1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Find current flowing through the wire

  1. Feb 13, 2012 #1
    1. The problem statement, all variables and given/known data

    A wire of total length 34.00 m, consists of equal lengths of Cu followed by Al, both having the same diameter of 0.80 mm. A voltage difference of 86.0 V is placed across the composite wire. What is the current flowing in the wire? Use rho=1.72x10-8 ohm-m for the resistivity of copper and rho=2.82x10-8 ohm-m for the resistivity of aluminum.

    2. Relevant equations

    RT = RCu + RAl
    R = rho*L/(pi*r2)
    R = V/I

    3. The attempt at a solution

    Solve R = rho*L/(pi*r2) for both Cu and Al and add together for RT

    (34*1.72*10^(-8))/(pi*.04^2) + (34*2.82*10^(-8))/(pi*.04^2)
    = 0.000307089463

    I = V/RT

    = 280048.684 ohm

    Keep getting the wrong answer, what am I doing wrong?
  2. jcsd
  3. Feb 13, 2012 #2


    User Avatar
    Science Advisor

    Three things:

    (1) If the total length is 34m, and it is made up of two pieces of equal length, how long is each piece?

    (2) How many meters is 0.4mm?

    (3) What are the units of current?
  4. Feb 13, 2012 #3
    Writing ohm was just a typo (was entering A before) and for some reason I was reading .80 mm as 80 mm, so I converted 80 mm -> .08 m. Completely forgot about halving the length though, thanks.

    (17*1.72*10^(-8))/(pi*.0004^2) + (17*2.82*10^(-8))/(pi*.0004^2)
    = 1.53544731

    = 56.009737 A

    That right then? I have one try left to get it right :s
  5. Feb 13, 2012 #4


    User Avatar
    Science Advisor

    That looks right to me.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook