Mind blown... - Page 1
Forum section: electronics / projects

author: jmole
author_rank: Regular Contributor
timestamp: May 10, 2013, 06:17:15 am
content: Posted this in another thread, but I feel like it's worthy of it's own thread here.Take a an oscilloscope, hook it to the same points on a circuit, and get a totally different waveforms.When Kirchoff's Law doesn't apply:Part 1:Part 2:

author: krish2487
author_rank: Frequent Contributor
timestamp: May 10, 2013, 06:52:04 am
content: Non-intuitive at first glance but he gave the answer to it himself.The keyword beingpotential differenceand notvoltage across.It is just the way of looking and visualising the circuit in your head.Forget the integrals and the faradays laws for a moment.Just think of the phenomenon in such a way.The potential difference between the two points remains the same. It depends on what point you choose to take as reference and measurewith respect to it.Hence the +0.9V or -0.1V

author: jmole
author_rank: Regular Contributor
timestamp: May 10, 2013, 07:12:38 am
content: Quote from: krish2487 on May 10, 2013, 06:52:04 amYou're acting as if this is just a simple means of changing what you call "ground" in the circuit. It's not.Check out the supplement here:http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/lecture-notes/lecsup315.pdf

author: Harvs
author_rank: Super Contributor
timestamp: May 10, 2013, 07:25:50 am
content: Yeah well. The typical person when they draw a circuit will have straight connecting lines meaning the same voltage.What un-intuitive is the way he has drawn it.Instead of using circuit symbols (i.e. coupled inductors) for the magnetics, he's just drawn straight lines on the schematic and put a solenoid in the center, yet then connects a volt meter half way up his straight line and says it's the same potential as the resistor...

author: AndyC_772
author_rank: Super Contributor
timestamp: May 10, 2013, 07:37:11 am
content: I can see that I may have spent some time with a reel of copper wire and a scope by the end of the day...I think one reason why it seems so unintuitive is that the schematic consisting of just two resistors is incomplete. Normally when this effect is utilised, the changing magnetic flux is tightly concentrated in a single place, and we include a transformer symbol on the schemetic to indicate where it is. We then consider the transformer coil as a component in its own right, and it makes perfect sense to consider it as a discrete voltage source.In this case, only the two resistors are shown, and conventional circuit analysis shows us that the voltage across each one is zero. It's no surprise at all that the equations we use to calculate voltage and current break down if we start with an assertion (in this case, the non-zero loop current) which is inconsistent with the schematic we've drawn.

author: jmole
author_rank: Regular Contributor
timestamp: May 10, 2013, 07:44:28 am
content: Quote from: AndyC_772 on May 10, 2013, 07:37:11 amIt seems to me that adding a transformer to the schematic is much more misleading. The trick here is where the voltmeters are physically placed with respect to the circuit.

author: ConKbot
author_rank: Super Contributor
timestamp: May 10, 2013, 07:46:40 am
content: mind blown? poorly drawn schematics are confusing? If the inductance of the wire is going to be causing shenanigans, then either put down an inductor, or draw it as a transmission line.Shady trickery like that screams physics professor and not EE for sureedit: skimming over the lecture note for that, I'm not sure if thats trying to be an intro for inductors, or just confusing the kids with parasitics.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 10, 2013, 08:43:25 am
content: Quote from: jmole on May 10, 2013, 07:44:28 amPerhaps I didn't explain my point clearly enough. We draw, and analyse, electrical schematics as a set of individual, discrete parts joined together with "ideal" wires which have no resistance and where no potential difference exists from one end to the other. These diagrams are the language we use to record and study designs, and we're used to describing their behaviour.In this case the diagram isn't a good representation of the entire system, though. The professor's resistors aren't joined together by ideal wires, they're joined by conductors acting as secondary windings of a transformer. The resistors themselves are subject to the external field too, so arguably they should also each be shown as a number of discrete elements joined in series with transformer windings. That would give a better 'lumped' approximation of the real circuit being used in the demonstration, in the same way as we can model a transmission line as a series of discrete, small L, R and C elements.

