hi making this video makes me nervous
this is a serious and complex video
about an electronic topic and i have to
do it very carefully
see it's like you disagree with stephen
hawking who's likely wrong
you are and now i'm about to disagree
with dr walter lewin
if you don't know dr lewin he has a phd
in physics he's a former professor of
physics at the massachusetts institute
of technology or mit he has made many
discoveries has written
tons of books has received many awards
for his scientific achievements
has educated thousands in schools and
millions over the web through his online
lectures
just check his wikipedia page i on the
other hand have a master's degree in
electronics
have worked for a while as an engineer
and then turned youtuber where i
constantly get shocked my mom thinks i'm
mostly okay
dr lewin teaches in one of his courses
that the curse of voltage law or
kvl doesn't hold true in some cases and
i disagree with him
directive fire i tried to contact dr
lewin to discuss this but understandably
he doesn't want to talk about it anymore
because so many people online have
annoyed him with their stupid reasonings
so i humbly make this video hoping that
he or some other great professor sees it
and tells me what they think about my
results
if he's right more power to him i
couldn't be happier to be wrong and
learn something new
now what's kirchoff voltage law or kvl i
read the definition from wikipedia as my
alibi which is
the principle of conservation of energy
implies that the directed sum
of the electrical potential differences
or voltage
around any closed network is zero this
is also my understanding and how i
learned it
if you find any closed loop in your
circuit the sum of the voltages across
its components
at a given time is zero adding them up
in one direction
this has never failed me but dr levin
says kvl doesn't hold when there is a
changing magnetic field
involved in a circuit the potential
difference between
two points in a circuit is no longer
defined
when we have an induced emf two
voltmeters
connected to exactly the same two points
can measure different values i'll leave
links to his relative lectures in the
description and i suggest you watch them
but let me quickly summarize his
reasoning
imagine we have a circuit like this a
one volt supply in series with 100 ohm
and 900 ohms
so one volt divided by one kilo ohm
total means there is a one milliamp
running through the loop or
0.1 volt here and 0.9 volt there if we
calculate kvl in the loop in this
direction
entering positive of r1 we have plus 0.1
volt
and entering positive of r2 we have plus
0.9 volts and entering
negative of the supply we have -1 volt
and the sum of all these is 0.
kirchhoff's law holds fine here but
let's remove the supply and
short it and instead apply a changing
magnetic field through the loop coming
out of the page
we know that a changing magnetic field
creates current in a loop and according
to the lens law the direction of the
current
is this way let's say at a moment in
time the current reaches 1 milliamp
which means we will have
0.1 volt across r1 and 0.9 volts across
r2
the sum of the voltages in the loop is
not zero so dr lewin concludes so
kirchhoff is for the birds
and fire day is not then dr levin does
an interesting experiment to prove this
he measures across the exact same two
points in the circuit with two different
scopes
at the same time and shows that each
scope shows a different number
three two one zero
look on the left there's v one notice
it's negative look on the right there's
v2 it's about nine times
larger than v1 he then concludes that in
this
special case depending on which way you
look at the circuit you read a different
voltage
so this half of the circuit has a
different voltage than the other half
which contradicts the kirchoff voltage
law and it doesn't hold he then says
that it's the faraday's law that always
holds not the kirchhoff's law which is
just a special case of faraday's law
dr lewin says that the two points across
the wires are electrically identical
i want you to understand and appreciate
that a1 and a2
from an electrical point of view are
identical they are the same because the
resistance of the wire is extremely
small compared to the resistors which is
true
and also the inductance of the wire is
extremely small which is also true
so the changing current through the wire
creates very negligible voltage across
the wire if anything
i repeated the same test and i agree
with his measurements but not his
conclusions let me repeat the test for
you
like dr lewin i have a coil or solenoid
and i'll switch a battery through a one
ohm resistor
over my coil the resistor limits the
current to 10 to 12
amps the coil creates a rise of changing
magnetic fields and settles
and then i have my loop with one kilo
ohm resistor on one side and 10 kilo ohm
on the other side
we can agree the wire resistance and
inductance between them is negligible
compared to the resistor values
so when i apply voltage in my coil the
magnetic field in it
changes like this and settles the
electromotive force or voltage induced
in my loop is the rate of change of
magnetic flux
and so the induced voltage changes like
this
now i'll measure across these two points
in the loop using my
sense wire that connects to the scope
notice that i've twisted the sense wire
having twisted wire
is important because say we have
magnetic fields coming
out of the board the induced current in
the first twisted loop is in this
direction
and in the next loop is also the same
direction this means that say in the
black wire the current wants to go in
this direction in loop one
and in this direction in loop two and
they cancel each other
so using twisted pairs makes sure that
the voltage and current in the wires are
not affected by external fields
connecting the scope doesn't affect the
main loop behavior at
all because it has a hundred mega ohm
input impedance so we can safely say
there is no loading effect from
connecting the scope to the loop where
we measure
now the sense wires are on the 10 kilo
ohm side ready
click see i have a voltage pulse with a
peak of around 25 millivolts
now all i do is i flip the sense wires
to the one kilo ohm side of the loop
ready click there we have a negative
pulse around one tenth of the first
pulse
because although the current in the loop
is the same the resistance is one tenth
what does it mean we are reading two
different voltages across the same spot
maybe there is voltage across these
wires unlike what we thought
i moved the sense wires across the loop
wire let's measure
click huh
the scope just triggers a noise there is
no voltage across the wires
so does it mean that two different
voltages from two resistors are applied
across the same spot and the kirchhoff
flow doesn't hold
i'm sorry but this is where dr lewin and
i part ways
please don't hit me wait we are not back
in old iran you can't hit me
can you let me show you dr levin's
theory is not dependent on the location
of the resistors so let's rearrange them
here i move the resistors side by side
and very close
if i measure across these two points i
still read two different voltages
depending on which side i do the
measurement same as before
here i have both my resistors on one
side and i'm measuring across these two
points on the 10 kilo ohm side
there we go kelly
see we see a pulse around the same size
i flip the wire to the one kilo ohm side
and kill click see i see the same
one-tenth pulse like before so nothing's
changed there but instead let's measure
right across
r1 and r2 and both r1 and r2 with
minimal localized sensing loops
here's the voltage across the 10k bam
see it's around the same 25 millivolts
and the voltage across the 1k
the same two and a half millivolts and
the voltage across both resistors
bam see it's actually higher than 25
millivolts
pretty close to 27 and a half so you see
that the voltage across both r1 and r2
is equal to the voltage across r1 plus
the voltage across r2 which is the same
voltage across the loop
this shows that the voltage across the
loop is not zero unlike what we thought
but is equal to vr1 plus vr2
the loop is the secondary of a
transformer with the primary being my
coil
dr levin's model misses a transformer in
the loop
with the voltage across the winding
equal to vr1 plus vr2
kvl holds but then dr lewin would say
you stupid kid
well he wouldn't say that he's a nice
guy but he would say
if instead of measuring locally like
this we measure
across the loop on the other side
we would read zero volt across the loop
which is correct
so which one is right well let me show
you an extreme case let's assume our
resistors are infinite or
open and so there is no current running
through the loop
now if we measure the voltage across
this tiny gap on this side we see a
voltage pulse
here's the loop with no resistors
measuring right across the gap
and we have a voltage pulse and when we
move the sense wires around
like this we see zero jump and here i
have the same loop with the sense lines
running right on top of the loop
and we have no voltage jump just some
noise
of course if i make the sense lines a
little bit smaller or larger than the
main loop
i'll have voltage jumps so if in fact we
have a voltage v
across this gap for reference say zero
volt here and plus
v on this side why do we read zero volts
across the sense lines
let's take halfway through the loop if
the voltage here is plus v
the voltage here would be half a v so we
have
half a v across this half of the loop
and another half of v across the other
half
there is no current running through the
sense lines so it doesn't matter that
they are going to the scope
both the sense lines and the loop have
the same shape and are in the same
field so they behave the same which
means
we have half a v on this sense line
and another half a v on the other sense
line
so if the voltage of the loop on this
terminal is v it is reduced on the sense
line by half a v
here and on the zero terminal it is
increased by half a v
here so you see that the voltage
difference between the sense lines is
zero at this point and the scope shows
zero
so the entire reason dr lewin was
reading two different voltages was due
to bad probing
no matter where you cut your circuit the
voltage
across these two points is always a
unique number
the voltage on this half of the circuit
is the same as the other half
well this looks a bit complicated but if
you look at this half of the circuit the
voltage across this half
is v1 which is the voltage across this
piece of loop wire
plus the resistor voltage which is vr1
plus v2 of this piece of wire
now if you sense it like this the same
voltages of the loop wires are
induced on the sense lines which
subtract from the voltage
across these points and all you read at
the scope is vr1
and if you sense on the other side the
induced voltages on the sense lines
change to match that side and you read
vr2 now you might say this is different
than my extreme case because there is
current running through the loop but
according to the lens law this current
actually reduces the original field so
the induced voltage on the sense lines
is less
and they end up matching there were
never two different voltages across
these points
this emphasizes the importance of proper
probing now i'm not hundred percent sure
but if my quick analysis is correct to
measure the voltage
across say these two points properly
the sense lines must come straight
together
and twist all the way to the scope or
meter
and keep your scope probe away too
because even if this loop is close to
your fields
voltage is induced in it and will affect
your readings my conclusion
krish of laws hold and have held nicely
for me so far
if you see any discrepancy it's because
you're either missing a component in
your model or probing wrong
have i been wrong before well yeah just
look at my previous videos
am i wrong now i don't think so because
science and analysis and experiments
support my claim as well as a bunch of
textbooks written by smarter people
the sad part is that almost all college
physics book
halliday and resnick john coley ohanian
jung and freeman they all when they deal
with selfie doctors with solenoids they
all
say let us now use kiss of lupu
that statement is a crime
well either way i'm sure the science
will win think about this
give this some thought this is not easy
and have a good weekend
h