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Failing Physics! Help!

  1. Oct 27, 2011 #1
    Hello PF,

    I just took my 2nd physics midterm today and I didn't do so great. I just don't get how to pass this class! Its so hard. I have tried everything from doing all the problems to reading the book a few times. I understand the concepts fairly well, I say this because I do the homework just fine. But when it comes to exams, I just totally have no idea what to do. I am currently taking electromagnetism and the math is very challenging.

    I think one of my problem is basic math. I get confuse when as to use what and where. Things like V=0 at r=infinity, and what potential is at what point (reference points and coordinates?)

    I'm sure I am not the only person that has this problem. When I take an exam, its hard for me to piece together what concepts to use. I understand SOME of the problem and a little bit of what to do but I cannot truly understand it. It bugs me because I do problems in the book just fine.

    So I guess my main question is, are there any ways to practice these basic math concepts? Things like: What integrand to use, equations like WORK=-DELTA U (minus signs really mess me up because of vectors). Or any other tips you guys might have for me. Personal experience to how you guys got through physics would be great!
  2. jcsd
  3. Oct 27, 2011 #2
    Read the problem, then draw a picture and lay out your known data. Then put it together into a solution.

    Problem -> (Pictures <-> Data) -> Solution

    I'm guessing that your book has quite a few visualizations in it and your exam does not.
  4. Oct 27, 2011 #3
    Easier said than done. I always draw pictures, write out my knowns and unknowns but doesn't help me solve the problem. This technique works well with book problems, but when exams come, I think i need more than this.
  5. Oct 27, 2011 #4

    1) What physics course are you in? I'm guessing the second course (Electricity and Magnetism) in a calculus-based format. A LOT of students find this tough because it's really the first time they might be really required to use calculus in a physics course (rather than just observe calculus being done in a derivation or example) ... and some things (like field theory) are really conceptual... added to the fact that there are positive and negative charges(that are often invisible to our naked-eye -- instead of just positive mass). If we know the course.... maybe we can point to some additional materials.

    Edit: oops -- you did mention EM... so I'll first point you to Walter Lewin's MIT lectures if you're in the calc-based version (google this... or ask again for the link).

    Also: if you've passed through a physics course before this (which presumably you did)... how did you "survive" it, so to speak? What techniques worked then that aren't working now?

    2) What is the format of your course? Do you have lab or recitation... and maybe a TA? TA's can be a great help (or not... depending on what they have been assigned and requested to do by the coordinating faculty member)... and we might be able to give you some guidelines as to how to best use the TA's help (if you've tried), office hours (if you've tried to talk to your professor about this), recitation hours (if you have any). What text is used? (Many of us might have materials for certain texts or similar... for example I have a bunch of old tests with solutions scanned for my calc-based classes.... but you'd have to judge their usefulness a bit since even with the same texts professors can have different expectations, and mine are pretty straightforward but probably pretty demanding).

    3) What is the exam format? Does your professor make available copies of old exams... maybe with solutions? Or other practice materials? Are you allowed a card or given an equation sheet (maybe even given it in advance)? Maybe we can then help you use these to best advantage...

    Some general hints without knowing this information:

    Know what is a vector, what is a scaler. Your post talks about signs of work and energy (which are scalers) but then relates problems with signs to vectors (which hints at a misunderstanding in the concepts of work and energy -- which in my class examples, I always looks for the absolute value of... and then look at how the work being done by one object is adding or taking out energy from the second to manually put in signs).

    Know your units. You should know that electric potential is Volts = Joules/Coulomb = Newton meters/Coulomb... Electric field is V/m and that a Volt/m = Newton/Coulomb, etc. Then -- learn to carry the units in every math equation you use, do the math with the units (aka "dimensional analysis") and make sure your final units are right.

    Make yourself an equation sheet (even if you can't use it on tests). On your equation sheet, make notes about units, when/how to use the equation, whether or not the concept is a scaler/vector, etc. Use it when you work practice problems... then see if you can put it aside and work a similar problem (if you can't use it on tests).

    Work as many practice problems as you can until you feel you really understand the material. Do assigned homework. Do practice materials the professor assigns. Do problems around the assigned homework problems. Maybe even try to write your own problem and solve it.
    Last edited: Oct 27, 2011
  6. Oct 27, 2011 #5
    Thanks for the tip, I will probably go to my teacher and ask him this same problem. For my first physics class, I took it two times, first time I dropped and second time i got a C.

    My class usually goes like this. teacher expects us to watch lectures online, then we have this workbook we do in class and discuss the problems. There are no TA's unfortunately, just a bunch of confused students and the teacher.

    On our exams, the teacher will give us a formula sheet that has the same information every time, so equations are not a problem. I do try to get a deeper understanding of the equation so I think I'm good in that area.

    I usually start studying by reading the chapter and making note cards of stuff that I find important. Stuff such as equations and why its that way and rules and what not. Then i attempt the problems but I never seem to get the correct answer the first time (I usually go to the solution guide and see how they did it and try to remember the steps so I can apply it to the next problem). I think I do need to start studying earlier though since I always find my self doing problems the day before exams.

    My classmate told me about Walter Lewin's online lectures and they are great! I wished I knew about it before my exams >.<...

    How else would you guys suggest I study for exams? Like if I finished all the problems, should I do more? Or go over concepts? Also, I find my self taking a long time trying to figure out what concepts to use for what problems. Should I go in the exam knowing exactly what to use? Maybe I am not doing enough problems. What are some of your guy's experience when taking an exam, do things just come really quick?
  7. Oct 28, 2011 #6
    Did you take vector calculus?
  8. Oct 28, 2011 #7
    I found that once you have a solid understanding of the topics, there is really no other way to study (the courses material anyway) than to just do problems. I'll work through every problem in the text on a subject that I am not to good at. Then I like to just make up my own or look on physicsforums homework help and just do someone elses.
  9. Oct 28, 2011 #8
    If you only got a "C" in basic physics after dropping it once, you don't belong in E&M, IMO. The "C" second time around is a very bad sign. The fact that you are having math issues is the second very big sign you don't belong in E&M. You need to get the heck out of ALL physics until you can beef your math up to where it is a trivial part of your physics learning. As was noted elsewhere in these forums, math is the key to physics. You will never NEVER NEVER unlock physics until you can breeze past the math.

    It will only get MUCH worse.

    The advice given by Physics_girl_phd is spot on for someone that has the fundamentals, which isn't you, by the sounds of things. Bail while you can, IMO.
  10. Oct 28, 2011 #9
    Although ThinkToday is quite blunt, I believe he is right. Unless of course you aren't in physics, and are only taking supplementary courses.

    If the latter is the case then I recommend that you take more of an exam approach to your problem solving. Gather a handful of questions from your book that you deem similar to the questions you've seen on your exams, and hammer them out without other resources. This will familiarize you with getting part marks, and help you deal with the panic that strikes when you have no idea how to do an exam question. If you get the question wrong, and then look at the back of the book and do it again, that won't necessarily help you. You have to be able to do questions from bottom to top, with no discontinuity in your understanding.
  11. Oct 28, 2011 #10
    Thanks for all the tips guys, unfortunately, I cannot drop this class. I am transferring next year and I absolutely need to take physics. I am currently taking the last calculus course right now. I guess I just need to work on my math and get my basics down. Any ideas as of how I should start working on this math? Also, some logic seems to be my weakness too
  12. Oct 28, 2011 #11
    To reinforce this: I think making an equation sheet as a study guide for a test has been very helpful for me over the past year. It's not there to just 'stare at' but the act of making the sheet can be helpful in itself (kind of like rewriting notes, but more pointed). As part of a greater study routine, it has been the biggest difference between Bs and Cs and all As for me.

    As part of my exam study routine, the first thing I do is go through and write down any theroems, shortcuts, forumlas and other 'you should know...' type of things (generally at least a condensed version of everything in a shaded box in a math text). For my first Calc III exam this sheet was 4 pages, my first diffeq exam it was only 1 page - the size of my physics sheets have been generally 2-3 pages. I found it especially helpful to 're-remember' things from earlier in the term. This process of writing a sheet is generally the best 30 minutes of studying that I spend for each exam (out of the several hours generally dedicated to the exam).

    After making this sheet (and where the bulk of my studying time is spent) I'll go through a sequence of problems. I follow a rule: I do every problem ending in 3 and 7 for a midterm and every problem ending in 5 for a comprehensive final (from the lessons covered). If I come across a problem that I don't know how to answer, I'll find the rule/law/theroem that supports a solution and write that on my sheet.
  13. Oct 29, 2011 #12


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    I'm a math major, so I won't speak very much about physics, but I will offer what I can from a mathematical perspective.

    I consider some basic, important math concepts to be algebraic manipulations and visual reasoning (geometric). I know that's a pretty hand-wavy description, but I think it will help understand what I'm trying to say. Your level of comfortability and skill with those major areas plays a big role in computational courses. Also, can you elaborate on what you mean by "logic" in this context?

    You mentioned having difficulty setting up an integral properly. Setting-up integrals should've been covered in your first calculus course, or the third course if we're talking about double and triple integrals. However, I'm not clear about what aspect of setting up integrals you're struggling with. Is it choosing the bounds, integrand, etc? Or do you struggle with carrying out the computations? Struggles with the former tend to be more conceptual/visual, while the latter is mechanical/algebraic.

    Another perspective is that calculus should really be included under the "basic math" umbrella for a math, physics or engineering major. Integration techniques need to be second nature to you. That's not to say you won't occasionally forget some uncommon identities and have to look them up, but you've said your struggles are more basic, so I'm assuming that to mean you're having issues with integration and differentiation rules/techniques. If this is the case, I'm afraid that you'll have to grab an old calculus text and start chugging through exercises. Not the most exciting thing to do, I know, but it will help prevent the little mistakes like the issue you mentioned having with negative signs.

    You also claim to be comfortable with drawing pictures, but you mentioned that you're taking the last calculus course at the moment. I'm assuming that by "last" you mean multi-variable calculus. Usually this is the first course in which one is introduced to integration (and differentiation) in 3-dimensions. This might help explain your difficulty in your E&M course, where I believe many of the concepts require functions of multiple variables. At my university, one isn't allowed to even register for E&M without first having obtained a "C or better" in multi-variable calculus and differential equations.

    The last thing I can suggest is that you take your exams to your instructor's office hours and go over everything you did incorrectly. This is extremely important! Even if you can figure out what you did wrong by yourself, the instructor will usually be able to give a better analysis of your methods/reasoning and should provide insightful tips on how to avoid the same errors in the future, and that's what you're after.
    Last edited: Oct 29, 2011
  14. Oct 29, 2011 #13
    There probably aren't enough problems in the basic textbook to hone your skills sufficiently - unless you are a Feynman-manque you won't be able to think on your feet in the exams sufficiently well to pass well. That's how the profs weed out those who *aren't* Feynmans - the ones they *really* want to find. The rest can get Cs and move into business school as far as they are concerned... The only non-Feynman way to succeed is *practice*, and work your socks off, work like an investment banker... that is, every hour..... Go through the Schaum problem books, repeatedly, until you can solve all the problems at the drop of a hat. Go through past papers, if you can get them. Check out problems in similar textbooks - especially those in the library (teach might get his exam questions from them ...:) Practice - lots of it, and then lots more! - makes perfect....
  15. Oct 29, 2011 #14
    I haven't taken E&M, I'm only on Calculus-based classical mechanics. Nevertheless, I disagree with what you said, you don't have to be a "Feynman-manque" whatever that means. The most important thing is knowing how to set up the problem and how to manipulate variables to find the unknown. Once you get the scheme of it you will solve many problems! I can probably write at least a 3 page guide on schemes needed to master calculus-based introductory physics. Its all about knowing what things to look for, and you certainly don't have to be "Feynman" level to master your physics course.

    There aren't enough problems in the book? You don't have to do countless problems, what is needed is knowing how to approach and go about your problem. Once that is picked up it can be generalized to many different things.
  16. Oct 29, 2011 #15
    Thanks for the response! practicing Algebra and the visual math sounds exactly like what I need. As for as integration, its the setting up part that I usually don't understand. I know the steps to get through most integration problems.
  17. Oct 30, 2011 #16
    That sucks.

    Anyways, as a student who is also studying E/M, the advice I can give you is this:

    Don't do ANY practice problems until you've mastered a "concept". By this, I mean that you can derive the next step from the previous step. I'm not sure how far your physics course goes, but I do know that the aim of most E/M courses is to show the discovery of light as an electromagnetic wave. This is found by combining Maxwell's equations in such a way that it forms a differential equation, which can be solved to reveal the constant c, the speed of light.

    The road to that discovery is a long and harsh one full of mathematics. Without the math background, your E/M education will be choppy at best. I remember trying to do this stuff in high school and it was pretty bad...

    I asked you if you took vector calculus up above. I'm not sure if you saw the question, but regardless, the point is that vector calculus and E/M go hand-in-hand. Electric fields are found through Coulomb's law, which requires multivariable integration (albeit it's only single-variable integration done once for each axis). Potentials are found via line integrals and are understood through gradient theory. Gauss' law is a result of the divergence theorem and can't be truly understood unless you study a mathematical proof of it. Studying its proof helps justify which situations it can be used for and which ones it can't. Studying Gauss' law also shows why charge concentrates itself on the surfaces of conductors, a critical idea when studying capacitors. These ideas are passed on to circuit theory. Magnetic fields require a very solid understanding of curl-typed vector fields. So far my course has gone up to here, but later on, I'm sure an application of Stoke's theorem will come in handy.

