# Practical Investigation Into Viscosity

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Introduction

Practical Investigation Into Viscosity Aim: To investigate the rate of descent of an object falling through a liquid due to gravity and the factors which affect the viscosity of the liquid. Theory: Viscosity is the resistance a material has to change in form. This property can be thought of as an internal friction. Something which is very important when investigating viscosity is laminar flow. If a fluid or gas is flowing over a surface, the molecules next to the surface (the ones clinging to the walls) have zero speed. As we get farther away from the surface the speed increases. This difference in speed is a friction in the fluid or gas. It is the friction of molecules being pushed past each other. You can imagine that the amount of clinging-ness between the molecules will be proportional to the friction. This amount of clinging-ness is called viscosity. Thus, viscosity determines the amount of friction, which in turn determines the amount of energy absorbed by the flow. Viscosity can be determined in the following way: Work is force times distance and it takes energy to do work whilst power is the energy times time. Imagine a school laboratory filled knee deep with oil. On top of the oil is a large plate of metal that we want to slide across the surface to the other side of the room. If you think about the cube of oil under the metal plate resisting the motion we can determine a unit for viscosity: o Friction is a force (in Newtons) acting along the direction of travel times the distance (in meters) so it is a Nm. o This frictional force obviously scales with the surface area (in m2) of the top of the cube, which brings us to Nm/m2. o We move the plate a distance (in meters) so now we have Nm/m3 of work. o Multiplying by time (in Seconds)

Middle

All five ball bearings seemed to reach their terminal velocity at the same timed interval. Viscosity of liquid The second set of experiments was to show the affects of a more viscous liquid (engine oil) on the decent of a ball bearing. Using the same method as the experiment for surface area, the ball bearing took longer to descend through the liquid. Temperature On the third experiment I varied the temperature of the oil. The difference was at a higher temperature the ball bearing descended faster through the liquid. Angle of descent Finally the fourth experiment compared angles of which the ball bearing descended through a liquid. The results showed a greater angle to the vertical reduced the speed of decent through the liquid. Initial apparatus The following apparatus was used to complete the investigation: 2.2 metre plastic tube (colourless) Diameter 0.03 metre 900ml Water 900ml Engine oil Five ball bearings with diameter and masses as follows: Class Mass (g) Diameter (mm) Very small 0.12 2.96 Small 0.88 6.98 Medium 4.07 9.98 Large 8.96 13.98 Very large 16.69 16.9 Clamp and clamp stand. Tape measure Stop clock Tray Bung Magnet Bunsen to heat oil (in a pan). Note: Each experiment shall be repeated 3 times and an average calculated to plot a graph. Practical investigation into Viscosity in liquids(Stokes Law) * Bookmark this Page * Email this Page * Plagiarism and How to Cite From www.essaybank.co.uk Introduction When dealing with fluid/mechanical systems, it is important to know what affects the rate of descent of an object through a liquid. There are many factors that affect the descent of an object through a liquid such as: 1) Temperature of the liquid 2) Mass* of object 3) Size/surface area of object 4 Viscosity of liquid 5) Angle of descent Temperature I would like to investigate the correlation between temperature and time of descent. Reading suggests that the colder the liquid the longer it will take for the object to reach the bottom.

Conclusion

From the discussion I mentioned the way in which the liquid flows around the ball bearing as it descends. This proved along with my experiments that the larger the ball bearing the slower the rate of descent. This occured in both engine oil and water. 2) The second major factor was the viscosity of the liquid in which the ball bearing descends. The more viscous liquid made the ball bearing descend at a slower rate. I can also conclude that the temperature of the liquid in which the ball bearing descends through can increase or decrease the rate of descent. With a higher temperature the liquid becomes less viscous and the ball bearing descends at a faster rate. 3) I have found that the angle in which the ball bearing descends through will decrease the speed of when it will reach the bottom. However as I mentioned in the discussion an extra force was acting upon this and therefore made this experiment invalid. 4) The final conclusion to be drawn from my investigation, is that the ball bearings seemed to reach their terminal velocity in the same timed interval. For experiment 1 it was 40-60 cm and for experiment 2 this was also 40-60. Therefore I would be able to conclude with a third liquid that it may be possible that the liquid does not effect the point in which a ball bearing reaches its terminal velocity. However I can conclude that the size of the ball bearing and also the mass does not effect where it reaches its terminal velocity. As you already know, if two objects of the same size but with different masses are dropped from the same height they will descend and hit the ground at the same time. It is only air resistance that will affect the descent if the objects size is slightly different. I can relate this to my experiments in finding the terminal velocity of the ball bearings through the liquid, and therefore explain why the occurance happened with only a slight varience with the very large ball bearings.

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