The aim of my investigation will be to analyse the relationship between several variables, which are defined by stokes law, and conclusively to apply these in order, to calculate the viscosity of the fluid from my results. To see how successful my experiment was, I will be comparing my results with those from external sources such as a Textbook; this would therefore determine how accurate my experimental results were. All fluids, from liquids to gases illustrate the property of viscosity to some degree.
Viscosity is caused by internal friction due to the strong intermolecular forces; hence it is affected by temperature in liquids. It is measured in Pascal’s per second unless the viscosity is kinematic. Viscosity can be thought of as fluid friction, just as friction between two solids resist the motion of one over the other, also possible to cause an acceleration of one fluid relative to the other. Liquids will vary from “thin” having a lower viscosity like water, to “thick” having much higher viscosity like honey or treacle.
There are many ways to measure viscosity, for example “the line spread test”, which involves a fixed quantity of liquid being allowed to flow out of a container and spread onto a flat surface or “redwood viscometer”, which involves the liquid to flow through a narrow tube driven by its own head of pressure, but due to the lack of apparatus, I chose to do the falling ball viscometer, as this experiment gave an absolute measurement of viscosity, and was too, the most feasible method to be performed in a lab.
I will be unable to calculate the viscosity using the above equation, because “F” will not be able, to be measured directly with the method I will be using. So I would rearrange the above equation to: When the ball bearing is travelling at terminal velocity the net force is zero, because there is no acceleration. This is known when used in the equation F=ma. For the ball bearing to be in terminal velocity all forces acting on the ball bearing should be in equilibrium. This is to be achieved by the spheres weight acting downwards, and in addition to the drag force produced, there is either an up thrust or buoyancy force produced.
Up thrust/Buoyancy always acts when an object is immersed in a fluid, the object displaces fluid around it, and hence it arises. Below is an image showing “Forces acting on a falling ball”. [pic] So, what I will be expecting is, after the ball has been dropped into a liquid, it should first accelerate due to the downward force of its weight, I then predict that the viscous drag will increase due to the increase in speed, the downward force due to gravity will decrease, this is where acceleration stops.
From this point the object is in terminal velocity, and will remain at this velocity, if no other force acts upon this object. Before conducting the experiment there are several measurements I will have to record to calculate the viscosity of the fluid. Measurements and Formula’s regarding Ball bearing …….. The volume of fluid displaced is theoretically equivalent to the volume of sphere. We can Write Down an expression for the forces acting on the ball bearing, which will combine to give a resultant force of zero.
We can now return to the experiment itself, as we have understood the theory lying behind calculating viscosity. I was conducting two experiments, one with varying temperature, and the other with different size ball bearings. Experiment Planning, Equipment, 3 Different size ball bearings, Stopwatch, 3 Measuring Cylinders, 3 Water baths, set at 35oc 45oc, 25oC, Golden syrup, Weighting scale, Micrometre, 2 Spoon, and Tray I used the micrometre for measuring the diameter of my ball bearing; this was he most appropriate instrument to use. The precision of the micrometre was to 0. 01mm, which was So, for my first experiment, I started off with “the varying temperatures”. I firstly filled three 100ml3 measuring cylinders with Golden syrup, half an hour in advance, so that air bubbles trapped and other uneven surfaces in the fluid can even out, this will prevent any eddies or turbulence while the ball bearing is falling through the cylinder.
The test tubes were placed in water baths, 1st cylinder was placed in a water bath, having a temperature of 35oc and the second cylinder was placed in a water bath, having a temperature of 45oc and the third cylinder was the control which was exactly 25oC. The Cylinders had to be left in the water bath until it usually maintained an approximate constant temperature. I had to be cautious when reading of the thermometer as there is always a possibility of parallax errors. I had to also make sure that I read of the thermometer from eye level, as this prevented parallax errors and reduced inaccuracy in the experiment.
The scale of the thermometer went up 1 degree at a time; the scale did not affect my experiment because all my measurements were all Whole numbers, but if they were between two degrees the scale of the thermometer would have been inconvenient. The next step involved me placing a big white piece of paper behind each cylinder as an ordinary backdrop, this will enable me to identify the ball easier and distinguish while it is falling. I then had to choose a ball bearing, which was an average size, to measure the ball bearing I used an micrometre as I thought this piece of equipment was very accurate in measuring very small objects.
I had to wash the ball bearing under water, and wipe with a paper towel, I did this because I thought the charge on the ball bearing could affect its velocity. I repeated this procedure for all ball bearings so that my experiment was not criticised for unreliability. The next part of my experiment involved me dropping the ball bearing through the fluid, but I had to think carefully and take into consideration my reflex reactions, because I was going to time the ball bearing falling between two points with a stopwatch.
After doing several online tests my reflex reactions averaged out to be 0. 2 seconds. I was planning to use light gates at first, as this would have been much more accurate in measuring the time of the fall, but the tube was rather large to have light gates around it, however we will not have access to such expensive equipment in the first place. I selected two points on the measuring cylinder, 20 and 80; I thought that this was reasonable because, the ball bearing had approximately 15 cm to reach terminal velocity.
So when the bottom of the ball bearing when past the 80 mark at terminal velocity I activated the stopwatch, and when the bottom of the ball bearing went past the 20 mark I deactivated the stopwatch. I personally think that my method did not have any significant risks or hazards. The only minor risk I faced was keeping all the measuring cylinders from knocking over and spilling. To prevent this I put a few weights on the base of the measuring cylinder. Another safety precaution was to keep all ball bearings in a dish; if they were on the floor it could be very dangerous to me and others around me.
The above graph shows a strong negative correlation, the increase in temperature shows the decrease in time. From this graph I can say that this graph is inversely proportional. From these results I am also able to construct a distance- time graph. The gradient of the graph will show the velocity of the ball.
It was necessary to repeat readings, because this would keep my experiment reliable and accurate. I however did not have any odd results, if I was to have any outliers in my results, it would be most likely to be activating and deactivating the stopwatch. The ranges of results were within 1 degree of each other results.
Independent and Dependant Variables
The variables that I will have to keep constant in my first experiment (temperature effect on viscosity). I will be keep the temperature as the dependant variable and keep all other variables as the independent variables, for example the size of the ball bearing, the same liquid and the same atmosphere. All my variables will be kept constant, the same ball bearing will be used each time, and I will have to use a magnet to take out my ball bearing from the fluid, and then be washed under cold water and then dried on a paper towel. This will keep my experiment fair.