author: SebG
author_rank: Regular Contributor
timestamp: May 10, 2013, 08:47:24 am
content: Quote from: Harvs on May 10, 2013, 07:25:50 am"Straight connecting lines" DO mean same voltage.The changing magnetic field induces a current to flow.The current in either resistor or the "straight connecting lines" is the same. Using ohms law with 1mA current you get a voltage of 0.1V through 100 Ohm resistor and 0.9V through 900 Ohm. The wires have very very low resistance (maybe even 1 mOhm or so) so the voltage is negligible. The intuition is that two resistors in parallel have the same voltage across it but that is not the case if they are within a changing magnetic field.

author: mikes
author_rank: Regular Contributor
timestamp: May 10, 2013, 10:30:36 am
content: Quote from: SebG on May 10, 2013, 08:47:24 amThe windings in a welding transformer have very very low resistance, yet produce voltage.

author: Korken
author_rank: Regular Contributor
timestamp: May 10, 2013, 10:31:25 am
content: Quote from: SebG on May 10, 2013, 08:47:24 amNot true in this case, you must take into consideration the inductance of the wires connecting the resistors.Remember, the current is changing very rapidly.

author: jmole
author_rank: Regular Contributor
timestamp: May 10, 2013, 10:37:11 am
content: Quote from: mikes on May 10, 2013, 10:30:36 amThey don't produce anything, they transform a changing magnetic field into a current.

author: mikes
author_rank: Regular Contributor
timestamp: May 10, 2013, 11:47:31 am
content: Quote from: jmole on May 10, 2013, 10:37:11 amYou can pedantically argue semantics all you want. They're low resistance. You can measure voltage across the ends. They cannot correctly be drawn as simple lines on a schematic, just as with the case at hand.

author: penfold
author_rank: Frequent Contributor
timestamp: May 10, 2013, 07:27:20 pm
content: I'm a big fan on that experiment, its beyond totally pointless! and those are always the best experimentsIt does however highlight that what you're measuring is not necessarily what is in the circuit, even if it does it in a way that canot be transferred to the real world.

author: amspire
author_rank: Super Contributor
timestamp: May 11, 2013, 12:28:02 am
content: I had a look at the video and there was no "mind blowing" as promised. I think he was using a trick of educators - deliberately trying to confuse the students by leaving something out that is easily overlooked to force them to learn. I have heard educators say that confusion is a great stimulus for the brain to develop. That is the only excuse I can think of. Deliberately say something that is wrong to force the students to resolve the confusion in their brain for themselves. It would probably work - I doubt there were students asleep in that lecture.What he was saying was just plain wrong. Perhaps he said that deliberately, but it is still plain wrong. He said that Kirchhoff's Law fails. In Kirchhoff's Second Law, you have to include the voltage sources in the loop and the lecturer did not. The wire has a voltage across each length due to the magnetic induction, so you have to include that voltage as a voltage source.In trying to measure the voltages in practice, you do have to take into account that the loop connecting the meters may also have a induced voltage if the field is changing in the metering loop. So you have to either eliminate any magnetic field changes in the metering loop or compensate for the magnetic field changes. An error in understanding how to measure a voltage is not a reason to say that Kirchhoff's Law fails.Kirchhoff's Second Law says that the sum of the voltages in the loop is zero.+1V induced - 0.1V across the 100 ohms - 0.9V across the 900 ohm resistor = 0V. The Law applies.

author: jmole
author_rank: Regular Contributor
timestamp: May 11, 2013, 12:32:45 am
content: Quote from: amspire on May 11, 2013, 12:28:02 amWhat he said was exactly correct."KVL is based on the asssumption that there is no fluctuating magnetic field linking the closed loop. This is not a safe assumption for AC circuits. In the presence of a changing magnetic field the electric field is not a conservative vector field. Therefore it cannot be the gradient of any potential. That is to say, the line integral of the electric field around the loop is not zero, which directly contradicts KVL."

author: amspire
author_rank: Super Contributor
timestamp: May 11, 2013, 12:53:44 am
content: Quote from: jmole on May 11, 2013, 12:32:45 amI know that is in Wikipedia, but it is wrong too. Where did they get that nonsense from? Kirchhoff's Seconds Law states that the voltages around a closed path in a circuit must sum to zero. In this case it does. There is 1V induced into the wire, so it must be included as a voltage source. It is a voltage source.I have never heard Kirchhoff's Second Law saying that a wire by definition must have 0V across it. You can only assume that if the wire actually does have 0V across it. If a wire does not have 0V across it because it has resistance or has an induced voltage, that must be included in your calculations of the loop voltages.Next you will be telling me that a thermocouple also breaks the law as you have a voltage and a current in a circuit with just two wires and a resistor. It doesn't.If you misquote a Law to make it easy for students to understand, then yes, it is easy for your misquoted Law to be wrong.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 01:04:43 am
content: Quote from: amspire on May 11, 2013, 12:53:44 amOh god. I remember this one a few weeks back in Circuits I. Half the students in my class couldn't grasp that a wireusuallyhas approximately zero volts across it (that is, they didn't even realize what it meant to draw a line between components on a schematic). The "instructor" (and I use that term very lightly) spent so much time drilling into the class that a wiremeansa defined zero volts between components that they are going to have a very rude awakening when they start to discover that every wire is a resistor, an inductor, a transformer winding, and one plate of a capacitor.I realize that in a "mathematical" circuit, this is what a wire is, but there was no attention at all given to the distinction between real wires and idealized ones. Hopefully most of the students are smart enough to recall electrostatics and electromagnetics from Physics the previous semester. I doubt it.

author: amspire
author_rank: Super Contributor
timestamp: May 11, 2013, 01:21:38 am
content: Quote from: c4757p on May 11, 2013, 01:04:43 amYes. If you draw a circuit with connections representing ideal zero volts - zero ohm connections, it is your job as the person drawing the circuit to make sure that actually matches reality. In the case of a wire in a changing magnetic field, the wire no longer behaves like a zero volts- zero ohms connector, so you have to modify your circuit to model the behavior of the actual physical wire.If your circuit does not match the physical reality you are trying to represent, then the circuit is wrong. Your calculations based on your circuit will be wrong.If you have a transformer in a circuit, you draw it as a transformer and allow for the voltages from the windings in your calculations. If some lamebrained professor is going to tell you transformers are just bits of wire, so you may as well delete the transformer in the circuit and replace it with simple shorted wire connections, then you are going to have a hard time getting any circuit to work.

author: BravoV
author_rank: Super Contributor
timestamp: May 11, 2013, 02:12:09 am
content: So all of this fuss is just a prank violating on the very basic of 101 in electical drawing rule ?

author: skipjackrc4
author_rank: Regular Contributor
timestamp: May 11, 2013, 02:20:44 am
content: Quote from: c4757p on May 11, 2013, 01:04:43 amI remember those classes. Sadly, a lot of these morons will wind up with the same degree as you... I have 1 semester left to get my BS, and I can't believe that half of my class is graduating.Anyway, this video is full of crap. Somebody will try to base a perpetual motion generator on this concept now...

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 02:26:16 am
content: Quote from: skipjackrc4 on May 11, 2013, 02:20:44 amEh, don't call them morons. In my experience it's almost exclusively the fault of the shitty excuses for instructors the school hires.* The only reason I'm doing better is that I came into the program already having a lot of experience with electronics as a hobby. Of course, I haven't been to your school, but I don't imagine it's too different unless you go to a particularly good school. (I remember being very impressed with the math faculty at my previous school, RIT. Would never have left if I had a choice...)QuoteI sure hope so! I'm usually depressed by idiotic things like that, but perpetual motion stuff is just... entertaining. That's all I can really say about it - it's fun to watch.*The school likes to insist they'd do better if they'd justask questions. "That's what office hours are for." The damned professors are even more useless in office hours when they don't have a script to read off of. Many of the students try so hard to get help at the beginning, and just end up giving up after they realize nobody could help them if their lives depended on it.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 02:31:25 am
content: I typically come to lab every week with "lecture materials" prepared, and I end up teaching my lab partners the week's material before we do whatever the work is. I spent three hours preparing documentation on hysteresis last week. Want to know how long it takes me to go through a week's worth of useless lecturesand actually accomplish instruction? Usually twenty minutes, and that's with them having no clue what we're even talking about at the beginning. We still finish all the work before every other group, and no, I'm not doing it all for them.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 02:37:47 am
content: I had to show the lab guy how to do single-shot on the oscilloscopes last week, after watching him fuck around with the holdoff and level controls for three minutes straight in the hopes that the feature would jump out and kick him in the nuts if he twiddled the knobs enough.I stopped short of trying to explain why a single-shot capture wouldn't have even been necessary if he had any clue how the trigger worked.

author: amspire
author_rank: Super Contributor
timestamp: May 11, 2013, 02:44:11 am
content: Considering that the lecturer got Ohms Law totally wrong - something that a professional electronics designer would never do, I can only assume he is not a good designer. He could be a Physicist or something.He probably wouldn't know which way round a resistor was meant to go in a circuit, or how if you put an IC upside down on a PCB, the electrons fall out. (See I am learning something from Dave's blogs).

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 02:47:03 am
content: Quote from: amspire on May 11, 2013, 02:44:11 amI told someone "don't hold it that way, you're losing all the electrons" the other day and he actually believed me for a moment!The look on his face was priceless, both when he was trying to figure it out and after he realized I was messing with him...I'm going to give him the benefit of the doubt and assume it was just a slow day...

author: skipjackrc4
author_rank: Regular Contributor
timestamp: May 11, 2013, 03:08:37 am
content: Quote from: c4757p on May 11, 2013, 02:37:47 amSounds familiar. I've had to show TAs how to use spectrum analyzers also. I will be getting my MS in electromagnetics, and working with a well-known lab. It will be a nice change, I think.Quote from: amspire on May 11, 2013, 02:44:11 amThis was my thought. Probably a guy who specializes in a different field who was assigned an entry-level physics class because everybody else was busy. That happened all the time during my first 2 years, though nobody was ever this wrong...

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 03:11:34 am
content: Quote from: skipjackrc4 on May 11, 2013, 03:08:37 amA spectrum analyzer is at leastsomewhatspecialized. Not being able to locate one of the most basic functions of one of the most basic pieces of test equipment within three minutes is absolutely unforgivable, especially for someone who's been trusted to be present to help students with problems.Especiallysince the entire school appears to have only two models of oscilloscope. How in hell has he not looked for it before?

author: sleemanj
author_rank: Super Contributor
timestamp: May 11, 2013, 03:13:39 am
content: What I got from that video, is that his students must be completely glazed over, not a single one of them said "excuse me, that's not Ohm's law".I remember when I was at uni back in the 90s occasions where lecturers would make a mistake and they were pointed out by the assembled students, sure it might be a small slip-of-the-wrist but at least if it's pointed out then they knew that you were listening, following along, and that you understood.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 03:16:40 am
content: Quote from: sleemanj on May 11, 2013, 03:13:39 amDepends on the school. When I was at RIT (a big science/engineering school near me - notMIT), the students would correct the professors full of what can only be described as arrogant glee. The professors would respond with profuse thanks, and occasional horror at the thought that they might have taught something incorrect. At the little state school (mostly liberal arts, with a sad excuse for an engineering program) that I'm now forced to attend for financial reasons, nobody corrects anybody. I've tried, myself, but even I have given up, because I'd be doing it every thirty seconds if I bothered at all.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: May 11, 2013, 03:28:43 am
content: I must admit, I didn't notice the Ohm's Law error at first. I was only half paying attention.

author: XynxNet
author_rank: Regular Contributor
timestamp: May 11, 2013, 09:23:43 am
content: Keep in mind, that Mr Lewin teaches physics, not EE.His topic is traps of non conservatives fields.His circuit diagramm is in so far a good one, as it starts at the knowledge-level of his students. Afterwards he points out the traps of modelling that circuit this way.EE drawing rules would not have helped his lecture at this point because he wants to teach the basic physics, not the EE drawing rules derived from basic physics.@Mistakes in lectures:Try lecturing 15-20 hours a week, week after week, without making any mistakes...If you did better, then you can talk about bad lecturers/teachers singling out their mistakes.

