somewhere else, because everything slowly leaks down to the same potential because of very slow charge transfers.

You wanna know the real reason we connect things to ground for safety?  Its because whatever the potential the ground is is the potential our feet are at, and if something shorts inside the electrical equipment, you want to make sure there's something there to equalize the case of the equipment to ground.

It doesn't matter whether the thing that's malfunctioning has a surplus of electrons or a deficit, or in the case of mains-powered equipment, a time-varying surplus and deficit that sloshes back and forth at 60Hz.  The important thing is that at any instant, the extra electrons will travel from the broken thing to ground OR from the ground to the broken thing through the wire and not through you.  The earth has nothing to do with it, really.   

The only time we can ever get some new electrons from somewhere to put into the earth is when lightning strikes.  Then you really are trying to conduct that charge safely into the earth.

But you wanna know something?  Our planet is about 300,000 to 400,000 volts negative with respect to the ionosphere because thunderstorms keep draining them from the ionosphere and depositing them on earth.  The charge slowly leaks back to the ionosphere.

It's not really at all important whether the Earth is positive or negative with respect to anything that can't leak back to it fast.  What's really important is that parts of you never get more than, I dunno, a hundred volts from thing you're standing on.  We're all already carrying around a surplus of electrons, because we're charged to about the same voltage as the Earth, and the Earth is very negative with respect to the ionosphere.  It doesn't hurt us to be charged up.

What hurts us is suddenly conducting a bunch of charge from one place to the other, and it doesn't really matter which direction it's going.


 
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
NK6Q
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Posts: 202




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« Reply #7 on: October 06, 2010, 11:15:10 PM »

If you can find one (Ebay perhaps), see if you can get a copy TutorText "Introduction to Electronics".  A very fun and unique textbook.  It's a little dated, but the basic material is still valid.  And, yes, in this book electron movement is from negative to positive (that really screwed me up when I took physics in college and they taught electrical flow goes from positive to negative!)

IMO, it's a great primer for electronic theory, resistance, capacitance, inductance, even thermionics.  I still have my copy (gifted to me new when I was thirteen back in '66) on my night table.  A year or so ago, it was my pathfinder in returning to ham radio.  In less than 4 months I went from Tech, to General, to Extra.  I couldn't have done it without my TutorText!
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