Not sure why my name is showing in the latest responses....
Found another and independant (?) presentation (if you can log in) regarding KVL and non-conservative electric fields...
Plus this paper with a similar test circuit for this discussion...
nts. That is not correct.
sical situation.
I'm not going to review the entire video to find the statement, but I'm pre tty sure the professor states this at one point.
its
and
That's as simple as it could be, integral calculus. The EMF is distributed around the loop exactly opposing the resistance which is also distributed around the loop. If the resistance is more lumpy exactly that portion of t he voltage corresponding to ohms law becomes apparent.
This was the nature of the experiment. The professor was claiming there wa s no voltage on any of the wires, so the only voltages measurable in the lo op were on the resistors. He then attached his probe to two midpoints in t he wires and claimed to be measuring the voltage on the two resistors by mo ving the probe to one side or the other. In fact, that *is* what he was do ing, but the assumption that the voltage in the wire was null was not valid . This was proven by the fact that there was voltage in the probe wires wh ich countered the voltages in the loop wires he wanted to cancel out so the measurement was only of the resistor! If this was not the case moving the probe would have had no effect.
ElectroBOOM showed all this very clearly.
Rick C.
Tesla referral code --+
On Saturday, November 24, 2018 at 6:39:49 AM UTC, snipped-for-privacy@gmail.com w rote:
te:
ineering, believes Prof Walter Lewin is wrong on Kirchoff's law not holding in non-conservative circuits:
e leads
y & yet
at
ect.
e Lewin's correct.
oltages at the same two points like Lewin, with minimum probe area -- so it isn't a probing problem. Yet he goes on to say:
t"
ing 1 amp in his set up; he needs to crack open a book on basic EM theory a nd study how shielding works.
it is obvious the professor doesn't understand anything about it. The mea surement is *LITERALLY* of the same two points only moving the probe wires.
e voltage introduced in the probe wires by the magnetic field.
t be any voltage induced.
form a semi circle which constitutes a fair sized loop.
The problem here is we have a difference in opinion in what is meant by the "probe"; I take it as the ends of the measurement probe, you seem to be in cluding parts of the circuit it's attached to. Even so, again this highligh ts the problem of performing measurements of voltages in parallel on the en d of the measurement probe: in conservative circuits you can say the voltag es are the same, but not in non-conservative circuits hence the ambiguity.
Larry Harson
A demonstration of measurable 'voltage' that depends on the circuit elements and not on a probe-wiring/magnetic-fluctuation description is what is required, and that is missing. Usually, in a circuit diagram, a region with significant changing magnetic flux is a black box labeled 'transformer', 'generator', or 'motor', and the circuit characterized by the external terminal voltages.
One can imagine this to be a problem with sloppy wiring, but it is not soluble with neat wiring. A million discrete subcomponents into your calculus, it's hard to see that the real problem of measurement is not locked away, it's just lost in the maze.
rote:
ngineering, believes Prof Walter Lewin is wrong on Kirchoff's law not holdi ng in non-conservative circuits:
ope leads
hly & yet
r at
bject.
nce Lewin's correct.
voltages at the same two points like Lewin, with minimum probe area -- so it isn't a probing problem. Yet he goes on to say:
ght"
rying 1 amp in his set up; he needs to crack open a book on basic EM theory and study how shielding works.
is
nd it is obvious the professor doesn't understand anything about it. The m easurement is *LITERALLY* of the same two points only moving the probe wire s.
the voltage introduced in the probe wires by the magnetic field.
n't be any voltage induced.
e form a semi circle which constitutes a fair sized loop.
he "probe"; I take it as the ends of the measurement probe, you seem to be including parts of the circuit it's attached to. Even so, again this highli ghts the problem of performing measurements of voltages in parallel on the end of the measurement probe: in conservative circuits you can say the volt ages are the same, but not in non-conservative circuits hence the ambiguity .