author: amspire
author_rank: Super Contributor
timestamp: May 11, 2013, 12:34:35 pm
content: Quote from: XynxNet on May 11, 2013, 09:23:43 amI think he is probably a fabulous teacher, and he is probably a brilliant physicist. I made the point that I think he was using a deliberate tactic to involve the minds of the students. I think if most of us tried to teach Kirchoff's Laws, we would bore the students silly.I actually don't think it is the job of a teacher to always tell the students the correct facts. They have to stimulate the minds, so the students get to see the truth for themselves. The teacher is not a crutch. I think Mr Lewin is probably very good at turning on the minds of his students.I don't mind that he made a mistake with Ohms law, but I just made the point that an electronics designer just would never have said E (Voltage) = I/R. So he was looking at the circuit as a Physicist rather then an electronics designer. The problem with the circuit is it didn't accurately model the reality so it is not a useful circuit for use by an Electronics designer.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 11, 2013, 01:07:34 pm
content: Quote from: amspire on May 11, 2013, 12:34:35 pmI do. A professor at a reputable institution is in a very privileged position, people will quite rightly look up to him, and regard what he says as definitively accurate. I'd consider it a major impediment to learning if someone who really ought to be correct says one thing, yet it contradicts a student's own measurements or outside experience.At the very least, if a topic is going to be simplified for the sake of clarifying one aspect of what's being taught, whoever is teaching that topic should make it clear that the simplification is taking place, and that there may be further consequences if a more complex or accurate scenario is being considered. That would go some way to removing the self doubt that can otherwise cloud and hinder the learning process.I still feel that in this particular case, the professor has confused himself and his students by mixing up a real wire - which in this case is a current carrying conductor being subjected to a magnetic field - and a schematic representation which implies a perfect electrical connection between two or more discrete components. His incomplete schematic is the problem, and if his students walked away from that lecture questioning the usefulness and applicability of Kirchhoff's laws, then that's a problem too.

author: c4757p
author_rank: Super Contributor
timestamp: May 11, 2013, 01:14:05 pm
content: Quote from: amspire on May 11, 2013, 12:34:35 pmI don't think it's the job of a teacher to always tell the studentsfacts. That is, lectures should do more to "stimulate the minds", as you say, and less to drill in datum after datum. The students should be inspired to then go learn the information, which is quite readily accessible these days, themselves. They should, however, be able to trust their professor as a credible source for the information that he does say. If classes were slower and more careful, I might agree with you, but at the pace that most of these go, the onus of fact-checking the professor should not be on the students.

author: amspire
author_rank: Super Contributor
timestamp: May 11, 2013, 01:28:26 pm
content: Quote from: c4757p on May 11, 2013, 01:14:05 pmThe thing is a good teacher should make it clear that something is not true because the teacher says it is true. A good teacher prepares the student to possibly be even better then the teacher. A good student should question everything the teacher says. The best the teacher can do is to present what the teacher believes to be fact when looked at from a particular point of view. It is common to see a different view of the truth when looked at from a different point of view and it is often the new and original point of view that make a genius a genius.The really key thing is it is impossible for a teacher to give understanding to a student - that is something only a student can do for him/herself. The teacher is the guide that takes the student along a path that will make the student experience the truth much faster then without a guide. At the end, the teacher stands aside and leaves it to the student to make their own way.

author: lewis
author_rank: Frequent Contributor
timestamp: May 11, 2013, 02:42:30 pm
content: It makes complete sense when you put all the symbols on the schematic.... Something like this? (attached)edit - that's one of my best drawings ever.

author: amspire
author_rank: Super Contributor
timestamp: May 11, 2013, 03:50:11 pm
content: Quote from: lewis on May 11, 2013, 02:42:30 pmI think you deserve an honorary doctorate for those very fine drawings. A Phd in Circuitology.