A few elementary questions

Hello everyone, I don't know if this is the best place to ask, but I just had a few questions about elementary wiring / circuits topics

Firstly, when looking at a circuit diagram with a voltage sourse in it, which terminal of the voltage source / battery is the flow of electricity coming from? I was told that the positive terminal is the terminal with a higher potential and is thus the origin of the electricity, but I was also told that electrons flow from the negative terminal. Which is correct? The flow of electricity is essentially the flow of electrons, right? I've heard of "conventional current" which is the flow of the holes left by the electrons as they move in the opposite direction (er, I think.). Does this somehow factor into this topic?

Secondly, What purpose does a ground/earth connection serve in an electrical circuit? I've heard its a safety device. If so, how does it function as a safety device, and if not, what is the significance of it?

Thanks a lot everyone!!

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Hi, MDL. You're in the right newsgroup to ask these questions -- actually, they're two of the most popular questions, and have been asked and answered many times.

Since you're a Google Grouper (join the throng!), look at the group homepage. In the "search this blog" box, type in:

"conventional current"

with the quotes for answers to the first question, and

earth +ground

for answers to the second.

Good luck Chris

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The answer depends on whether you are doing electronics or physics. Electricity and electronics define current as the net movement of charge, without worrying about what is the actual sign of the charge that is moving. It assumes positive change is moving, so passive current is a movement from the more positive to the less positive node. Inside the source, charge is assumed to be actively pumped back from the less positive end to the more positive end, by some energy source. If the current is actually made up of electrons (and in metals, this is the case), they go the other way.

Ground has at least 3 purposes.

The node called ground is a common reference point for many measured potentials, regardless of whether it is actually connected to the Earth, or not. Surrounding a circuit with a metal box at the common potential keeps external noise out and internal noise in. The Earth can also be a signal or power return path between separate pieces of equipment. The old telegraph signals were sometimes sent along one wire by this method, which required a good ground connection at each end.

An earth ground connection drains away static charge build-up.

If the circuit includes dangerously high voltage that is sourced from a supply that has one end r the other connected to Earth, somewhere else, surrounding that circuit with a grounded conductive container prevents you from touching any dangerous voltage in the circuit. where you would complete a circuit through the Earth. back to the source.

Reply to
John Popelish

The flow of electrons travels from the negative terminal, through the circuit, to the positive terminal.

The "flow" of "holes" (places in the atoms where electrons would be, if there were there, but they're not) goes the other way.

"voltage" is a made-up concept we use to measure the force behind these flows; by convention, electrons flow towards higher (more positive) voltages, and holes flow towards lower (more negative) voltages. Diode symbols, for example, point in the direction of hole flow.

Earth ground serves as a dump for excess current which enters the circuit through unexpected sources, such as lightning or wiring shorts. It's a safety device for you if/when it has lower resistance than your body (at least, restance between it and other things you might be touching, like the floor), so it's important to get it right. For ESD protection, it can act as a safety device for your circuit by giving spikes somewhere to go other than through your circuit.

Reply to
DJ Delorie

By convention, charge flows from positive to negative. This is "conventional current", the opposite from electron flow. Electrons have negative charge. That's the way it's done, so you'll just have to get used to it.

Suppose you touch, say, a washing machine, and internally a wire inside has become shorted to the frame. If the frame is grounded, the current flows through the ground wire away from you, and a fuse probably pops somewhere. Without the ground wire, the current path would be through *you*, and by the time a fuse blew you'd be long dead.


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John Larkin

Another analogy you can use: the positive side is like a vacuum that sucks up electrons, the electron movement is called current. Look at the arrow on diodes and transistors as pointing toward the vacuum source. By thinking of the positive as a vacuum, it leads to the term Depletion meaning less of or an absence of. So depletion in electronics means an absence of electrons. Anyways Grab hold of the idea that electricity flows from the negative to the positive and see the arrows as the side that sucks the electrons in. The arrow side of a diode will be where the electrons are going as in a voltage rectifier. Think of holes as an absence of electrons and as such is positive. And the arrow points to where the electrons are coming from. Hope that helps some. JTT

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James Thompson

Thank you for the responses everyone, I think I've gotten it. I would just like to clarify a couple of things:

  1. Is this current a flow of electrons being suctioned up from the earth through "you", or the opposite, the electrons are sucked out of the device into the earth? Since electricity flows from areas of high potential to lower potential, and is always trying to move to the Earth (I think), would I be correct to say that the (conventional) current flows from the device to the Earth? (when actually electrons are moving from the Earth to the device?)
  2. Also, is the ground wire in this example connected to the frame of the device, or rather the circuit inside of the device?
  3. Also, when an electrical device has a standard outlet plug with two terminals and a ground, where does that ground wire ultimately connect to? Does it connect to the circuit contained in the device, or rather some portion of the frame of the device? A good example I suppose, would be a typical power bar.
  4. Finally, again concerning the washing machine example in particular, in the case that there is no ground wire (or even where there is), how can the electricity flow through "you" between Earth and the device if there is no return path?
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The neutral and safety ground are bonded at the main service disconnect, near the meter and grounding system for a building. If they are tied together anywhere else, it defeats the concept of a safety ground.

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Michael A. Terrell

Doesn't make any difference, not in the slightest. I'd have to take some issue with the statement that "electricity flows from areas of high potential to lower potential, and is always trying to move to the Earth." The latter part of that is incorrect, or at the very least irrelevant - "electricity" will always flow (given a conducting path) between any two points of differing electrical potential. If one of those points happens to be the Earth, fine, but there's nothing magical about the Earth that somehow "draws electricity" other than the fact that it's a very large (and therefore convenient) reference point. And which point is of "higher" potential really is only relevant if you ARE concerned with which way the current (or charge carriers) are moving. You often are concerned about that, but not in this case.

In the case of most U.S. home situations, the ground wire at the breaker box end connects to an actual "grounded" point - a conductor which is buried in the earth. This is the same point to which the neutral (white) wire ultimately connects to as well. At the appliance end, the ground (green) wire most typical conducts to the conductive frame and/or case parts of the device. If there is no such overall frame/case connection, there may not be a dedicated "safety" ground, or else it may connect to a dedicated "safety ground" area of the PC boards, etc., within the device. U.L. (and others) have some very strict requirements about where and how these connections are made when a safety ground wire is provided/required.

The Earth IS the return path, by virtue of the neutral connection going to the ground point at the breaker box, per the above.

Bob M.

Reply to
Bob Myers

Appliances are generally powered by Alternating Current, so the electrons flow in both directions alternately.

Also, it should be understood that it doesn't matter what the charge carriers are, or if they are positive or negative. Current flow whether "conventional", that is assumed to be positive charges moving from

  • to -, or if they are electrons moving from - to +, makes no difference to the analysis of circuit action.

The ground wire for appliances is always connected to the frame of the device.

Ultimately the ground wire will connect back to the entry panel (fuse or circuit-breaker panel) and hence to a grounding rod driven into the Earth.

There would need to be a complete circuit. But such can be provided by you standing in a puddle of water leaking from the (dangerously deteriorated!) washing machine and contacting another grounded object, the concrete floor, or by you happening to also touch a water tap or another grounded appliance.

Reply to
Greg Neill

Same wires are common to different grounds. This post will demonstrate many different ground that share common connections. Each ground may have different center reference points.

Center point of safety ground is found in the mains breaker box. All 'grounded' appliances connect to this bus bar. For example, that washing machine frame connects (via third prong and green ground wire) hardwired to that bus bar. Earthing electrode (ground rod) and cold water pipes connect to that safety ground bus bar. All neutral (AC white wires) also connect to that bus bar.

Only common connection between safety ground wires and neutral wire is that bus bar. That bus bar is the center of safety ground - and why anything connected (bonded) to ground can cause a circuit breaker to trip - for human safety. Everything that connects directly to that bus bar will stay voltage limited (typically less than 2 volts) which means human is not harmed.

Meanwhile, to interconnect electronics, electronic appliance voltages should not 'float' excessively. Best way to interconnect electronics - especially those using common mode signaling such as printers and RS-232 devices - is for all electronic components to share a common ground. In this case, we are using the same safety ground wires for another ground purpose. But this time, electronics 'common' ground is (for example) a three prong power strip. Since all appliances share a single point ground (power strip) - and wire distance here is important

- then ground loops do not exist. Therefore hum or noise that would interfere with audio or data signals would (should) not exist.

A same green safety ground wire is also routed to become a 'common' ground between electronic boxes. In this case, center of that ground system is a power strip.

Meanwhile, inside electronics appliance may be two separated ground systems on same printed circuit board. Analog ground and digital ground may remain separated on the pc board except at one point where both meet. Both grounds remain separated to reduce noise problems. Generally (but not always), that pc board ground then connects to safety (green wire) ground. Just another ground that centers, this time, at a point where analog and digital grounds meet. A ground that may or may not also connect to building safety ground.

Utility is responsible for another ground that should be inspected:

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Owners of jacuzzis and swimming pools who suffered electrical shocks may be familiar with this ground. Another and different ground that is also connected to that breaker box 'bus bar' safety ground.

Different ground that share comm> Thank you for the responses everyone, I think I've gotten it. I would

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In metals, electrons flow. In other types of conductors (such as human bodies) there are no electrons flowing, instead there are positive particles and negative particles both flowing, but in opposite directions. The positive particles are mostly sodium ions, and the negatives are mostly chloride.

So, how would we describe the direction of an electric current in a human body (or in salt water, or in battery acid?) In that case there is no single direction for flowing charges. If we choose either direction is the "real" direction, half the carriers will still be flowing the "wrong" way.

Or in other words, we cannot determine the direction of current by only looking at the physical motion of flowing charges.

On the other hand, if positive charges flow forward, this gives us a positive current, while if negative charges flow backward, this also gives us a positive current.

So the answer is simple: positive current adds the two flows together, then ignores the actual motion of the charges. It adds the forward positive charge motion to the backwards negative charge motion.

And finally, note that ammeters cannot detect the motion of the moving carriers. An ammeter only measured the magnetism surrounding the conductors, so it has no way to tell whether the current is made of backward-moving electrons or forward-moving protons (or made of two populations of opposite ions which flow in opposite directions.)

So what is the *real* direction of electric current? Simple: it's the direction that your ammeter measures. And if you really want, you can reverse the leads of your ammeter, then measure all currents as negative numbers. But obviously the reversed terminals and the negative numbers cancel out (they get multiplied together, negative times negative equals positive.)

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty

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Research Engineer UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@chem.washington.edu Box 351700, Seattle, WA 98195-1700 ph:206-543-6195 fax:206-685-8665

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As John Poplish mentions, an earth ground was used in telegraphy as a common ground but supposedly not for very long since the ground conditions varied by location. Some telegraphers allegedly had to pour water on their grounding rods to make them work. And there were other problems associated with using an earth ground as a common circuit ground.

An earth ground is often used as a sort of land fill for disposal of unwanted, unused, or unexpected energy. Sometimes it's used as a circuit's common ground, but that's not common (sorry, resistance is futile).

An earth ground apparently has many other uses, shielding, reflecting, ...

What is "flowing through" wires is mainly energy, not electrons or positive ions. Sometimes in A.C., potentials might be attributed a linear momentum through wires.

Also "flow through" has questionable meaning for resistors and capacitors since what didn't "flow through" them may have been converted to heat, potentials, ...

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Note that dirt is an electrolytic conductor, so during an electric current withing the Earth, no electrons flow. Instead, atoms with too many protons flow one way, while atoms with too many electrons flow the other way. You can *try* to create electron currents in the Earth, but the electrons will only survive for a few nanoseconds before being grabbed by atoms and turning into negative ions. And then, the presence of all those negative ions will attract the positive ions already in the dirt, causing a flow of positives. (And of course the excess negatives start repelling each other, so they flow the other way as they spread out.)

Confusing? Situations like the above are the reason why we use "conventional charges" which flow as a "conventional current." We assume that during any currents, all the flowing charges have the same sign. Centuries ago we declared the conventional charges to be positive. (During WWII a large group of technicians tried to change this to negative conventional charges, but they just succeeded in confusing thousands of students, as well as creating a population of technicians who hate those of the "opposite religion": they hate the community of scientists and engineers.)

If you employ conventional charges (the positive ones,) then your statement is right. But if you're a scientist and the *actual* polarity of flowing charges is important, then you might better say "a difference in potential causes a flow of charges." That way you don't have to discuss how many positives flow one way and how many negatives the other.

Electrons are NOT trying to move from the Earth to the positively-charged device. The ground wire sucks in negative ions while simultaneously spitting out positive ions. Electrons do flow up the wire towards the device, and at the surface of the ground-rod there are chemical reactions. The chemical reactions are the spot where positive ions flow out into the dirt, negative ions flow from the dirt to the metal, and electrons flow from the reaction sites and migrate into the metal.

There always has to be a return path. If you're barefoot on damp concrete, you're electrically connected to ground. If you're wearing shoes, but the floor is wet and your shoe soles are wet, then you're also grounded. Better wear some rubber-soled platform shoes, the kind Nikola Tesla wore. (Hmmm, Tesla's lab was like the one in the Frankenstein movies, even having a retractible roof and giant lightning towers. So why did Frankenstein's Monster end up wearing the thick-sole rubber shoes? It shouldhave been Dr. Frankenstein who was clomping around in those things!)

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http//staff.washington.edu/wbeaty/

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