Page 42 of 49 FirstFirst ... 17323334353637383940414243444546474849 LastLast
Results 821 to 840 of 976

Thread: Need help with science/math schoolwork? Post here!

  1. #821
    Logios's Avatar Senator
    Join Date
    Aug 2010
    Location
    Copenhagen, Denmark
    Posts
    1,212

    Default Re: Need help with science/math schoolwork? Post here!

    How many pure serial, pure parallel and combined did you get?

  2. #822
    edse's Avatar Domesticus
    Join Date
    Jun 2011
    Location
    Finland
    Posts
    2,293

    Default Re: Need help with science/math schoolwork? Post here!

    4 pure serial, 4 pure parallel and 6 combined.

  3. #823
    Logios's Avatar Senator
    Join Date
    Aug 2010
    Location
    Copenhagen, Denmark
    Posts
    1,212

    Default Re: Need help with science/math schoolwork? Post here!

    I found 17. Of the combined s-p I got 3 par. within ser. 3 ser within par. and 3 par within par. Maybe one is redundant.

  4. #824
    edse's Avatar Domesticus
    Join Date
    Jun 2011
    Location
    Finland
    Posts
    2,293

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by Logios View Post
    I found 17. Of the combined s-p I got 3 par. within ser. 3 ser within par. and 3 par within par. Maybe one is redundant.
    3 parallel within parallel, isn't they all the same?

  5. #825
    Logios's Avatar Senator
    Join Date
    Aug 2010
    Location
    Copenhagen, Denmark
    Posts
    1,212

    Default Re: Need help with science/math schoolwork? Post here!

    You are right, they are not different, and the resistance is the same (0.977 Ohm) when calculated separately in spite of the extra junction. Then I get 14 possibilities as well. A good thing I did not choose electronic engineering.

  6. #826

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by edse View Post
    4 pure serial, 4 pure parallel and 6 combined.
    It's these plus the Y connection and the Δ connection

    Edit: No,the Δ is redundant

    Edit 2: I don't know, I tend to think it's not redundant after all because you can have 3 terminals
    Last edited by Timoleon of Korinthos; February 06, 2013 at 03:24 PM.
    "Blessed is he who learns how to engage in inquiry, with no impulse to hurt his countrymen or to pursue wrongful actions, but perceives the order of the immortal and ageless nature, how it is structured."
    Euripides

    "This is the disease of curiosity. It is this which drives to try and discover the secrets of nature, those secrets which are beyond our understanding, which avails us nothing and which man should not wish to learn."
    Augustine

  7. #827
    edse's Avatar Domesticus
    Join Date
    Jun 2011
    Location
    Finland
    Posts
    2,293

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by Timoleon of Korinthos View Post
    It's these plus the Y connection and the Δ connection

    Edit: No,the Δ is redundant

    Edit 2: I don't know, I tend to think it's not redundant after all because you can have 3 terminals
    Is it possible to calculate equivalent resistances for those? It seems to be more of a tool to straighten out unclear connections.
    Last edited by edse; February 06, 2013 at 03:37 PM.

  8. #828

    Default Re: Need help with science/math schoolwork? Post here!

    You are right, they don't have an equivalent because they involve three terminals, not two. I can't think of another 2 combinations though, I also get 14. Unless you count each resistance by itself as a combination, in which case you would have 17. Do you need to calculate equivalent resistances or just draw the different combinations?
    Last edited by Timoleon of Korinthos; February 06, 2013 at 03:50 PM.
    "Blessed is he who learns how to engage in inquiry, with no impulse to hurt his countrymen or to pursue wrongful actions, but perceives the order of the immortal and ageless nature, how it is structured."
    Euripides

    "This is the disease of curiosity. It is this which drives to try and discover the secrets of nature, those secrets which are beyond our understanding, which avails us nothing and which man should not wish to learn."
    Augustine

  9. #829
    edse's Avatar Domesticus
    Join Date
    Jun 2011
    Location
    Finland
    Posts
    2,293

    Default Re: Need help with science/math schoolwork? Post here!

    Draw them and calculate them. I'll leave those two as the last connections but without calculations. I bet the teacher has done something wrong.

  10. #830
    edse's Avatar Domesticus
    Join Date
    Jun 2011
    Location
    Finland
    Posts
    2,293

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by edse View Post
    Draw them and calculate them. I'll leave those two as the last connections but without calculations. I bet the teacher has done something wrong.
    The teacher had done a mistake.

