Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Calculating resistance

  1. Mar 31, 2016 #1
    Hi,

    I am just starting out learning about DC curcuits and I have been presented with a formula in my learning materials to calculate the resistance of a 1km length of 0.6mm diameter copper wire.

    The formula is attached as an image with the Answer 60Ω

    My question is how would you enter this formula into a calculator as I can't seem to get the answer 60. Do I have to set the calulator to a specific setting? do we I have to break the calculation into different stages?

    I don't know whether the format of my notes is messed up or its just me doing something wrong.

    Thanks for the help
     

    Attached Files:

  2. jcsd
  3. Mar 31, 2016 #2

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    I believe that it's a matter of order of operation (you know, putting parentheses in the proper places). I punched in the values and got 60.1252--which is the correct answer rounded.
     
  4. Mar 31, 2016 #3

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    How did you do it? Could you post what you typed into the calculator?
     
  5. Mar 31, 2016 #4
     

    Attached Files:

  6. Mar 31, 2016 #5

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    That's odd. I did the same thing. This is basic arithmetic, so calculator settings should not really matter. What answer did you get? I see that your calculator is in degrees, but radians vs. degrees doesn't matter here at all. What are you're other settings?
     
  7. Mar 31, 2016 #6
    Its set to Normal Mode in degrees
     
  8. Mar 31, 2016 #7

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    What was answer did you exactly get? From there, I can work backwards from your answer.
     
  9. Mar 31, 2016 #8
    3.757825045
     
  10. Mar 31, 2016 #9

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    Hold on, I'm thinking . . .
     
  11. Mar 31, 2016 #10

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    The only thing I could think of was that perhaps your calculator was on a different base system, but I don't think so . . . These are my all settings:

    Display Digits: Float 6
    Angle: Degree
    Exponential Format: Normal
    Real or Complex: Real
    Calculation Mode: Auto
    Vector Format: Rectangular
    Base: Decimal

    Compare this with your calculator. And here's mine, for the sake of it:
    calc.png
     

    Attached Files:

    • calc.png
      calc.png
      File size:
      105.3 KB
      Views:
      34
  12. Mar 31, 2016 #11
    Im going to get a calculator app for my phone and give it another try. Thanks for your help at least I know Im not entering it in wrong
     
  13. Mar 31, 2016 #12

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    You know, I'll try that too. I really want to know what's wrong now!
     
  14. Mar 31, 2016 #13

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    I received the same answer :eynman:
     
  15. Apr 1, 2016 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Ahh the youth of today. I would have thought it quite possible to put down the calculator and work out a simple sum like that on paper.
    There's only a Pi (use three or four sig figs), a 60, a 1.7 and a 4 involved.
     
  16. Apr 1, 2016 #15

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    Don't worry, I don't know about the OP, but I still love paper and No. 2 pencils as much as you do (and if I'm not youth, I don't know who is)
     
  17. Apr 1, 2016 #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I get the impression that almost everyone is youth - relative to me (and a few other old PF gimmers):smile:
    So few people would even consider doing any arithmetic on paper, these days. It's refreshing to read that you would, though PQ.
     
  18. Apr 1, 2016 #17
    To the OP: In your calculator, you have entered ##R = \rho \frac{(L/\pi D^2)}{4}## instead of ##R = \rho \frac{L}{\pi D^2 / 4}##. Notice that the first one gives ##R = \rho \frac{L}{4 \pi D^2}## and the second one (the correct one) gives ##R = \rho \frac{4L}{\pi D^2}##
     
  19. Apr 1, 2016 #18

    Merlin3189

    User Avatar
    Gold Member

    Although I'm in SophieCentaur's camp, I got as far as 1700/9π without anything. Then I did reach for a calculator. But when I got down to SC's comment, I tried that with pencil and back of envelope, just to see what happened, and with Pi at 22/7 it comes out at 60.1

    But, since you are asking about calculator methods, I'd say your big mistake is to try to put it all into a single calculation. You CAN do that, but as far as I can see, even using a calculator, there is nothing to be gained in time nor accuracy, let alone mistakes, in combining the two calculations. Just work out A, then press reciprocal and enter the rest of your second formula, pausing only to note that 2.8x10-7 m2 for A is about right. If the second expression were more complex, just pop your first answer into a memory and use it wherever is convenient.

    D sq x pi / 4 = recip x ρ x L = is one more keystroke than ρ x L / ( D sq x pi / 4 ) = but requires no thought nor planning and provides a small check along the way.

    Even now Axmls has pointed out your possible error, I can't tell from your calculator display whether you did what he says or whether you put in something else. I can't read it as Axmls does. But he's probably correct, since your answer is a factor of 16 out.
    If someone wrote that expression on paper as shown on your screen, I'd say it was wrong, but my reading of it does not give your answer.





     
  20. Apr 1, 2016 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Goooood boy. If I could reach, I'd give you a gold star in your book. :smile:
     
  21. Apr 3, 2016 #20

    ProfuselyQuarky

    User Avatar
    Gold Member
    2016 Award

    :DD (when I said youth, I meant <18)
    Thanks, I just get annoyed when I see 8-year-olds and 9-year-olds with TI-89 calculators doing math homework :eek:

    I just want to peer over and say, "Umm, 321 plus 447 is called mental math for a reason!!"
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted