1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding New Resistance

  1. Apr 25, 2009 #1

    Steven7

    User Avatar

    A wire has uniform cross sectional and resistance R. Then the wire is pulled so that the length increases by 10%. How to find the new resistance? Assumption: The wire thins uniformly.
     
    Steven7, Apr 25, 2009
  2. jcsd
    Phys.org - latest science and technology news stories on Phys.org
  3. Apr 25, 2009 #2

    LowlyPion

    User Avatar
    Homework Helper

    What equation would you think to apply to figure this?
     
    LowlyPion, Apr 25, 2009
  4. Apr 25, 2009 #3

    Steven7

    User Avatar

    R=rho*l / A
    I just don't get the part how the length of the wire affects the cross sectional area/radius.
     
    Steven7, Apr 25, 2009
  5. Apr 25, 2009 #4

    LowlyPion

    User Avatar
    Homework Helper

    Don't you figure that there is a conservation of mass?

    What happens to the volume of wire when you stretch it?

    Volume = L * A

    if the volume is constant, then if L = 1.1*L then A must equal A/1.1
     
    LowlyPion, Apr 25, 2009
  6. Apr 25, 2009 #5

    Steven7

    User Avatar

    I got it. I thought the increase of L decrease the r to r/1.1.
    Can I ask another question about resistance?
    A square carbon film of thickness 5x10^-7m, rho 4x10^-5 ohm m is formed on an insulator rod of diameter 3mm.What is the length of the rod so that the carbon film on its curve surface has a resistance of 100 ohm.

    I tried:
    RA/rho = L
    100[3.142*(1.5mm+5x10^-7)^2]/4x10^-7 = L
    But the answer is 1.18cm which is different from mine. Where did it gone wrong
     
    Last edited: Apr 25, 2009
    Steven7, Apr 25, 2009
  7. Apr 25, 2009 #6

    LowlyPion

    User Avatar
    Homework Helper

    Consider that your cross sectional area is the cross section of a thin shelled tube.

    A = L * T = π * d * 5*10-7

    So:

    100Ω = 4*10-5 * L / (3.14*.003*5*10-7)
     
    LowlyPion, Apr 25, 2009
  8. Apr 25, 2009 #7

    Steven7

    User Avatar

    I'm sorry but according to your equation, I don't get the answer which is 1.18cm. By the way, I also don't understand the statement "carbon film on its curve surface" means. Do you mind explaining it?
     
    Steven7, Apr 25, 2009
  9. Apr 25, 2009 #8

    LowlyPion

    User Avatar
    Homework Helper

    It's an insulating rod, so only the surface foil conducts.
    If you unwrap the foil off the rod its cross sectional area is width (π * d) and its thickness is 5*10-7

    Area then is W*T = (π * d) *( 5*10-7 ) = 3.14*.0003*5*10-7

    Substituting into

    R = 100 = p*L/A

    yields

    100*3.14*.0003*5*10-7 / 4*10-5 = L = 1.1775*10-2m
     
    LowlyPion, Apr 25, 2009
  10. Apr 25, 2009 #9

    Steven7

    User Avatar

    I struggle with A = π r^2 and you cleared those. Thanks a lot.
     
    Steven7, Apr 25, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Finding New Resistance

  1. Calculate the resistance, then find the current at the new voltage (Replies: 2)

  2. Finding resistance (Replies: 1)

  3. Resistance: find the equivalent resistance. (Replies: 4)

  4. Finding Resistance (Replies: 2)

  5. Find the resistance? (Replies: 6)

Loading...