    I have a system of equations that I don't know how to solve properly.

    a + b = 1/2
    c + d = 1/2
    a + c = 2/3
    b + d = 1/3

    a, b, c and d are all probabilities and their sum is 1

  11. #831
    John Doe's Avatar Primicerius
    Join Date
    Apr 2009
    Location
    Scotland
    Posts
    3,530

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by edse View Post
    I have a system of equations that I don't know how to solve properly.

    a + b = 1/2
    c + d = 1/2
    a + c = 2/3
    b + d = 1/3

    a, b, c and d are all probabilities and their sum is 1
    I tried with a =0, the result is (a,b,c,d)=(0,1/2,2/3,-1/6), I found it weird to have a negative probability though.
    then with a=1, the result is (a,b,c,d)=(1, -1/2,-1/3,5/6), weirder...

    Seems there are infinate solutions, but you want the one with all 4 being positive. I can't help you more.

  12. #832

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by edse View Post
    The teacher had done a mistake.

    I have a system of equations that I don't know how to solve properly.

    a + b = 1/2
    c + d = 1/2
    a + c = 2/3
    b + d = 1/3

    a, b, c and d are all probabilities and their sum is 1
    This is a linear system of equations. The simplest treatment for such problems is the Gaussian elimination.

    The issue in this case is that after the first steps you end up with the following equivalent system:

    1a + 1b + 0c + 0d = 1/2
    0a + 1b + 0c + 1d = 1/3
    0a + 0b + 1c + 1d = 1/2
    0a + 0b + 1c + 1d = 1/2

    This shows that essentially you have 4 unknowns but only 3 linearly independent equations, consequently one degree of freedom, so if you want a solution you have to choose a value for one of the unknowns yourself, plug it in the equations and solve for the other three. This means of course that you can obtain infinite sets of solutions, as your parameter can take an infinite number of values.

    Your physical constraints are that since a,b,c,d represent probabilities, the value of each of them must lie between 0 and 1. So one pretty straightforward solution is to set c=1/3, which will bring about d=1/6, b=1/6 and a=1/3. All of them within the [0,1] range, therefore having a physical meaning. I suspect that this is what your teacher expects.


    At this point I want to ask, have you been taught linear algebra in your course, or are you supposed to be familiar with it? If no, skip the rest. If yes, the fact that there is no unique solution can be proven:

    In your system A = [1 1 0 0; 0 0 1 1; 1 0 1 0; 0 1 0 1;]

    and b = [1/2 1/2 2/3 1/3]T

    You will note that m=n=4, det(A)=0 and rank(A)=rank([A b]) = 3, so a solution exists and depends on n-rank = 4 -3 = 1 parameters
    Last edited by Timoleon of Korinthos; February 10, 2013 at 10:02 PM.
    "Blessed is he who learns how to engage in inquiry, with no impulse to hurt his countrymen or to pursue wrongful actions, but perceives the order of the immortal and ageless nature, how it is structured."
    Euripides

    "This is the disease of curiosity. It is this which drives to try and discover the secrets of nature, those secrets which are beyond our understanding, which avails us nothing and which man should not wish to learn."
    Augustine

  13. #833
    edse's Avatar Domesticus
    Join Date
    Jun 2011
    Location
    Finland
    Posts
    2,293

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by John Doe View Post
    I tried with a =0, the result is (a,b,c,d)=(0,1/2,2/3,-1/6), I found it weird to have a negative probability though.
    then with a=1, the result is (a,b,c,d)=(1, -1/2,-1/3,5/6), weirder...

    Seems there are infinate solutions, but you want the one with all 4 being positive. I can't help you more.
    I had better luck in the begining trying with b=0 as well as d=0 getting (1/2,0,1/6,1/3) and (1/6,1/3,1/2,0)

    Quote Originally Posted by Timoleon of Korinthos View Post
    This is a linear system of equations. The simplest treatment for such problems is the Gaussian elimination.

    The issue in this case is that after the first steps you end up with the following equivalent system:

    1a + 1b + 0c + 0d = 1/2
    0a + 1b + 0c + 1d = 1/3
    0a + 0b + 1c + 1d = 1/2
    0a + 0b + 1c + 1d = 1/2

    This shows that essentially you have 4 unknowns but only 3 linearly independent equations, consequently one degree of freedom, so if you want a solution you have to choose a value for one of the unknowns yourself, plug it in the equations and solve for the other three. This means of course that you can obtain infinite sets of solutions, as your parameter can take an infinite number of values.

    Your physical constraints are that since a,b,c,d represent probabilities, the value of each of them must lie between 0 and 1. So one pretty straightforward solution is to set c=1/3, which will bring about d=1/6, b=1/6 and a=1/3. All of them within the [0,1] range, therefore having a physical meaning. I suspect that this is what your teacher expects.


    At this point I want to ask, have you been taught linear algebra in your course, or are you supposed to be familiar with it? If no, skip the rest. If yes, the fact that there is no unique solution can be proven:

    In your system A = [1 1 0 0; 0 0 1 1; 1 0 1 0; 0 1 0 1;]

    and b = [1/2 1/2 2/3 1/3]T

    You will note that m=n=4, det(A)=0 and rank(A)=rank([A b]) = 3, so a solution exists and depends on n-rank = 4 -3 = 1 parameters
    I did the Gaussian elimination and with one more step I got

    1a + 0b + 0c - 1d = 1/6
    0a + 1b + 0c + 1d = 1/3
    0a + 0b + 1c + 1d = 1/2
    0a + 0b + 1c + 1d = 1/2

    If I put d=t it leads to

    a = 1/6 + t
    b = 1/3 - t
    c = 1/2 - t
    d = t

    From here I can only make the constraint 0 ≤ t ≤ 1/3 and I guess that's the most I can get out of it.

    This is only part of an exercise, another part implies that d = 1/3 and since that is one possible solution I believe it is solved.

  14. #834

    Default Re: Need help with science/math schoolwork? Post here!

    Well, I don't know, yes, if you need to find a set of specific figures go for d=1/3 or whatever makes sense in the context of the exercise.

    But I would say that the way you have expressed every unknown as a function of t would be the best way to formulate the solution in the general case, because it displays the parametric dependence from the outset as well as making obvious the range of values that your parameter can take in order for the physical constraints of the problem to be respected.
    "Blessed is he who learns how to engage in inquiry, with no impulse to hurt his countrymen or to pursue wrongful actions, but perceives the order of the immortal and ageless nature, how it is structured."
    Euripides

    "This is the disease of curiosity. It is this which drives to try and discover the secrets of nature, those secrets which are beyond our understanding, which avails us nothing and which man should not wish to learn."
    Augustine

  15. #835

    Default Re: Need help with science/math schoolwork? Post here!

    Yo!

    What raw materials are used to make USB flash drives? Its a pain in the ass, i searched whole internet but didnt find anything..

  16. #836

    Default Re: Need help with science/math schoolwork? Post here!

    Magic.

  17. #837

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by Sphere View Post
    Magic.
    Thanks for your help biaatch, but i already got my answer.

  18. #838
    Praepositus
    Join Date
    Mar 2009
    Location
    California
    Posts
    5,616

    Default Re: Need help with science/math schoolwork? Post here!

    Probably a stupidly obvious chem question which I have trouble answering.
    "Separate samples of a solution of an unknown soluble ionic compound are treated with KCL, Na2SO4, and NaOH. A precipitate forms only when Na2SO4 is added. Which cations could be present in the unknown soluble ionic compound?"
    I don't really need an answer, just an explanation of how to progress through this problem would be greatly appreciated.

  19. #839
    Logios's Avatar Senator
    Join Date
    Aug 2010
    Location
    Copenhagen, Denmark
    Posts
    1,212

    Default Re: Need help with science/math schoolwork? Post here!

    Look up which ions would make precipitations (look for numbers for solubility) with KCl (not many if any) and NaOH (some here). Precipitations are expected if the solubility of any new possible species is lower than those already in solution. The cations forming precipitations with Cl+ and/or OH- is not expected to be present in the solution. Then look up which sulphate salts that have a low solubility and discard those not having high solubilities as hydroxides or chlorides.

    Edit: If this was unclear then please let me know, and I will try to give a better explanation.
    Last edited by Logios; February 20, 2013 at 06:45 AM.

  20. #840

    Default Re: Need help with science/math schoolwork? Post here!

    Could somebody help me with my electricity & magnetism homework?

    Question 1:

    Two condensers connected in series will be charged such that one gets +Q charge on one of its plates, and the other -Q.

    Schematic: ------(-Q)| |(a)------(b)| |(+Q)-------

    Can somebody prove (or at least show) intuitively to me why the plate "a" gets a +Q charge, and the plate "b" -Q? Ie, why do the inner plates have charges equivalent to the outer ones?

    Question 2:

    A dialectic of length d is inserted into a condenser made up of two plates a distance D apart (d<D). Afterwards, the condenser acts like it is two condensers connected in series. Why?
    Last edited by Nikitn; February 24, 2013 at 03:02 AM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •