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Homework Help: Linear Thermal Expansion

  1. Oct 15, 2013 #1
    1. The problem statement, all variables and given/known data

    A brass bar and an aluminium bar are each attached opposite each other to immovable walls. (There is a diagram, but I think the idea is clear enough). There is a gap between the two bars of 1.3x10-3m at 28°C.

    At what temperature will the gap be closed?

    coefficient of linear expansion of brass = 19x10-6 (C°)-1
    aluminium = 23x10-6 (C°)-1

    Length of brass bar = 2.0m
    Length of aluminium bar = 1.0m

    2. Relevant equations

    ΔL = [itex]\alpha[/itex]LoΔT

    3. The attempt at a solution

    I'm not sure where to begin really. Obviously part of the problem is that the two rods are not going to meet in the middle as they expand.

    I have 2 unknowns for both rods in the equation for linear expansion, so it's not really helpful to rearrange for ΔT and set them equal to each other.

    If you could get me started that would be great,

  2. jcsd
  3. Oct 15, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What must the sum of the ΔL's equal?
  4. Oct 15, 2013 #3
    1.3 x 10^-3 m

    Can I say:

    ΔL + ΔL = [itex]\alpha[/itex]LoΔT + [itex]\alpha[/itex]LoΔT

    1.3x10^-3 = (3.8x10^-5 + 23x10^-6)ΔT

    ΔT = 21.3 degrees C

    28 + 21.3 = Temp when bars will touch
    Last edited: Oct 15, 2013
  5. Oct 15, 2013 #4

    Doc Al

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    Staff: Mentor

    I didn't check your arithmetic, but that is definitely the way to solve it.
  6. Oct 15, 2013 #5
    I'll check and double check.

    Thanks a lot!
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