    Everything I mentioned in the previous paragraph is taught in vector calculus. My physics course actually had vector calculus as its prerequisite. (Of course, I'm taking the version for physics majors, so I'm not sure how choppy the other two versions of this same course is.)

    I'm sure there are ways of getting around learning all the essential math concepts that I've mentioned above, since I know that all biology majors at my university are allowed to take the "easy" physics sequence, which tries to avoid calculus as much as possible. I'm not sure how they do it, but it's not pretty, and most people just end up forgetting after the course is over. I've seen their books and they're pretty poorly written.

    This concerns me, because you said that the way your course works is that you're required to watch video lectures online. I think the biggest roadblock in education is a bad teacher. Really, for any science course, your teacher should be supplemental. If you're learning purely from a teacher, you're not going to get it. The only way you're every going to understand material is if you gather all the facts together and organize them in such a way that you can look at the first fact and derive the rest (with very few reminders or "new" facts in between).

    Anyways, practice problems aren't all that important. You should be doing some practice problems, but physics is about generalization. Each fact leads to the next. The most important problems you'll ever have to face are those bringing you from one step to another, and that requires all the mathematics in between.

    If you're having trouble now, just drop it and take it at your new university. If that is impossible and you persist in pushing through, I would highly suggest looking through a vector calculus book and trying to teach yourself in this order: (curl and divergence), gradients, scalar and vector line integrals, scalar and vector surface integrals, Green's and Stoke's (curl) theorem, and Gauss' (divergence) theorem.

    Take a break from physics and just enjoy the mathematics.

    Are you having a tough time thinking about objects as collections of infinitesimal elements, or of magnetic fields as the summation of infinitesimal vector (found via cross product) elements? Exploiting symmetry and having knowledge of the spherical and cylindrical coordinate systems helps tremendously.
    Last edited: Oct 30, 2011
  18. Oct 30, 2011 #17
    This is an example of what I get confused about.

    in this problem we need to find the difference in potential to find the capacitance.

    I understand that difference in potential is just delta V and that is just V_final - V_initial.

    Why is V_a used as final and V_b used as initial in this problem? Shouldn't it be the other way around. I say this because r_b has a bigger radius than r_a.
    This is exactly the kind of thing that gets me really confused on tests.


    Last edited by a moderator: Oct 30, 2011
  19. Oct 30, 2011 #18
    By convention, you count the positive plate first. It has nothing to do with outside/inside. This is because in circuits, you get some symmetrical situation that looks like this:


    But which way is the voltage pushing the current? You don't really know, so it's necessary to define a convention. Imagine that a positive current i is being pumped clockwise around the circuit situation. (In other words, positive charge elements +dq are flowing in the clockwise direction.) When the flow reaches some equilibrium the top plate will have a total charge of +Q and the bottom plate will have a total charge of -Q. We say that the positive plate has a higher potential than the negative plate, and that the voltage drop across the capacitor in this direction is equal to the voltage applied from the battery. Let the top plate be Va and the bottom plate be Vb. Then the voltage drop Vab ≡ Va - Vb.

    The same convention applies to your spherical capacitor, even though there's no battery.

    Maybe you should examine the common conventions and how they're defined? Simple reasoning processes will always lead you to the correct convention.

    For example, suppose I get the convention wrong. Then after solving the for the voltage, I'll find V= -Q/4πε₀ * (1/ra-1/rb). To check whether my sign is correct, I'll examine the potential of the inside shell alone, since the two shells are superpositions of one another, and by the shell theorem the outer shell doesn't contribute to electrical forces inside it. The voltage of the interior shell is V= Q/4πε₀ * 1/ra. This contradicts the wrong convention I previously found, so I know that I made an error in the sign convention and I can correct for it.

    The images you provided are huge, by the way.
    Last edited: Oct 30, 2011
  20. Oct 30, 2011 #19


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    In terms of integration, the challenge is basically finding out what is "changing", and what kind of measure you are trying to calculate.

    If you can a) describe the object (line, surface, etc) b) define what is changing and c) Define the domain over which this happens then you should be ok for most problems.

    When you do your problems take a bit of time out to think about these things and hopefully you should get a better understanding of what is going on when these integrals are being setup.
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