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Viscosity and Temperature

AS PHYSICS. Viscosity and Temperature. We can measure the viscosity of a liquid from its terminal velocity using Stokes’s law: F = 6  η rv If a falling sphere has reached its terminal velocity, then, as we have seen before: W = F + U

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Viscosity and Temperature

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  1. AS PHYSICS Viscosity and Temperature We can measure the viscosity of a liquid from its terminal velocity using Stokes’s law: F = 6ηrv If a falling sphere has reached its terminal velocity, then, as we have seen before: W = F + U Where W is the weight of the sphere and U is the upthrust acting on the sphere. Using the notation that we have used before, we can substitute into this equation to give the terminal velocity of the sphere in oil as: v = 4/3 r2 g (ρsphere - ρoil ) / 6η © TPS 2008

  2. AS PHYSICS Viscosity and Temperature We can measure the radius of the sphere, the density of the sphere and the density of the oil, at any temperature we choose. We can then calculate the viscosity at that temperature, simply by measuring the terminal velocity v. The diagram shows a possible setup. The three bands are used to delineate two equal distances that a ball bearing is allowed to fall through. If it takes the same time to fall through each, it must be travelling at its terminal velocity.

  3. AS PHYSICS Viscosity and Temperature Don’t forget that if you use a steel ball bearing, you can remove it from the measuring cylinder using a strong magnet.

  4. AS PHYSICS Viscosity and Temperature Timing the fall, also allows us to calculate the terminal velocity of the ball (as long as we know the distance the ball has fallen). v = (d1 + d2) / (t1 + t2) Several repeat runs will allow us to get a more meaningful, average value and show us how repeatable the value we are using is. Having the information outlined in the previous slide, we can then calculate the viscosity of the liquid. Time t1 Distance d1 Distance d2 Time t2

  5. AS PHYSICS Viscosity and Temperature Finally, you need to think of a good way of presenting your results. Variations like this are usually treated graphically. The independent variable should be on the abscissa (x-axis) and the dependent variable on the ordinate (y-axis). With some liquids, there might be a large variation in viscosity with temperature over several orders of magnitude (factors of 10). Such results are best represented using a log graph.

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