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Homework Help: Linear expansion coefficient of glass

  1. Nov 13, 2008 #1
    Given: The linear expansion coefficient of glass is 9 × 10−6 (◦C)−1. An automobile windshield has dimensions of 60 cm by 400 cm. What minimum spacing around the wind-shield is needed to prevent the windshield from breaking if the temperature changes by 150◦F? Answer in units of mm.

    I understand I use the equation alpha*Length*delta(t) but it won't pop out the right answer and I think the dimensions are what is throwing me off. Any advice is appreciated!
     
  2. jcsd
  3. Nov 13, 2008 #2

    Doc Al

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    Show what you did. I would have chosen the 400 cm dimension to work with, as it will change the most. Did you convert the temp change to degrees C?
     
  4. Nov 13, 2008 #3
    I did not change to degrees Celsius. The conversion is (5/9)*(150-32) so my new temp is 65.556. Also, I have to change cm -> mm. so the dimension would be 4000 mm. So, my final equation would be (9e-6)(4000)(65.556) ??
     
  5. Nov 13, 2008 #4

    Doc Al

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    Yes, but don't forget that that's the total increase in that dimension. So what minimum spacing would you need at each end?
     
  6. Nov 13, 2008 #5
    My answer from that comes out to be 2.36 but I don't know how to find the minimum spacing piece that you're talking about.
     
  7. Nov 13, 2008 #6

    Doc Al

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    If the entire glass expands by 2.36, how much space do you have to allow at each end?
     
  8. Nov 13, 2008 #7
    the given dimensions + expansion value.
     
  9. Nov 21, 2008 #8
    so i dont know if you allready figured out your problem.... but i just did that exact problem. the error that you made was that the temp that is given is a (delta t). to convert from F to C it is just (delta t)*(5/9).... you dont subtract 32. that should fix it! after that it is just:

    delta L = alpha*Lo*delta t and that should be your answer.
     
  10. Nov 22, 2008 #9

    Doc Al

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    Good catch! (I wasn't paying attention when the OP did the conversion. :uhh:)

    Fix that conversion. You're converting a temperature change, not a temperature.
     
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