Another basic Q - trace charge or current in a circuit?

[Kris Krieger]

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

 

[Kris Krieger]

John Popelish <jpopelish@rica.net> wrote in
news:A8udnTL5JqfQFDHVnZ2dnUVZ_t_inZ2d@comcast.com:

Kris Krieger 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'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)?

The electrons are not like bullets fired through a pipe, but
more like a train track loop with so many cars on it that
they connect all the way around. The resistor drags the
whole train, regardless of where it is in the loop.

Oh! OK. That makes a lot more sense (yeah, embarassingly enough, I'd had
more of a "bullet" picture in my mind).

I've been looking in various sources to try to "get" it, [snip]
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.

The battery is like n electron pump that sucks them in at
one end and pushes them out the other. The wire and
components are made of electrons. When current flows in the
loop, all the mobile electrons (which are vibrating all
around in every direction) drift around the loop in unison,
at least as long as things change slowly enough that you
don't have to worry about the speed of light letting some
part of the loop get ahead of other parts.

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...?

Current is the net movement of charge past some arbitrary
reference point in the loop. individual charged particles,
like electrons or protons are attracted by oppositely
charged particles and repelled from similarly charged
particles.

So the trick with the battery is to set it up in a way similar to how
Diodes are described - one end has an electron surplus, the other has an
electron deficit, and (if I understand this correctly) the way the Universe
works, is that electrons have a much greater tendency to move towards an
electron deficit, than to move away from a material to create such a
deficit.

Normal matter (with no surface charge (static)
or current passing through it, contains a balance of
positive and negatively charged particles, over distances
larger than molecules. Non uniform balance over distances
smaller than molecules is what holds molecules together by
electrostatic attraction.

Ok, that makes sense to me, because I understood molecular polarity in
Chemistry. I guess I got confused by the terminology; I kept thinking of
current in the sense of the common useage of water current or air current,
which the average person experiences or interprets as a pressure against
oneself (or some object) - but, with electronic current, it seems like it's
the direction in which the attraction propagates, so to speak (i.e. the
lack of electrons that pulls, towards itself, the surfeit of electrons from
the negative end).


Thank you for the explanation! - I've printed it out and put it into my
notebook.


- Kris

 

[Kris Krieger]

Peter Bennett <peterbb@somewhere.invalid> wrote in
news:c1ipa4ds589d9ml6bkvbal96no8eieai8b@news.supernews.com:

On Wed, 20 Aug 2008 15:51:17 -0500, Kris Krieger <me@dowmuff.in
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

 

[stan]

Kris Krieger wrote:

Peter Bennett <peterbb@somewhere.invalid> wrote in
news:c1ipa4ds589d9ml6bkvbal96no8eieai8b@news.supernews.com:

On Wed, 20 Aug 2008 15:51:17 -0500, Kris Krieger <me@dowmuff.in
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.

It's not so much mythical. For example it's very possible to have
positive ions in a gas or liquid and they are certainly mobile and
actually physically move. In a solid though it's very correct that
positive charges or charge carriers basically don't physically move. In
those cases the books typically refer to "effective" charge flow. The
idea is that when an electron leaves an atom it leaves behind a "hole"
and since the atom now lacks an electron is has a net positive charge so
the hole is considered to represent a positive charge carrier. When
another atom loses an electron it may come fill the original hole and
you can consider the effect as if the electron physicall moved from one
atom to the next and the "hole" acts as if it moves in the opposite
direction at the same time. So some consider that the hole has caried a
positive charge from one atom to another.

The concept isn't really hard but it seems like a very convoluted way to
think about things. Thats a very real observation. I think I should
also say that what I just wrote was for the benifit of the op.

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.

Actually when you consider things from an energy standpoint the
conventional current starts to shine. When you consider that the
positive charge ends represents a higher potential energy than the
negative end, the conventional current has charges moving from a higher
energy to a lower energy which is more satisfying than imagining a
charge climbing up hill for no reason.

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.

I can still remember my first brush with semiconductors and conventional
current and thinking what moron came up with that? The idea spawned a
very interesting response or spoof. Try googling for phrase "Bell Labs"
dark sucker. Turns out some clever person cam up with the theory that
light bulds didn't actually emit light or photons. Instead they actually
suck the dark out of the air. Conventional current seems like Alice in
Wonderland logic to nearly everyone who is exposed to it but you
eventually get over it.

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.

You could think of it as an excess of electrons or negative charges
collected together at the negative terminal, and a scarcity of electrons
at the positive terminal. The electrons aren't very social and they
don't want to stay together, what with like charges and all. When you
give them an opportunity or path to get from the negative terminal to
the positive they really, really want to spread out and so they will
travel from the negative to the positive. The trick is that in order to
move towards the positive terminal they have to move a hole or positive
as they progress. So the electrons move along the path from negative and
they will "effectively" move the holes from the positive to the
negative.

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.

Now quite correct. Positive ions are positively charged and are very
common. Actually the positive particles in the nucleus are protons and
not positrons and you don't need a freestanding protron to have a
moveable positive charge. Say you have an atom with two protrons and two
electrons. It will have a net neutral charge. Let that same atom loose
an electron and you have a moveable electron or negative charge and the
remaining atome has two positive charges and one negative charge so it's
not balanced and has a net positive charge. In a solid the atom is fixed
in place and can't move even when it loses an electron so your thinking
is correct in solids. But consider a rain cloud for example. The are
many moveable atoms and molecules in a cloud and they easily lose
electrons creating moveable positive charges. Sometimes you can see the
consequences of this net charge as lightning.

In a liquid based battery like the one in your car a chemical reaction
causes molecules to lose electrons to seperate from molecules and the
positive charges move through the liquid while the electrons move in the
other direction.

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.

Actually if you look at it from an energy standpoint on the crowded side
the molecules have higher energy and the vacuum side has lower energy.
When you open the valve they will flow from higher energy to lower
energy untill the energy differential doesn't exist. So basically the
energy is pushing them into the vacuum. It's kinda like falling downhill
form higher energy to lower energy rather than being pulled down hill by
the lack of altitude.

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.

Actually the location of the resister doesn't matter. As long as the
resister is in the path of the current it has the same effect. The idea
is that resisters convert energy into heat so any charge traveling
through a resister will lose some of its energy to heat. In fact it will
lose exactly the same amount of energy no matter where it gets placed.

If that idea doesn't work then try this. Consider the path from the
negative terminal to the positive terminal. Lets say the wire, the
resister, and the led all look like uphill climbs to the electron. If
the climb is high then not many electrons will make the journey and if
you lower the climb then many will make it. Now also consider that each
electron has some energy currency as it sets out on the climb. Lets say
the resister each require a toll in eneergy to be paid. The resister
uses the eneergy it gets to make heat while the led uses the toll to
make light. The lower the hill the more electrons make the journey and
the more tolls paid so the more heat and light you get. When you make
the climb higher you get less electrons making the journey and paying
tolls so you get less heat and light.

It's the total climb that determines how many make it and how many tolls
will be collected and it really doesnt matter if you climb the resister
or the led first. In this climb it's strictly single file so theres no
way to go part way up the climb and stop. Every electron that starts the
journey will finish the journey.

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

 

[Kris Krieger]

John Popelish <jpopelish@rica.net> wrote in
newseudnQyveu--KzDVnZ2dnUVZ_hadnZ2d@comcast.com:

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.

 

[Kris Krieger]

stan <smoore@exis.net> wrote in news:2a81o5-bds.ln1@invalid.net:

I'm topposting becasue I don;t have a reply per se, but wanted to asy
Thanks! for the additional information - I'm saving all of these great
answers and explanations


If I seem "overly enthused" sometimes, well, I guess I am, but it's only
because I like to express appreciation to people when they're helpful ((and
there are certainly quite a lot of people who jump at the chance to express
crankiness, hostility, demanding judgementalism, and/or other negative
things - and hey, I enjoy bucking that trend <g!> ))

- Kris

Kris Krieger wrote:
Peter Bennett <peterbb@somewhere.invalid> wrote in
news:c1ipa4ds589d9ml6bkvbal96no8eieai8b@news.supernews.com:

On Wed, 20 Aug 2008 15:51:17 -0500, Kris Krieger <me@dowmuff.in
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.

It's not so much mythical. For example it's very possible to have
positive ions in a gas or liquid and they are certainly mobile and
actually physically move. In a solid though it's very correct that
positive charges or charge carriers basically don't physically move. In
those cases the books typically refer to "effective" charge flow. The
idea is that when an electron leaves an atom it leaves behind a "hole"
and since the atom now lacks an electron is has a net positive charge so
the hole is considered to represent a positive charge carrier. When
another atom loses an electron it may come fill the original hole and
you can consider the effect as if the electron physicall moved from one
atom to the next and the "hole" acts as if it moves in the opposite
direction at the same time. So some consider that the hole has caried a
positive charge from one atom to another.