I want to understand this. Your claim is that two probes (the curved wires that get twisted once together) attached to the same points in the circuit , A and B, will see two different voltages, not because the probes add or s ubtract voltage due to their shapes, but because the actual voltages on A a nd B are different depending on the probe used?
Rick C.
Tesla referral code -+-
pretty sure the professor states this at one point.
long its
law and
uted around the loop
n the loop wires he wanted to cancel out so the measurement was only of the resistor! If this was not the case moving the probe would have had no eff ect.
nts
quired, and that is missing. Usually, in a
ack box
erized by
luble with neat wiring.
t the real problem
I don't follow your objection. ElectroBOOM drew the circuit with a transfo rmer coil. If you prefer, consider the circuit to have four transformers, one for each wire segment, resistor A to measurement point 1, measurement p oint 1 to resistor B, etc. If you want to be more accurate, consider each resistor to have an embedded transformer since they have non-trivial size i n this case.
Rick C.
Tesla referral code -++
Especially if they are wire-wound resistors
A parasitic circuit element is still a circuit element. If you don't model the entire circuit, the results are less than meaningless. Got it?
Therein lies the problem:
If you are going to draw a schematic of a circuit, stick to what the schematic /is/.
A schematic is a graph, in the abstract. A set of nodes connected by edges.
A graph is NOT a function of space, it is a set. There is no x, y or z on a graph. That is why we draw inductors and coupling factors on the schematic, and why we cannot integrate a field over a schematic loop.
The instant you invoke fields and loops on a schematic, you have violated that agreement.
Then it becomes a /geometric diagram/, and you must add dimensions for all wire sizes, paths, connections and so on, so that the measured voltages can be meaningful.
After all, if we draw a path through a uniform magnetic field, and find it generates 2V of EMF when we were expecting 1V, that is just as much in error as when it generates 1V when we expected zero! If the size and shape is not defined, we have nonsense.
Whereas, if we abstract away from the geometry, say by drawing a transformer element, we aren't interested in how we arrived at the EMF, just that it is present, and that what we build is representative of that abstraction.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
es that get twisted once together) attached to the same points in the circu it, A and B, will see two different voltages, not because the probes add or subtract voltage due to their shapes, but because the actual voltages on A and B are different depending on the probe used?
No, what I'm claiming is that the phrase 'the actual voltages' is a trap. In the presence of a changing magnetic field, it doesn't allow a unique voltage representation , there's lots of ways to probe, and they ALL have problems (the physics requires more inform ation than can be supplied as 'actual voltage' data).
Voltage doesn't exist in nature; it is an abstraction we use when dealing w ith electric fields and that abstraction leaves out much interaction in these cases. If you t hink voltage is fundamental, you have to believe it can be determined from physical test ing; it cannot (if only because the ground reference point is arbitrary), and changing mag netic field is yet another flaw in the assumption.
ires that get twisted once together) attached to the same points in the cir cuit, A and B, will see two different voltages, not because the probes add or subtract voltage due to their shapes, but because the actual voltages on A and B are different depending on the probe used?
In the presence of
on, there's lots of
rmation than
with electric fields
think voltage
sting; it cannot
agnetic field
I think this is a physics/ EE thing. In physics class you learn about this(Faraday's law). And it's hit or miss if you are shown it as an EE. Hopefully electroboom will learn, I like him and W. Levin (who has some amazing lecture video's. The man is a master with a piece of chalk, not so good about the sexual stuff.) The transformer 'analogy' is right.
George H.
:
engineering, believes Prof Walter Lewin is wrong on Kirchoff's law not hol ding in non-conservative circuits:
scope leads
ughly & yet
sor at
subject.
since Lewin's correct.
nt voltages at the same two points like Lewin, with minimum probe area -- s o it isn't a probing problem. Yet he goes on to say:
ought"
arrying 1 amp in his set up; he needs to crack open a book on basic EM theo ry and study how shielding works.
his
and it is obvious the professor doesn't understand anything about it. The measurement is *LITERALLY* of the same two points only moving the probe wi res.
r the voltage introduced in the probe wires by the magnetic field.
ldn't be any voltage induced.
ire form a semi circle which constitutes a fair sized loop.
the "probe"; I take it as the ends of the measurement probe, you seem to b e including parts of the circuit it's attached to. Even so, again this high lights the problem of performing measurements of voltages in parallel on th e end of the measurement probe: in conservative circuits you can say the vo ltages are the same, but not in non-conservative circuits hence the ambigui ty.
es that get twisted once together) attached to the same points in the circu it, A and B, will see two different voltages, not because the probes add or subtract voltage due to their shapes, but because the actual voltages on A and B are different depending on the probe used?
Yes, but more generally: my claim is that the voltage between two points de pends upon the path taken where there's a changing magnetic field from Fara day's law. In conservative circuits where there isn't a changing magnetic f ield, saying that the voltage between A and B is V isn't ambiguous because it's independent of the path.
Although the probe wires will pick up induced EMFs, these will be countered by an internal static electric field giving a net electric field close to zero. Think of a conductor place in a static electric field which initially causes the charges to move, eventually creating a static electric that con tributes to a net zero internal electric field and EMF, and a static charge distribution.
Once one understands this, it's easy to see that the scope will give differ ent measurements depending upon the orientation of the probes which reflect s the path dependence of voltages in the video. But those EEs who believe t here is only an induced electric field inside a wire, including that of an inductor or transformer, will come up with an incorrect physical interpreta tion of what's actually happening by adding induced EMFs around the KVL loo p.
Larry Harson
ires that get twisted once together) attached to the same points in the cir cuit, A and B, will see two different voltages, not because the probes add or subtract voltage due to their shapes, but because the actual voltages on A and B are different depending on the probe used?
In the presence of
on, there's lots of
rmation than
with electric fields
think voltage
sting; it cannot
agnetic field
I'm afraid I don't agree that voltage is not fundamental because you have t o pick a zero point. That's like saying elevation is not fundamental for t he same reason.
Saying Kirchoff's law doesn't hold is one thing, saying a voltage measureme nt has no meaning is entirely different. If nothing else, the fact that th e voltage measurement can be explained perfectly is justification to the id ea that the voltage is defined.
Rick C.
Tesla referral code +++
rote:
te:
ic engineering, believes Prof Walter Lewin is wrong on Kirchoff's law not h olding in non-conservative circuits:
s scope leads
roughly & yet
essor at
e subject.
n since Lewin's correct.
rent voltages at the same two points like Lewin, with minimum probe area -- so it isn't a probing problem. Yet he goes on to say:
thought"
carrying 1 amp in his set up; he needs to crack open a book on basic EM th eory and study how shielding works.
in his
up and it is obvious the professor doesn't understand anything about it. T he measurement is *LITERALLY* of the same two points only moving the probe wires.
her the voltage introduced in the probe wires by the magnetic field.
.ouldn't be any voltage induced.
wire form a semi circle which constitutes a fair sized loop.
by the "probe"; I take it as the ends of the measurement probe, you seem to be including parts of the circuit it's attached to. Even so, again this hi ghlights the problem of performing measurements of voltages in parallel on the end of the measurement probe: in conservative circuits you can say the voltages are the same, but not in non-conservative circuits hence the ambig uity.
ires that get twisted once together) attached to the same points in the cir cuit, A and B, will see two different voltages, not because the probes add or subtract voltage due to their shapes, but because the actual voltages on A and B are different depending on the probe used?
depends upon the path taken where there's a changing magnetic field from Fa raday's law. In conservative circuits where there isn't a changing magnetic field, saying that the voltage between A and B is V isn't ambiguous becaus e it's independent of the path.
ed by an internal static electric field giving a net electric field close t o zero. Think of a conductor place in a static electric field which initial ly causes the charges to move, eventually creating a static electric that c ontributes to a net zero internal electric field and EMF, and a static char ge distribution.
erent measurements depending upon the orientation of the probes which refle cts the path dependence of voltages in the video. But those EEs who believe there is only an induced electric field inside a wire, including that of a n inductor or transformer, will come up with an incorrect physical interpre tation of what's actually happening by adding induced EMFs around the KVL l oop.
I believe ElectroBOOM drew a circuit with BOTH probe wires which would show two different voltages at the same time (or maybe he actually built the ci rcuit, I don't recall). So how can the two readings be different if there isn't a voltage imposed over a length of the wires in some way?
Rick C.
Tesla referral code ----
wires that get twisted once together) attached to the same points in the c ircuit, A and B, will see two different voltages...
p. In the presence of
tion...
ng with electric fields
to pick a zero point. That's like saying elevation is not fundamental for the same reason.
The real thing in nature is the electric field. And, in the presence of c harges and matter, the vector electric field at all points in space has three components. It a lso is a sum of pure divergences (which, mathematically, means a scalar field, V, can be the progenitor of a ll the E vector fields, in the formula E =grad(V).
But, we never see V directly, we only see that charged objects have forces on them, and we can explain almost all of those forces with E = grad(V).
This breaks down when we add in induction by changing B field, because that 's a force on a charge that CANNOT be grad(V) for any possible 'V'. It cannot, because this fiel d is NOT a pure divergence, it has curl.
If you start learning with voltmeters, it's not obvious that V is poorly de fined: if you start with third-year calculus, it's all clearer. I'd consider the E field fundament al, and V derived from it.
ment has no meaning is entirely different. If nothing else, the fact that the voltage measurement can be explained perfectly is justification to the idea that the voltage is defined.
Unsatisfying, if that 'explanation' is a black box called 'transformer' wit hout probe-able internals. It just hides the changing B field in a box rather than understanding it. Light is E-field completely created by B-field change; Kirchoff can't handle that, either.
e:
ed wires that get twisted once together) attached to the same points in the circuit, A and B, will see two different voltages...
rap. In the presence of
tation...
ling with electric fields
ve to pick a zero point. That's like saying elevation is not fundamental f or the same reason.
charges and matter, the
also is a sum of pure divergences
all the E vector fields,
s on them, and
at's a force on a charge
eld is NOT a pure
defined: if you start with
ntal, and V derived from it. Hmm well it's sorta an open question if the fields or the potentials are 'fundamental'. Though it's been years since I did any 'real' E&M problems, most times it's easiest to figure out the potentials, and then calculate the fields from them. (Statics are easy.)
George H.
rement has no meaning is entirely different. If nothing else, the fact tha t the voltage measurement can be explained perfectly is justification to th e idea that the voltage is defined.
ithout probe-able internals.
Light is E-field
e:
ed wires that get twisted once together) attached to the same points in the circuit, A and B, will see two different voltages...
rap. In the presence of
tation...
ling with electric fields
ve to pick a zero point. That's like saying elevation is not fundamental f or the same reason.
charges and matter, the
also is a sum of pure divergences
all the E vector fields,
s on them, and
at's a force on a charge
eld is NOT a pure
defined: if you start with
ntal, and V derived from it.
I think I understand now. You are confusing knowing V with knowing E. Not hing you have written above disputes the idea that V is defined.
rement has no meaning is entirely different. If nothing else, the fact tha t the voltage measurement can be explained perfectly is justification to th e idea that the voltage is defined.
ithout probe-able internals.
Light is E-field
Rather than hiding it, it includes it in an otherwise incomplete model.
Rick C.
Tesla referral code ---+
Whit3rd is almost right. The fields are fundamental, but in classical E&M the potentials aren't, at least not at AC. However, the reason isn't that you need to pick a zero point--its more physicsy and mathy than that.
In source-free regions, the curl of the time-dependent E field generates H, and the curl of H generates E. For nontrivial problems, it's usually much easier to work with potentials: the familiar electric potential phi and the magnetic vector potential A.
B = curl A, (1)
E = -grad phi - dA/dt. (2)
Now for today's math fact: for any scalar field F, curl grad F = 0.
Thus since only the curls matter, you can add the gradient of an arbitrary scalar field to A and not change the B field. This operation is called a _gauge_transformation_ and is also important in quantum field theory.
In electrodynamics two common choices are the Lorentz gauge (which simplifies Maxwell's equations a lot) and the Coulomb gauge (which is good for I forget what).
When you apply a gauge transformation, you also need to change phi, because of equation (2)--changing the gauge doesn't change E and B, so phi has to change to compensate. (This isn't hard to do.)
Thus the circuits idea of voltage isn't well defined except in the limit of small size and low frequency, which is what I said early on in the thread.
In quantum mechanics, the Aharonov-Bohm effect shows that the phase of the wave function of a particle with nonzero magnetic moment couples to the magnetic vector potential, _even in field-free regions_, which is pretty cool. You can show this with an atom interferometer.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
rote:
onic engineering, believes Prof Walter Lewin is wrong on Kirchoff's law not holding in non-conservative circuits:
his scope leads
horoughly & yet
ofessor at
the subject.
win since Lewin's correct.
ferent voltages at the same two points like Lewin, with minimum probe area
-- so it isn't a probing problem. Yet he goes on to say:
e thought"
or carrying 1 amp in his set up; he needs to crack open a book on basic EM theory and study how shielding works.
ae in his
etup and it is obvious the professor doesn't understand anything about it. The measurement is *LITERALLY* of the same two points only moving the prob e wires.
ather the voltage introduced in the probe wires by the magnetic field.
nt.
shouldn't be any voltage induced.
be wire form a semi circle which constitutes a fair sized loop.
t by the "probe"; I take it as the ends of the measurement probe, you seem to be including parts of the circuit it's attached to. Even so, again this highlights the problem of performing measurements of voltages in parallel o n the end of the measurement probe: in conservative circuits you can say th e voltages are the same, but not in non-conservative circuits hence the amb iguity.
wires that get twisted once together) attached to the same points in the c ircuit, A and B, will see two different voltages, not because the probes ad d or subtract voltage due to their shapes, but because the actual voltages on A and B are different depending on the probe used?
s depends upon the path taken where there's a changing magnetic field from Faraday's law. In conservative circuits where there isn't a changing magnet ic field, saying that the voltage between A and B is V isn't ambiguous beca use it's independent of the path.
ered by an internal static electric field giving a net electric field close to zero. Think of a conductor place in a static electric field which initi ally causes the charges to move, eventually creating a static electric that contributes to a net zero internal electric field and EMF, and a static ch arge distribution.
fferent measurements depending upon the orientation of the probes which ref lects the path dependence of voltages in the video. But those EEs who belie ve there is only an induced electric field inside a wire, including that of an inductor or transformer, will come up with an incorrect physical interp retation of what's actually happening by adding induced EMFs around the KVL loop.
ow two different voltages at the same time (or maybe he actually built the circuit, I don't recall). So how can the two readings be different if ther e isn't a voltage imposed over a length of the wires in some way?
The two parallel paths contained different resistors giving different volta ges through them; the 1K giving -2.5mV the 10K 25mV.
Larry Harson
wires that get twisted once together) attached to the same points in the c ircuit, A and B, will see two different voltages, not because the probes ad d or subtract voltage due to their shapes, but because the actual voltages on A and B are different depending on the probe used?
ap. In the presence of
ation, there's lots of
nformation than
ing with electric fields
you think voltage
testing; it cannot
g magnetic field
ve to pick a zero point. That's like saying elevation is not fundamental f or the same reason.
rement has no meaning is entirely different. If nothing else, the fact tha t the voltage measurement can be explained perfectly is justification to th e idea that the voltage is defined.
Coulomb gauge is good for static type problems.
Do you keep this all in your head? I sorta know it, but have to look it up... This was OK.
I was going to ask if there was the equivalent with the scalar potential, and then I found this paper.
George h.
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