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Coeff. of Linear Exp. of glass from vol expansion of content

  1. Jun 12, 2015 #1
    1. The problem statement, all variables and given/known data
    The coefficient of volume expansion of olive oil is 0.68 × 10-3 K-1. A 1-liter glass beaker is
    filled to the brim with olive oil at room temperature. The beaker is placed on a range and the
    temperature of the oil and beaker increases by 25 C°. As a result, 0.0167 liters of olive oil spill
    over the top of the beaker. What is the coefficient of linear expansion of glass?

    2. Relevant equations
    ΔV = βViΔT
    ΔV = (3α)ViΔT
    β = Coefficient of Volume Expansion
    α = Coefficient of Linear Expansion

    3. The attempt at a solution
    Vi = 1.0 L (for both olive oil and the beaker)
    Ti = 20°C = 293.15 K
    ΔT = 25°C = 298.15 K
    Vlost = 0.0167 L
    βolive = 0.68x10-3 K-1
    αglass = ?

    Calculate volume expansion of beaker
    ΔVolive = (0.68x10-3 K-1) * (1.0 L) * (298.15 K)
    = 0.202742 L

    ΔVglass = ΔVolive - Vlost
    = 0.202742 L - 0.0167 L
    = 0.186042 L (Volume increase for glass beaker)

    Calculate Coefficient of Linear Expansion for Glass Beaker
    0.186042 L = (3αglass)(1.0 L)(298.15 K)
    0.186042 L = 894.45 L⋅K * αglass
    αglass = 0.186042 L / (894.45 L⋅K)
    = 0.000208 / K = 2.1x10-4 K-1

    This is very close to one of the actually answers (there are 5). One of them is 2x10-5 K-1. The teacher has said that he has removed the correct answer for some of the questions, and that we are to explain why are answer is correct if that is the case. Is my answer correct? If not, what did I do wrong?
     
  2. jcsd
  3. Jun 12, 2015 #2

    SteamKing

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    Hint: think significant figures. Does your result carry the correct number of significant figures?
     
  4. Jun 12, 2015 #3
    I believe so. There is 1 significant figure (the 1 liter volume), so the answer I would have is 2x10-4.

    But the issue isn't the sig. figs. (at least I don't think), it's the exponent. My answer has 2x10-4, while the closest answer is 2x10-5. I've gone over my calculations and I believe I have done it correctly.
     
  5. Jun 12, 2015 #4

    SteamKing

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    You're getting confused on how to deal with temperatures in your calculations.

    Remember, degrees Kelvin and degrees Celsius use the same temperature scale, but each scale has a different starting point. For the change in temperature ΔT of 25 °C, you have assumed that this is the same as a temperature of 298.15 °K, which is wildly incorrect for calculating a change in volume.
     
  6. Jun 12, 2015 #5
    But if 20°C (293.15 K) is the initial temperature and the final temperature is Ti + ΔT = 20°C + 25°C = 293.15 K + 298.15 K = 591.3 K, and ΔT is Tf - Ti, then isn't ΔT = 591.3 K - 293.15 K = 298.15 K? What am I missing? Don't they give the change in temp right away? :(
     
  7. Jun 12, 2015 #6

    SteamKing

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    You're treating a temperature reading and a change in temperature as the same thing, which is not correct.

    If an initial temperature reading is 20 °C and the temperature increases by 25 °C, then the final temperature is 45 °C, which is not equivalent to 591.3 °K. You can check this by converting °K to °C. Remember, a temperature reading of 0 °C on the Celsius scale is the same as a temperature reading of 273.15 °K on the Kelvin scale, but a change in temperature of 25 °C is the same as a change in temperature of 25 °K.

    On both the Celsius and Kelvin temperature scales, the difference in temperature between the freezing point of water and the boiling point of water is 100°.
     
  8. Jun 13, 2015 #7
    Ah, my mistake. I understand what I was doing wrong. Thank you.
     
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