Another basic Q - trace charge or current in a circuit?
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Posted by Kris Krieger on August 20, 2008, 4:51 pm
Please log in for more thread options Hi, All, another basic/learner's question: When you're looking at a circuit, and trying to analyse how it works, are you supposed to trace current, or charge? It's my understanding that current is traced by starting at the + end, but charge is traced by starting at the - end. I'm asking because I'm trying to figure out why components are placed where they are. IOW, taking something almsot stupidly simple: |---------/\/\/\------| | | | | ___ + |-----| _ |) <---let's say this is an LED ___ |-----| _ | - | | | |---------------------| My question is this: if charge (electrons) is moving from - to + , why is the resistor placed "behind" the load (an LED in the example)? I've been looking in various sources to try to "get" it, but so far, the only thing I can assume (because I haven't seen a good explanation, tho' I've checked several references) is that what " + " actually represents is a *PULL* - IOW, the lack of electrons at the + end acts more like a sort of, well, "electron vacuum", rather than a current in the common sense of the term (because, if you're standing in a river for example, the current *pushes* you, but from all I've read, what's "pushing", in a circuit, is coming from the other end, i.e. the - end, and therefore, the word "current' seems to be totally counterintuitive). Anyway, this idea of "pull", rather than "current flow", is the only thing that makes any sense to me in terms of why, if the electrons are coming from the " - " end, the resistor seems to be "behind" the load in terms of flow control. So, is "pull" (or even "attractive force") what is meant by "current", or have I gotten it totally bassackwards...and if so, what's actually going on...? Many Thanks in Advance! - Kris | |||||||||||||
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Posted by Peter Bennett on August 20, 2008, 9:57 pm
Please log in for more thread options wrote: I think that most people now (certainly scientists and engineers) think in terms of "conventional current" - the flow of mythical positive charges form the positive terminal of the voltage source, through the external circuit, and returning to the negative terminal of the voltage source, although we know that, in most circuits, current is actually a flow of negatively charged electrons. Long ago, when electricity was invented, subatomic particles such as electrons were unknown, so the early scientists arbitrarily declared that current was a flow of positive charge, and based all their calculations on that belief. During the vacuum tube era, technicians (who were thought not to be as smart as scientists and engineers) were taught using negative (electron) current, as it is difficult to explain the operation of vacuum tubes (particularly cathode ray tubes!) using conventional current. It doesn't really matter whether you use conventional (positive) current or electron (negative) current when analyzing a circuit, as long as you don't change concepts in midstream. It is probably best to use conventional (positive) current, as that is what most textbooks will use these days. >
>I'm asking because I'm trying to figure out why components are placed where >they are. IOW, taking something almsot stupidly simple: > > > > > |---------/\/\/\------| > | | > | | >___ + |-----| > _ |) <---let's say this is an LED >___ |-----| > _ | > - | > | | > |---------------------| > > >My question is this: >if charge (electrons) is moving from - to + , why is the resistor placed >"behind" the load (an LED in the example)? Remember Kirchoff's current law - since there is only one path through this circuit, the current will be the same at all points in the circuit, regardless of the order of the components in the circuit. -- Peter Bennett, VE7CEI peterbb4 (at) interchange.ubc.ca GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver.powersquadron.ca | |||||||||||||
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Posted by Kris Krieger on August 21, 2008, 2:11 pm
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> wrote:
> >>Hi, All, another basic/learner's question:
>> >>When you're looking at a circuit, and trying to analyse how it works, >>are you supposed to trace current, or charge? It's my understanding >>that current is traced by starting at the + end, but charge is traced by >>starting at the - end. >
> I think that most people now (certainly scientists and engineers) > think in terms of "conventional current" - the flow of mythical > positive charges form the positive terminal of the voltage source, > through the external circuit, and returning to the negative terminal > of the voltage source, although we know that, in most circuits, > current is actually a flow of negatively charged electrons. > > Long ago, when electricity was invented, subatomic particles such as > electrons were unknown, so the early scientists arbitrarily declared > that current was a flow of positive charge, and based all their > calculations on that belief. > > During the vacuum tube era, technicians (who were thought not to be as > smart as scientists and engineers) were taught using negative > (electron) current, as it is difficult to explain the operation of > vacuum tubes (particularly cathode ray tubes!) using conventional > current. > > It doesn't really matter whether you use conventional (positive) > current or electron (negative) current when analyzing a circuit, as > long as you don't change concepts in midstream. It is probably best > to use conventional (positive) current, as that is what most textbooks > will use these days. What happened is that, more I thought about it, the more I got confused by more common experiences of current (wind and water), which is the pressure created by the flow of a substance (wind, water, oil) past a sensor (or person ;) ), becasue the flow of electrons moves from the negative terminal, whereas thre is no such thing as a flow of "positve particles" - which then confused me more because that made it seem like resistors, etc., were "in behind" the components they're supposed to regulate. So the only thing that makes sense to me is the idea, explained by a few kind folks in addition to yourself, of "attractive force". That makes sense to me in terms of, say, the resistor in my simple example controlling, *not* the electron flow, but rather, the strength of the attractive force from the + terminal. I know it's a picayune/"split hair" detail ;) , but I sometimes can't accept/"get"/comprehend "the usual explanations" that most people just accept, because "the usual" just doesn't make sense to me. So, I got totally confused by people simply saying "well that's how the current flows", becasue I could never understand how something can flow when it ("positive particles") doesn't exist, and "positve stuff" doesn't exist because positrons are locked into atomic nuclei, and therefore can't "flow" anywhere unless the atoms themselves are smashed apart in an acceletrator. So the common everyday simplistic explanation got me completely confused. In university, when the instructor just gave a heavy annoyed sigh and said "Just accept it!", all that happened was that (typical of my response to simplistic non-explanations), I rejected the subject and lost any and all interest in the subject - until recently that is ;) ! So, teh idea of attractive force, as opposed to flwo of some non-existent "positive stuff", and threfore the idea of regulating attractive force, I can understand. In a way, it's like air, if you have two connected chambers, with one bign at sea-level pressure and the otehr being at vacuum - if there is a valve between the two, the flow of air will be "attracted" by the vacuum because there is a density gradient of air molecules, and, since teh one chamber is at normal pressure/temperature, it obviously is not "pushing", whcih means that the vacuum is "pulling" - and the air flwo will be regulated not only by the valve, but also, by the degree of vacuum, because a strong vacuum relative to the normal chamber means there is a higher/stronger difference in densities, i.e. a higher density gradient. Anyhoo, that concept helps me understand that the resistor is placed where it is becasue it controls, not the electrons, and not some supposed "flow" of something ("positive thingies") that doesn't exist, but rather, it the amount of attractive forceor (created by a density gradient of charge i.e., electron density) that one is allowing to be exerted by the low-density area upon the high-density area. Dunno whether that makes sense to anyone else, but it's the only way I finally "got" what teh resistor is actually doing... At any rate, I definitely appreciate everyone's input on this :) ! >>I'm asking because I'm trying to figure out why components are placed
>>where they are. IOW, taking something almsot stupidly simple: >> >> >> >> >> |---------/\/\/\------| >> | | >> | | >>___ + |-----| >> _ |) <---let's say this is an LED >>___ |-----| >> _ | >> - | >> | | >> |---------------------| >> >> >>My question is this: >>if charge (electrons) is moving from - to + , why is the resistor placed >>"behind" the load (an LED in the example)? >
> Remember Kirchoff's current law - since there is only one path through > this circuit, the current will be the same at all points in the > circuit, regardless of the order of the components in the circuit. > Good point, thanks! - Kris | |||||||||||||
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Posted by John Popelish on August 21, 2008, 2:42 pm
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Kris Krieger wrote: > What happened is that, more I thought about it, the more I got confused by
> more common experiences of current (wind and water), which is the pressure > created by the flow of a substance (wind, water, oil) past a sensor (or > person ;) ), becasue the flow of electrons moves from the negative > terminal, whereas thre is no such thing as a flow of "positve particles" - > which then confused me more because that made it seem like resistors, etc., > were "in behind" the components they're supposed to regulate. But flow of water or air is not normally thought of as pressure (at least, not in science and engineering) but as mass or volume per unit if time passing a given observation point. Think of gallons per minute going over a dam. Amperes of current are coulombs per second of charge passing through a point in a circuit. That charge can be pushed by repulsion from one direction and pulled by attraction from the other direction. The effects Are additive and interchangeable. All the stuff of the circuit is full of charges that move essentially as a nearly incompressible fluid, in most situations. Static charge is an exception, where the surface charge acts like a compressible fluid. > So the only thing that makes sense to me is the idea, explained by a few
> kind folks in addition to yourself, of "attractive force". That makes > sense to me in terms of, say, the resistor in my simple example > controlling, *not* the electron flow, but rather, the strength of the > attractive force from the + terminal. > > I know it's a picayune/"split hair" detail ;) , but I sometimes can't > accept/"get"/comprehend "the usual explanations" that most people just > accept, because "the usual" just doesn't make sense to me. So, I got > totally confused by people simply saying "well that's how the current > flows", becasue I could never understand how something can flow when it > ("positive particles") doesn't exist, and "positve stuff" doesn't exist > because positrons are locked into atomic nuclei, and therefore can't > "flow" anywhere unless the atoms themselves are smashed apart in an > acceletrator. So the common everyday simplistic explanation got me > completely confused. In university, when the instructor just gave a heavy > annoyed sigh and said "Just accept it!", all that happened was that > (typical of my response to simplistic non-explanations), I rejected the > subject and lost any and all interest in the subject - until recently that > is ;) ! > > So, teh idea of attractive force, as opposed to flwo of some non-existent > "positive stuff", and threfore the idea of regulating attractive force, I > can understand. In a way, it's like air, if you have two connected > chambers, with one bign at sea-level pressure and the otehr being at vacuum > - if there is a valve between the two, the flow of air will be "attracted" > by the vacuum because there is a density gradient of air molecules, and, > since teh one chamber is at normal pressure/temperature, it obviously is > not "pushing", whcih means that the vacuum is "pulling" - and the air flwo > will be regulated not only by the valve, but also, by the degree of vacuum, > because a strong vacuum relative to the normal chamber means there is a > higher/stronger difference in densities, i.e. a higher density gradient. A better way to think of voltage as pressure is to imagine a balance of forces on the movable charges in conductors. When you apply voltage across a conductor, you upset the balance of forces and the charges move in reaction to that imbalance. In the air pressure analogy, there is no suck. Vacuum does not pull on air molecules at a distance, like gravity does. It just pushes on them less than the higher pressure at the other end of the system. The pressure forces are unbalanced on opposite sides of any particular air molecule, regardless of whether any forces are negative or if all are positive but different in magnitude. The effect is the same. > Anyhoo, that concept helps me understand that the resistor is placed where
> it is becasue it controls, not the electrons, and not some supposed "flow" > of something ("positive thingies") that doesn't exist, but rather, it the > amount of attractive forceor (created by a density gradient of charge i.e., > electron density) that one is allowing to be exerted by the low-density > area upon the high-density area. There is a basic law of circuits (Kirchoff's law) that sys that the total voltage drop around any loop has to be zero. The battery pumps up the voltage difference across its terminals, and the rest of the circuit has to develop reverse voltage that uses that pressure up. Flow through a resistor requires (produces) voltage drop. The resistor in that LED circuit uses up all the extra voltage the LED did not need (drop), while doing so at some particular desired current magnitude. In effect, its resistance sets the magnitude of the current by having waste a particular amount of voltage and having a particular resistance. If the resistor did not fol.ow ohms law (was not a linear resistance) its voltage drop would not be proportional to its current, so it would be harder (more complicated math than simple proportionality) to predict its effect in the circuit. An LED is an example of a device that is not ohmic (not a linear resistance), since its voltage drop is not proportional to its current, but close to proportional to the logarithm of its current. -- Regards, John Popelish | |||||||||||||
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Posted by Kris Krieger on August 21, 2008, 7:01 pm
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I'm top-posting just to say Thanks, John, for the clarifications :) This is all helping a lot, and I appreciate that you took the time to explain the concepts - the formulas are having a lot more meaning now, and the circuits I've been looking at are making more sense. Thanks again :) - Kris > Kris Krieger wrote:
> >> What happened is that, more I thought about it, the more I got confused
>> by more common experiences of current (wind and water), which is the >> pressure created by the flow of a substance (wind, water, oil) past a >> sensor (or person ;) ), becasue the flow of electrons moves from the >> negative terminal, whereas thre is no such thing as a flow of "positve >> particles" - which then confused me more because that made it seem like >> resistors, etc., were "in behind" the components they're supposed to >> regulate. >
