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What is the independent variable in Boyle's Law?

  1. Jan 8, 2015 #1
    I asked my teacher, and I was told that volume is the independent variable, and that pressure is dependent. The textbook I'm using said the same thing. After looking up some graphs of the law, I found that both pressure and volume were used as independent variables.

    It seems counter-intuitive to me, that volume would be the independent variable. Does that mean that if I have two gasses, Gas A and Gas B (← lesser volume), that Gas B should be under greater pressure, because of it's lower volume? If pressure is measured as force in Newtons / unit of area, then why would volume matter, if we only care about the the 2D area in contact with the atmosphere? Unless, it is the surface area in contact with the environment that we care about for pressure, which would make sense.

    To me, having pressure be the independent variable makes much more intuitive sense, as a greater pressure should decrease the volume.

    Thanks for any help.
     
  2. jcsd
  3. Jan 8, 2015 #2

    Bystander

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    "Intuition" is the last method to use in the sciences. Two variables: pressure and volume. Which one could be controlled? Volume. Therefore volume is the "independent" variable for the series of measurements used as the basis for Boyle's Law. Some other experimental situation, pressure might be more easily controlled and be the "independent" variable. "Independence" is NOT an absolute property of any particular variable, it is a matter of what variable is selected to be varied independently to observe the behavior of a dependent variable.
     
  4. Jan 8, 2015 #3
    Is this assumption correct?
     
  5. Jan 8, 2015 #4

    Bystander

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    "... the surface area in contact with the environment that we care about for pressure ...."
    As stated, the "assumption" makes no sense.
    Try rewording it, and we'll see if we can't straighten out the confusion.
     
  6. Jan 8, 2015 #5
    When measuring pressure, it is measured as the force in Newtons/surface area.
     
  7. Jan 8, 2015 #6

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    More strictly, the normal force per unit area; "normal" meaning the component of force perpendicular to the surface. Close enough.
    Let's complete this question with the detail you need to understand it: if you have the same number of moles of two gases, A and B, or two same size samples, A and B, of a single gas, at the same temperature (that much was built into the discussion that introduced you to the concept), THEN, B is at greater pressure because of the lower volume. Make sense?
     
  8. Jan 8, 2015 #7
    Yes, thank you. That makes a lot more sense.
     
  9. Jan 9, 2015 #8

    DrDu

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    Which variable is dependent and which independent is not answered by the law but by your experimental setup. You could vary the volume and measure pressure or vary the pressure and measure volume.
    More precisely this is a question about the statistical error model of some statstical regression to determine the relationship between V and P.
    Both situations are highly idealized as one usually supposes the independent variable to be free of error while the measured or dependent variable is measured with some error.
     
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