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Viscosity and temperature, density is changing...

  1. Jun 20, 2017 #1

    MPZ

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    1. The problem statement, all variables and given/known data
    Hi, I am doing physics lab on finding coefficient viscosity of fluids and how it changes by temperature by dropping a marble into fluids, finding terminal velocity, then using stoke's law to find viscosity. (using density of fluid, sphere, sphere diameter etc). The equation is found here https://en.wikipedia.org/wiki/Stokes'_law under Terminal Velocity.... I have just realized something and encountered a problem. For this experiment to work, the fluid at all temperatures should have the same density, however changing the temperature will change the density!!! How can I keep the density constant? Does what I am saying make sense?

    2. Relevant equations
    The equation of stoke's law is found here https://en.wikipedia.org/wiki/Stokes'_law under Terminal Velocity. It is derived when the viscous drag force plus the buoyant force equal the weight of the ball at terminal speed

    3. The attempt at a solution
    Trying to find a way to keep density constant but no success yet!
     
  2. jcsd
  3. Jun 20, 2017 #2
    You can't keep density constant if the temperature is changing. For most common fluids, there should be tables of density vs. temperature that you could find. Failing that, you can just assume the density is constant - there will be some inaccuracy in your result, but it should not be huge. Generally viscosity varies with temperature much more than density. For example, between 10 and 80°C, the density of water decreases by about 3%, but its viscosity decreases by about 70%. (http://www.viscopedia.com/viscosity-tables/substances/water/) The density of the marble will also change, but much less than that of the fluid.
     
  4. Jun 20, 2017 #3

    MPZ

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    will this be ok for my lab. do you think I should calculate the density for every temperature of the fluid and substitute it into stoke's equation? thanks
     
  5. Jun 20, 2017 #4
    If you have reliable density-temperature data, yes.
     
  6. Jun 20, 2017 #5

    MPZ

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    what if i calculate the density myself?
     
  7. Jun 21, 2017 #6
    Again, if you have a reliable density vs. temperature equation. How were you going to calculate the density?
     
  8. Jun 21, 2017 #7

    MPZ

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    finding mass of fluid and using density=mass/volume for every temperature
    Do you have an idea of a procedure that will show the relation between viscosity and temperature??
     
  9. Jun 21, 2017 #8
    Ah, you're going to measure it. (To be really accurate, you'd have to account for the thermal expansion of your volume-measuring container, but this is likely to be small compared to that of the fluid.)
    Measure it. Isn't that what you were going to do?
     
  10. Jun 21, 2017 #9

    MPZ

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    Will it be ok to assume that the density of the marble and fluid is constant since the decrease will be 2 to 3% and use the value of the density at room temperature? Or will that make my lab report stupid?
    also, will this assumption still make me observe the relation between viscosity and temperature?
     
  11. Jun 21, 2017 #10
    I think that's OK for a lab, unless your lab instructions specify that higher accuracy is required. You should state the assumption you have made and the justification for it. Then your lab report will certainly not be stupid. (You might get the supplementary question: if you know the density varies by 3%, that implies you know how the density varies with temperature - as indeed it is tabulated for water - so why not use these accurate values?) You should also be aware that if your fluid is not water, the density change may be greater - the coefficients of thermal expansion are typically higher for organic liquids than water - but still much less than the change in viscosity.
     
  12. Jun 21, 2017 #11

    MPZ

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    The fluid I will use is Honey, here is its table (http://www.viscopedia.com/viscosity-tables/substances/flower-honey-blended/) I don't want to calculate the density or use the accurate values because i need to have one independent/dependent variable while others should be controlled.
    Independent: Temperature (40°C, 50°C, 60°C, 70°C, 80°C).
    Dependent: Viscosity
    BTW, I am a high school person who is doing this for the IB internal assessment
     
  13. Jun 21, 2017 #12

    MPZ

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    does what I said make any sense?
     
  14. Jun 21, 2017 #13
    This should be OK because the change in viscosity is very large over a relatively small temperature range, over which the change in density is only about 1%. I think the uncertainty in your viscosity measurements is likely to swamp any effect due to change in density.
    I'm not sure about this as a general principle, though. The density is controlled in the sense that it is determined by the temperature: if you set the temperature, you set the density. It cannot be "controlled" in the sense of being kept constant as you change the temperature, because that is physically impossible. As you are not measuring viscosity directly, but deriving it from the measured terminal velocity using a formula involving density, you will always get more accurate results using true density values than by assuming a constant value.
     
  15. Jun 21, 2017 #14

    MPZ

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    thank you so much for answering my concerns. I have to ask my supervisor for this, I think I will end up using a constant value but then write about it in the "Sources of error" section and how I should have used the true density values in the "Improvements" section. Thank you again legend! If I have more concerns can I just ask you? ;) <3
     
  16. Jun 21, 2017 #15
    You should also bear in mind that "honey" is not a pure substance; there will be inevitable variations in composition between batches, even of the same type of honey, so you should not be surprised if your density and viscosity values are different from those tabulated. In a case like this it would be better to make one accurate measurement of density and use it throughout, than to rely on tabulated values.
     
  17. Jun 21, 2017 #16

    MPZ

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    That is what I thought too. I will use the same type of honey/from same company. I will measure the density at room temperature and use it throughout, right?
     
  18. Jun 21, 2017 #17
    Sounds like your best bet.
     
  19. Jun 21, 2017 #18

    MPZ

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    ok thanks again
     
  20. Jun 21, 2017 #19

    DrClaude

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    Staff: Mentor

    Not only is honey not a pure substance, it is a natural substance with enormous variation. You should only use a single jar or, if that is not possible, a set of jars with the same batch number.
     
  21. Jun 21, 2017 #20

    MPZ

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    Ok, I will try to do that, thanks
     
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