> But flow of water or air is not normally thought of as > pressure (at least, not in science and engineering) but as > mass or volume per unit if time passing a given observation > point. Think of gallons per minute going over a dam. > Amperes of current are coulombs per second of charge passing > through a point in a circuit. That charge can be pushed by > repulsion from one direction and pulled by attraction from > the other direction. The effects Are additive and > interchangeable. All the stuff of the circuit is full of > charges that move essentially as a nearly incompressible > fluid, in most situations. Static charge is an exception, > where the surface charge acts like a compressible fluid. > >> So the only thing that makes sense to me is the idea, explained by a
>> few kind folks in addition to yourself, of "attractive force". That >> makes sense to me in terms of, say, the resistor in my simple example >> controlling, *not* the electron flow, but rather, the strength of the >> attractive force from the + terminal. >> >> I know it's a picayune/"split hair" detail ;) , but I sometimes can't >> accept/"get"/comprehend "the usual explanations" that most people just >> accept, because "the usual" just doesn't make sense to me. So, I got >> totally confused by people simply saying "well that's how the current >> flows", becasue I could never understand how something can flow when it >> ("positive particles") doesn't exist, and "positve stuff" doesn't exist >> because positrons are locked into atomic nuclei, and therefore can't >> "flow" anywhere unless the atoms themselves are smashed apart in an >> acceletrator. So the common everyday simplistic explanation got me >> completely confused. In university, when the instructor just gave a >> heavy annoyed sigh and said "Just accept it!", all that happened was >> that (typical of my response to simplistic non-explanations), I >> rejected the subject and lost any and all interest in the subject - >> until recently that is ;) ! >> >> So, teh idea of attractive force, as opposed to flwo of some >> non-existent "positive stuff", and threfore the idea of regulating >> attractive force, I can understand. In a way, it's like air, if you >> have two connected chambers, with one bign at sea-level pressure and >> the otehr being at vacuum - if there is a valve between the two, the >> flow of air will be "attracted" by the vacuum because there is a >> density gradient of air molecules, and, since teh one chamber is at >> normal pressure/temperature, it obviously is not "pushing", whcih means >> that the vacuum is "pulling" - and the air flwo will be regulated not >> only by the valve, but also, by the degree of vacuum, because a strong >> vacuum relative to the normal chamber means there is a higher/stronger >> difference in densities, i.e. a higher density gradient. >
> A better way to think of voltage as pressure is to imagine a > balance of forces on the movable charges in conductors. > When you apply voltage across a conductor, you upset the > balance of forces and the charges move in reaction to that > imbalance. > > In the air pressure analogy, there is no suck. Vacuum does > not pull on air molecules at a distance, like gravity does. > It just pushes on them less than the higher pressure at > the other end of the system. The pressure forces are > unbalanced on opposite sides of any particular air molecule, > regardless of whether any forces are negative or if all are > positive but different in magnitude. The effect is the same. > >> Anyhoo, that concept helps me understand that the resistor is placed
>> where it is becasue it controls, not the electrons, and not some >> supposed "flow" of something ("positive thingies") that doesn't exist, >> but rather, it the amount of attractive forceor (created by a density >> gradient of charge i.e., electron density) that one is allowing to be >> exerted by the low-density area upon the high-density area. >
> There is a basic law of circuits (Kirchoff's law) that sys > that the total voltage drop around any loop has to be zero. > The battery pumps up the voltage difference across its > terminals, and the rest of the circuit has to develop > reverse voltage that uses that pressure up. Flow through a > resistor requires (produces) voltage drop. The resistor in > that LED circuit uses up all the extra voltage the LED did > not need (drop), while doing so at some particular desired > current magnitude. In effect, its resistance sets the > magnitude of the current by having waste a particular amount > of voltage and having a particular resistance. If the > resistor did not fol.ow ohms law (was not a linear > resistance) its voltage drop would not be proportional to > its current, so it would be harder (more complicated math > than simple proportionality) to predict its effect in the > circuit. > > An LED is an example of a device that is not ohmic (not a > linear resistance), since its voltage drop is not > proportional to its current, but close to proportional to > the logarithm of its current. > | |||||||||||||
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>
>When you're looking at a circuit, and trying to analyse how it works, are you
>supposed to trace current, or charge? It's my understanding that current is
>traced by starting at the + end, but charge is traced by starting at the -
>end.