The concept isn't really hard but it seems like a very convoluted way to
think about things. Thats a very real observation. I think I should
also say that what I just wrote was for the benifit of the op.

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.

Actually when you consider things from an energy standpoint the
conventional current starts to shine. When you consider that the
positive charge ends represents a higher potential energy than the
negative end, the conventional current has charges moving from a higher
energy to a lower energy which is more satisfying than imagining a
charge climbing up hill for no reason.

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.

I can still remember my first brush with semiconductors and conventional
current and thinking what moron came up with that? The idea spawned a
very interesting response or spoof. Try googling for phrase "Bell Labs"
dark sucker. Turns out some clever person cam up with the theory that
light bulds didn't actually emit light or photons. Instead they actually
suck the dark out of the air. Conventional current seems like Alice in
Wonderland logic to nearly everyone who is exposed to it but you
eventually get over it.


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.

You could think of it as an excess of electrons or negative charges
collected together at the negative terminal, and a scarcity of electrons
at the positive terminal. The electrons aren't very social and they
don't want to stay together, what with like charges and all. When you
give them an opportunity or path to get from the negative terminal to
the positive they really, really want to spread out and so they will
travel from the negative to the positive. The trick is that in order to
move towards the positive terminal they have to move a hole or positive
as they progress. So the electrons move along the path from negative and
they will "effectively" move the holes from the positive to the
negative.

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.

Now quite correct. Positive ions are positively charged and are very
common. Actually the positive particles in the nucleus are protons and
not positrons and you don't need a freestanding protron to have a
moveable positive charge. Say you have an atom with two protrons and two
electrons. It will have a net neutral charge. Let that same atom loose
an electron and you have a moveable electron or negative charge and the
remaining atome has two positive charges and one negative charge so it's
not balanced and has a net positive charge. In a solid the atom is fixed
in place and can't move even when it loses an electron so your thinking
is correct in solids. But consider a rain cloud for example. The are
many moveable atoms and molecules in a cloud and they easily lose
electrons creating moveable positive charges. Sometimes you can see the
consequences of this net charge as lightning.

In a liquid based battery like the one in your car a chemical reaction
causes molecules to lose electrons to seperate from molecules and the
positive charges move through the liquid while the electrons move in the
other direction.

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.

Actually if you look at it from an energy standpoint on the crowded side
the molecules have higher energy and the vacuum side has lower energy.
When you open the valve they will flow from higher energy to lower
energy untill the energy differential doesn't exist. So basically the
energy is pushing them into the vacuum. It's kinda like falling downhill
form higher energy to lower energy rather than being pulled down hill by
the lack of altitude.

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.

Actually the location of the resister doesn't matter. As long as the
resister is in the path of the current it has the same effect. The idea
is that resisters convert energy into heat so any charge traveling
through a resister will lose some of its energy to heat. In fact it will
lose exactly the same amount of energy no matter where it gets placed.

If that idea doesn't work then try this. Consider the path from the
negative terminal to the positive terminal. Lets say the wire, the
resister, and the led all look like uphill climbs to the electron. If
the climb is high then not many electrons will make the journey and if
you lower the climb then many will make it. Now also consider that each
electron has some energy currency as it sets out on the climb. Lets say
the resister each require a toll in eneergy to be paid. The resister
uses the eneergy it gets to make heat while the led uses the toll to
make light. The lower the hill the more electrons make the journey and
the more tolls paid so the more heat and light you get. When you make
the climb higher you get less electrons making the journey and paying
tolls so you get less heat and light.

It's the total climb that determines how many make it and how many tolls
will be collected and it really doesnt matter if you climb the resister
or the led first. In this climb it's strictly single file so theres no
way to go part way up the climb and stop. Every electron that starts the
journey will finish the journey.

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

 

[stan]

Jasen Betts wrote:

On 2008-08-21, stan <smoore@exis.net> wrote:

It's not so much mythical. For example it's very possible to have
positive ions in a gas or liquid and they are certainly mobile and
actually physically move. In a solid though it's very correct that
positive charges or charge carriers basically don't physically move.

in P-type semiconductos there's a quantum entitiy called a 'hole'.
and they carry positive charge. the easiest way to demonstrate this
(and prove it's not electrons carrying the charge) is to construct a
and test hall-efect cell made from p-type semiconductor.

direction at the same time. So some consider that the hole has caried a
positive charge from one atom to another.

inside semiconductors holes are as real as electrons.

I'm aware. I was trying to move the newbies understanding along
incrementally. I think we both agree the water pressure model will only
get you so far and you encounter problems. The old effective hole flow
model will get you along pretty far before you need quantum concepts and
is arguably simpler. I could be wrong but I think the OP was basically a
learning on his own and not and engineering or physics student. When
presented a question about current through a resister and an led it
seems inappropriate to start with an Einstein-Bose layer discussion.
Do you disagree?

 

[Kris Krieger]

stan <smoore@exis.net> wrote in news:4nr5o5-rq2.ln1@invalid.net:

Jasen Betts wrote:
On 2008-08-21, stan <smoore@exis.net> wrote:

It's not so much mythical. For example it's very possible to have
positive ions in a gas or liquid and they are certainly mobile and
actually physically move. In a solid though it's very correct that
positive charges or charge carriers basically don't physically move.

in P-type semiconductos there's a quantum entitiy called a 'hole'.
and they carry positive charge. the easiest way to demonstrate this
(and prove it's not electrons carrying the charge) is to construct a
and test hall-efect cell made from p-type semiconductor.

direction at the same time. So some consider that the hole has caried a
positive charge from one atom to another.

inside semiconductors holes are as real as electrons.

I'm aware. I was trying to move the newbies understanding along
incrementally. I think we both agree the water pressure model will only
get you so far and you encounter problems. The old effective hole flow
model will get you along pretty far before you need quantum concepts and
is arguably simpler. I could be wrong but I think the OP was basically a
learning on his own and not and engineering or physics student.

As the OP, I can say, yup, you're exactly right. I did take 2 semesters of
physics in University, but that was back in 1978, plus we just skimmed over
the most basic concepts of electronics - and even at that, little was realy
explained; it was a matter of "here are a few formulas, figure out these
problems, can't do it?, too bad, time to move on to the next chapter".

I started off wanting to make some decently-bright solar light units (with
four to maybe six white LEDs) that I could put into my stained-glass pieces
- but I then started looking at more complex ideas (battery charging IC and
so on), and then, got curious as to what is actually going on in circuitry,
well, at least relatively simple circuitry.

There is no EE or even electronics hobbyist to whom I'll ever pose a job
threat <LOL!>, but seriously, I just got tired of teh typical consumer
bschtick wherein all electronic/electrical devices are pretty much seen as
"magical black boxes". That just doesn't satisfy me any more

When
presented a question about current through a resister and an led it
seems inappropriate to start with an Einstein-Bose layer discussion.
Do you disagree?

It's all grist for the proverbial mill, but, like a water-powered mill,
sometimes my brain grinds a bit slowly <g!>, which is what happend with the
question about current - which I do understand better now, thanks to people
generously taking time to offer explanations. In my book, any answer that
doesn't say I'm just stupid, and instead makes teh effort of trying to
explain concepts, is appreciated <G!>

- Kris

 

[stan]

Kris Krieger wrote:

stan <smoore@exis.net> wrote in news:4nr5o5-rq2.ln1@invalid.net:

Jasen Betts wrote:
On 2008-08-21, stan <smoore@exis.net> wrote:

It's not so much mythical. For example it's very possible to have
positive ions in a gas or liquid and they are certainly mobile and
actually physically move. In a solid though it's very correct that
positive charges or charge carriers basically don't physically move.

in P-type semiconductos there's a quantum entitiy called a 'hole'.
and they carry positive charge. the easiest way to demonstrate this
(and prove it's not electrons carrying the charge) is to construct a
and test hall-efect cell made from p-type semiconductor.

direction at the same time. So some consider that the hole has caried a
positive charge from one atom to another.

inside semiconductors holes are as real as electrons.

I'm aware. I was trying to move the newbies understanding along
incrementally. I think we both agree the water pressure model will only
get you so far and you encounter problems. The old effective hole flow
model will get you along pretty far before you need quantum concepts and
is arguably simpler. I could be wrong but I think the OP was basically a
learning on his own and not and engineering or physics student.

As the OP, I can say, yup, you're exactly right. I did take 2 semesters of
physics in University, but that was back in 1978, plus we just skimmed over
the most basic concepts of electronics - and even at that, little was realy
explained; it was a matter of "here are a few formulas, figure out these
problems, can't do it?, too bad, time to move on to the next chapter".

I started off wanting to make some decently-bright solar light units (with
four to maybe six white LEDs) that I could put into my stained-glass pieces
- but I then started looking at more complex ideas (battery charging IC and
so on), and then, got curious as to what is actually going on in circuitry,
well, at least relatively simple circuitry.

There is no EE or even electronics hobbyist to whom I'll ever pose a job
threat <LOL!>, but seriously, I just got tired of teh typical consumer
bschtick wherein all electronic/electrical devices are pretty much seen as
"magical black boxes". That just doesn't satisfy me any more


When
presented a question about current through a resister and an led it
seems inappropriate to start with an Einstein-Bose layer discussion.
Do you disagree?


It's all grist for the proverbial mill, but, like a water-powered mill,
sometimes my brain grinds a bit slowly <g!>, which is what happend with the
question about current - which I do understand better now, thanks to people
generously taking time to offer explanations. In my book, any answer that
doesn't say I'm just stupid, and instead makes teh effort of trying to
explain concepts, is appreciated <G!

Sounds like your professor at least sopke english. At one point I thouht
I might just have to move to asia and drift around to pick up some
languages before I could get through school. I'm not poking fun at
people with accents here, I admire people who speak multiple languages.
In this case there were two people from his native India and they
reported hes was very hard to understand in his native tounge and in
class he was somewhere in between and wasn't really speaking either
language. He was incredibly smart and a really good guy. To top it off
his handwriting was unintelligable to even his teaching assistant. It
was quite a semester

He did bring up a good point that hasn't been mentioned yet. If you're
going to try and get your head around this stuff I would stick to the
simpler passive stuff like resisters, capacitors, and inductors till I
wasn't feeling lost, then tackle semiconductors. Before anyone jumps on
me I'm not saying that electromagnetic fields are simple, I'm saying you
can develop a comfortable grip on capacitors and inductors and do quite
a bit of circuit analysis without really needing to solve a wave
equation or even knowing who Maxwell was. I'm also saying that figuring
out a pn junction can be a bit more challenging than a battery and a
resistor.

 

[John Popelish]

Kris Krieger 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'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)?

The electrons are not like bullets fired through a pipe, but
more like a train track loop with so many cars on it that
they connect all the way around. The resistor drags the
whole train, regardless of where it is in the loop.

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.

The battery is like n electron pump that sucks them in at
one end and pushes them out the other. The wire and
components are made of electrons. When current flows in the
loop, all the mobile electrons (which are vibrating all
around in every direction) drift around the loop in unison,
at least as long as things change slowly enough that you
don't have to worry about the speed of light letting some
part of the loop get ahead of other parts.

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...?

Current is the net movement of charge past some arbitrary
reference point in the loop. individual charged particles,
like electrons or protons are attracted by oppositely
charged particles and repelled from similarly charged
particles. Normal matter (with no surface charge (static)
or current passing through it, contains a balance of
positive and negatively charged particles, over distances
larger than molecules. Non uniform balance over distances
smaller than molecules is what holds molecules together by
electrostatic attraction.

--
Regards,

John Popelish

 

[Peter Bennett]

On Wed, 20 Aug 2008 15:51:17 -0500, Kris Krieger <me@dowmuff.in>
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.

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

 

[John Popelish]

Kris Krieger wrote:

John Popelish <jpopelish@rica.net> wrote in
newseudnQyveu--KzDVnZ2dnUVZ_hadnZ2d@comcast.com:

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

My pleasure. This sort of thing is what this group for.

--
Regards,

John Popelish

 

[Jasen Betts]

On 2008-08-21, stan <smoore@exis.net> wrote:

It's not so much mythical. For example it's very possible to have
positive ions in a gas or liquid and they are certainly mobile and
actually physically move. In a solid though it's very correct that
positive charges or charge carriers basically don't physically move.

in P-type semiconductos there's a quantum entitiy called a 'hole'.
and they carry positive charge. the easiest way to demonstrate this
(and prove it's not electrons carrying the charge) is to construct a
and test hall-efect cell made from p-type semiconductor.

direction at the same time. So some consider that the hole has caried a
positive charge from one atom to another.

inside semiconductors holes are as real as electrons.

Bye.
Jasen

 

[John Popelish]

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