# Is Walter Lewins Loop Trick. Correct or False?

• spsch

## Homework Statement

I posted this in the advanced section, but glancing over the problems there I realized this probably belongs here.[/B]
Hello everyone. I hope this is the right category. I am in desperate need of help. I have no High School Education. But a few weeks ago I participated in an entrance exam for a university (very well known for physics actually) in my country of residence.

Long story short, I passed all exams (11) except 2 math exams, did best in physics but am 0.1 short of passing overall.

Physics were two exams (oral and written). In the oral one the examiner made up problems on the fly. I solved them well and quick. But she made up this exact problem Starting at 33:33 Minutes that Mr. Walter Lewin uses.
TLDR;
is his way wrong?

Backstory:

I am very comfortable with circuits but I did Walter Lewins courses after my first completion of Giancolis Book and then Ohanian.
I found his way to be much faster.

The Examiner asked me for I 1 (Current through the left loop) and R3 said all other things (I 2, R1, R2, I3) are known.

I told them (a physics teacher and a professor of chemistry were my examiners) that usually one would solve this by using Kirchhoffs loop rule.
But I know a faster way from youtube. The Examiner asked me to go ahead then and use my way.

But while writing my equations (same as Lewin in the Video) she kept saying I was wrong as I1 and I2 don't go through R3.
While I was actually just one step (a total of 3 away from solving).
I got a B.
Though I was sure I wasn't wrong as I practiced the death out of this.
I went back that evening and luckily found a problem in Giancolis Book that is basically the same.
I worked this out and several others easily with the equations I had used at the exams.

I showed the professor and he agreed that my way gives the correct solution every time. But insisted it just gives the right answers but is still wrong. So they would end up only bumping me to an A if it would help me pass the overall exam.

I found out that they made a mistake during this too.
When I tried to reason that just three steps and two equations is faster.
I noticed the Professor realize a mistake they made, though he didn't admit it. He thought I 1 was directly solvable in one equation with Kirchhoffs rules. As the problem they had in mind was with R3 given. Luckily on the paper she wrote down R 3 and I 1 aren't given.
Which I think swayed him to give me the A.

Anyhow they didn't even give me the A, so I have a meeting tomorrow. A+ would push to 0.03 under Pass which qualifies as review (which could get me in).

I thought it's not wrong as the Potential Difference is the Integral of E x d. And the closed loop Integral or an Integral from A to A would always be zero. (Which is something I mentioned at our meeting).
And I3 is just the difference of I2 and I1.

But I'm not a hundred percent sure. Is his way actually scientifically wrong?

I am sooo sorry to burden you. I am 30 years old. I am trying to fulfill my dream here. I have another shot but financially it's a burden to wait another year. ( I saved a few years and quit my job to study for this exam and be able to attend Uni)

If it isn't wrong. HOW DO I politely but very indisputably convey this tomorrow?
I am eternally grateful to you.

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## Answers and Replies

That is a 50 minute video. A pointer to the right time would help. Even better would be a sketch of the problem presented to you in the exam and your solution method.

spsch
That is a 50 minute video. A pointer to the right time would help. Even better would be a sketch of the problem presented to you in the exam and your solution method.
Hi mfb

I chose the link so that the video starts at the exact moment he introduces the problem. At least it should and does for me 33:33 min

i'm also going to make sketch now. But if you follow him my equations and the problem match exactly.
it's quite the coincidence

Update: Sketch

The above is basically the sketch I received.
https://imgur.com/a/R1kbJBf

I proved (With three similar problems from Giancolis book where we had numbers to work with) and the professor agreed that the result is "numerically" always correct. Just the way he insisted is wrong.

And at the beginning, until he saw that R3 wasn't given he also insisted that with Kirchhoffs rules we could have solved it in one step.

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The forum doesn't seem to conserve the timestamp.

That works, sure. Just an application of Kirchhoff's laws. I don't see why you would use it in this case, however:
The Examiner asked me for I 1 (Current through the left loop) and R3 said all other things (I 2, R1, R2, I3) are known.
If you know two currents and you are asked about the third one, you can just add/subtract the two.

spsch
The forum doesn't seem to conserve the timestamp.

That works, sure. Just an application of Kirchhoff's laws.
Oh I'm sorry, I hope the Time helps now.

Is his way correct?
If yes, how?
According to them it's not correct as they wanted me to use three equations.

Which is in itself silly as they could have said I should use Kirchhoffs Rules when I said I know a simpler way, instead of asking me to show mine and stopping me.

In my equation what they said specifically was wrong was the - I2 R3 part. in the first equation and -I1R3 in the second equation.

If you know two currents and you are asked about the third one, you can just add/subtract the two.

this is indeed simpler. I used that as an argument why my way works without realizing it's an easier way to the solution.

That was very silly of me.

If lewins way however isn't wrong, can you explain to me how I could argue for it's validity?

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According to them it's not correct as they wanted me to use three equations.
Add 1=1 as third equation.
No, seriously: If they asked for one specific way to solve it then you should use this method. Otherwise a large range of methods will work.

As yet another option for the problem given in the video: Introduce the unknown voltage V3 and make two equations based on V3 and I3 coming from the two sides of the circuit. Also two equations and two unknowns. Solving for I1 would take a bit more time that way.
You can also do a Y-Δ transform...

What is done in the video certainly works.

spsch
Add 1=1 as third equation.
No, seriously: If they asked for one specific way to solve it then you should use this method. Otherwise a large range of methods will work.

This has been my main problem with the whole thing. She didn't ask me to use her method.
She only asked me to solve for R3 and I1.
She asked me to go ahead and use my Youtube method when I said I know an easier way (Altough I realized now it's not easier). But when I went ahead she kept saying I was wrong and in the end crossed out my -I1R3 and -I2R3 parts as wrong.

If it isn't wrong, what makes it right? The Professor insists this way is wrong. Only the solutions come out right.

I'm really distraught, I'm sorry to be so difficult.

what makes it right?
It is only an application of Kirchhoff's laws.
You could easily add I3 to the system and replace it later, that wouldn't change the overall approach, it would just give more intermediate equations.

spsch
It is only an application of Kirchhoff's laws.
You could easily add I3 to the system and replace it later, that wouldn't change the overall approach, it would just give more intermediate equations.
This is why I then thought my way is easier.

Before realizing now I have 2 of 3 variables in I3 = I2 + I1 ...

Yet in my equations: https://imgur.com/a/R1kbJBf they said that of V1 = I1R3 + I1 R1 - I2R3 the "-I2R3" part and in
V2 = I2R2 + I2R3 - I1R3 the "-I1 R3" part is wrong.

I really believe you it isn't. I believed it isn't wrong before too, but I wasn't sure anymore as they were so persistent that it's wrong. But I have to convince them.

Is there some way I can explain without doubt tomorrow that it doesn't violate Kirchhoffs Loop Rule or Laws of Physics and that it isn't wrong?
Or why in the above equations the two parts aren't wrong?

I think they mean it's wrong because I should have used a third equation for the outer loop in their opinion.

I am really sorry. I don't want to be difficult. And I really, really appreciate you taking your time to help.

But everything rides on me being able to convince them tomorrow that it isn't wrong. So they may review my exams again.
The Professor already admitted that this is the only reason I didn't get the full score. And it's all that I needed then.

If you are at the very bottom of the entrance requirement, I would question whether you are ready for university. You shouldn't be at the bottom, my friend. You should right up there competing with the best of them, trying your hardest. I would think about this because that's probably what they are thinking, it'll get him into university but he isn't ready.

If you are at the very bottom of the entrance requirement, I would question whether you are ready for university. You shouldn't be at the bottom, my friend. You should right up there competing with the best of them, trying your hardest. I would think about this because that's probably what they are thinking, it'll get him into university but he isn't ready.
Hi Verty. Thank you for your advice. I have.

While I'm sure everybody says this...

I am a lot better than I did at the exams. I am probably not ready for school as I haven't been in a school for 16 years and I only went up to seventh grade.
I don't think we had exams then.
About half the students that take this exams are University students from other unis in Europe. Overall about 2/3rd fail. I don't think all of them aren't ready. (They have to take the exams because they didn't meet the math and physics requirements likely).

However I am at the very bottom of passing because I completely froze in the second math exams. Things I can really do well. (Planes and Distances between them and Probability). So I left it completely out.
I just said there, shaking, sweating. I had trouble breathing. I went to the bathroom three times to put water in my face.

Anyhow, I got an F. I didn't do anything there. I wanted to, but I just couldn't start. I read the questions but I couldn't think about them.
Really, really simple stuff.
Problems I solved many times before. I could prove it but it doesn't mean anything.

Even with an E I would have passed comfortably in the overall. I could have attempted to solve half a problem.

I want to say. I didn't do terrible in all the other exams. Only in One. But extremely, extremely terrible.
If I had just attempted to solve one problem. I would have made it.
It was my inability to pull myself together at that one exam. Math is calculated with a weight of for times.

All the exams after that went extremely well.
But I think it was because the pressure was gone, I already knew I had an F on the second exam and I didn't expect to do as well in all the others as math is by far my strongest subject.
Giving up somehow lifted that pressure.

I feel if I could just get into the environment (school) I would do even better and not be as nervous on tests.

In the last month. I practiced with previous exams, I did them at home. Not all but many were more difficult than this one. I never cheated. I always solved all of them at home. Under time constraints. I passed all of them. Some barely, but many with a very healthy margin. I know it's my fault. I just don't know why I froze like that that day. Maybe it was the sleepless night. Maybe it was the early morning commute. Maybe it's because I read on the internet you shouldn't keep at one problem too long.

Last but not least. Quite a few get in with being about 0.05 short upon review. Actually from what I heard almost all.

I should have insisted on the bump which I deserved (since it isn't wrong). But I trusted I receive it when the professor said so. He said I probably won't need it though back then. (I don't think he was aware of that math exam I practically skipped).

Try this as an exercise. Write the three equations as the examiners wanted. If you use the "small" loops, the two voltage equations would each have an I3R3 term; the third equation should be a variant of I3 = I2 - I1 depending on how you choose the directions of the currents. If you substitute this expression for I3 in the other two equations, you should get Levin's equations. His method is just a shortcut that does the substitution automatically. If you want to be successful at university, I suggest that you try to concentrate on understanding what you are doing instead of blindly following a recipe that you found on the web. True understanding comes when you discover and apply your own recipes and shortcuts that work just as well.

spsch
Try this as an exercise. Write the three equations as the examiners wanted. If you use the "small" loops, the two voltage equations would each have an I3R3 term; the third equation should be a variant of I3 = I2 - I1 depending on how you choose the directions of the currents. If you replace this expression for I3 in the other two equations, you should get Levin's equations. His method is just a shortcut that does the substitution automatically.

Hi Kuruman. Thank you very much. This is actually exactly what I did to prove to the Professor (of Chemistry) it's the same. I solved it both ways and showed that the equations can be derived from one another. Using Problems from Giancoli to prove it qualitatively and quantitavely.

As mentioned. In the end, he agreed that my way works. But only numerically he said. The way is still wrong. (He actually got quite upset when I said my way is also right.) Hence half the bump. From B+ to A and not to A+.
I also feel like if the physics examiner who is a physics teacher was present things would have gone differently.

I felt I had very good understanding until that meeting and that exam. Because I could not understand why they said it was wrong.
And why he kept insisting that it is wrong after I showed that it gets the right answer every time and works out the same.
That's why I started this thread.

But since mfb said nothing is wrong. I feel like I'm having just very bad luck here.

As the A that was promised would have also gotten me into 0.05 Review territory (just not as clearly as the A+) I'm starting to suspect something, but I try not to think the worst. And hope he just forgot.

Is there a way other than deriving the equations and showing the solutions end up the same to prove to them quite politely that it isn't wrong?
It's really just that.
He insists it's wrong. I was sure it wasn't. I thought I was able to prove it.
Then unsure because who am I to question the professor?
Now I'm sure again.
But that doesn't help me, I have to convince him.
If there isn't another way, then thank you all. I will just have to live with it I guess.

But that doesn't help me, I have to convince him.
From what you have posted so far, you will not be able to convince him simply because he does not want to be convinced or he is already convinced, but will not admit it. Move on.

davenn
Hi mbf, I updated it, but am not sure how to rotate the image
https://imgur.com/a/R1kbJBf
Here's a copy of the image rotated:

That method of dealing with the currents is a fairly common method.

You do need to be careful about signs. It looks like you have used "the sum of the EMFs in any closed loop is equivalent to the sum of the potential drops in that loop. Notice that in the direction of Loop 1 (marked with I1) the current through R3 is I1 − I2. in the direction of Loop 1..I would encourage my students to write the Loop 1 equation with I1 − I2 grouped as follows.
##V_1 = (I_1-I_2)R_3 + I_1 R_1 ##​
but there's nothing wrong with what you did.

In an oral exam, my view is that if the student can explain what he/she is doing and why they're doing it, that's more important than doing it in some strictly prescribed way.

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