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Investigating the viscosity of liquids.

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Investigating the viscosity of liquids.


All liquids stick to a solid surface so that their velocity gradually decreases to zero as the wall of the pipe or the container is approached. A fluid is therefore sheared when it flows past a solid surface, and the opposition force set up by the fluid is called its viscosity. The speed of flow of liquids is dependent on their viscosity, which controls their resistance to flow. Viscosity can be explained as  internal friction exhibited to some degree by all fluids. A property of a fluid which tends to prevent it from flowing. It arises in liquids because the movement of a molecule relative to neighbouring molecules is opposed by intermolecular forces between them. This resistance acts against the motion of any solid object through the fluid or against motion of the fluid itself past stationary obstacles. Viscosity also acts internally on the fluid between slower and faster moving adjacent layers. High-viscosity fluids have higher resistance to flows and low-viscosity fluids have lower resistance to flows. Generally speaking, the lower the viscosity of a fluid, the ‘runnier’ it is (such as water) and the higher the viscosity of a fluid, the ‘thicker’ it is (such as and syrup).

Measuring Viscosity using Stokes’ law:

It has been decided to use the falling sphere viscometer, in order to find the viscosity or liquids during this investigation. It involves timing a sphere falling at terminal velocity through a fluid. From stokes findings, he was able to quantify viscosity by studying the force exerted on a spherical object as it moves through a fluid. This force is known as the viscous drag force and is described by the relationship known as Stokes’ law:

F = 6πrηv

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°C higher than the desired temperature, when it is carefully poured into the long plastic tube carefully using a funnel. This is due to taking into account that the temperature of glycerol will drop during the experiment. During the preliminary tests, it is found that on average during the experiment, the temperature of the glycerol drops by about 4°C. By starting at +2°C, to -2°C, of the desired temperature, avoids giving biased results.

        Prior to taking measurements, the distance travelled by the ball bearing will be marked. 0.3m will be marked from the bottom of the plastic tube, when the bottom of the ball bearing is seen to pass through this line, timing will start. When the bottom of the ball bearing touches the bottom surface of the plastic tube, the stopwatch will be stopped. As the terminal velocity of the ball bearing needs to be measured in this experiment, another line is drawn 0.1m above the 0.3m line. This is where the glycerol will be poured up to. This 0.1m of glycerol should be sufficient for the ball bearing to reach terminal velocity before timing starts at the 0.3m line.

        During the experiment, the 1cm diameter ball bearing will be used. Preliminary tests found that this ball bearing drops through the glycerol at a lower speed than larger ball bearings. This maybe due to having a lower weight in relation to the upthrust and viscous drag. This gives more accurate results as the timing is initiated by the operator, there is a greater percentage uncertainty if the ball bearing drops too quickly. The ball bearing used will not be released  from air, as the ball bearing will accelerate quicker in air and thus may enter the glycerol at a higher speed than the terminal velocity in glycerol.

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        I have also made the assumption that the ball bearing reaches terminal by the time it reaches the 0.3m line. However, this cannot be proven with our limited knowledge and equipment provided. But I believe that the error produced as a result of this should not be very large, as the ball bearing reaches terminal velocity very quickly.

        I think that I have conducted this experiment accurately within experimental limitations. I have tried my best to ensure that all the variables apart from temperature was controlled, and I think that my results should be accurate enough to draw a reliable conclusion.

        Next time, I could eliminate the error of using a narrow tube by containing the fluid in a wider container that would allow much more space around the ball bearing. Thus, error due to increased resistance at the side of the tube would be eliminated. Therefore, results obtained would be more accurate.

        I can also increase the distance that the ball bearing has to travel. As a result, the percentage of time the ball bearing is travelling at terminal velocity is increased. Thus, the inaccuracy due to the time when the ball bearing was accelerating would be less significant in my results. Therefore, the margin of error would be decreased, thus increasing the accuracy of my results. In addition, sets of phototransistors can be used next time to eliminate the operator errors I caused by timing the fall of the ball bearing with the naked eye and by hand.

        In the future, I would like to see how concentration of the fluid will affect its viscosity. I will change the concentration by mixing it with water. I think that as the density of water is much lower than glycerol fluid, the higher concentration of the fluid, the greater the viscosity.

A r t h u r   C h a n   1 2 L                Physics- Viscosity Investigation

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