Determining the viscosity of glycerol at 20*C using Stokes Law.

[candide94]

Homework Statement

I used a home made falling sphere viscosimeter to obtain:

terminal velocity = 0.06216 ms^-1

I used a micrometer to obtain:

radius = 0.001835m

and, because my value for glycerol viscosity was way off the known value of 1.495 Pa s, I decided to measure the density of the steel spheres and of the glycerol to make sure these weren't anomalous (compared to the 'standard' values):

density of steel sphere = 9775000 g/m^3
density of glyerol = 1205900 g/m^3

The steel density is a little too big, and the glycerol density a tiny bit too small, but this in fact got my answer closer to the known value. However its still a way off (see below)

Homework Equations

viscosity = (2*g*(r^2)*(density of sphere - density of glyercol)) / (9*terminal velocity)

The Attempt at a Solution

viscosity = (2*9.81*(0.001835^2)*(9775000-1205900))/(9*0.06216)
=1011.94 g m^-1
=1.01 kg m^-1
=1.01 Pa s

which is only about thirds of the known value! (1.495 Pa s). This is closer than what it was when I used the standard density values, however. But I can't figure out what has gone wrong. To work out terminal velocity I videod the falling spheres and used graphics analysis software to work out when they reached terminal velocity, calibrating from a mark on the screen. To keep the temperature constant I placed the test tubes in a water bath (using straight water from the tap) - the thermometer said it was at 20*C, but it was just a liquid in glass one, rather than digital... (maybe the temp was higher than I thought?) The glycerol density is pretty near to the expected value so I can't imagine its own properties having affected it. But i could be wrong.

If anyone could help me with sources of uncertainty I haven't thought of, please reply! If you know of any glycerol properties that explain these anomalous results, please reply!

Thankyou

 

[anathema]

Thank you for sharing your experiment and results on the use of a homemade falling sphere viscosimeter. It is always interesting to see new methods being used in scientific research.

From your post, it seems that you have taken great care in measuring the relevant parameters and have tried to troubleshoot any potential sources of error. However, there are a few things that may have contributed to the discrepancy between your measured viscosity value and the known value for glycerol.

Firstly, the use of a homemade viscosimeter may introduce some uncertainty in your results. It is possible that the dimensions of your apparatus, such as the size and shape of the container and the placement of the spheres, may not be precisely controlled. This could affect the accuracy of your terminal velocity measurements, which would in turn affect your calculated viscosity value.

Additionally, the temperature of your glycerol may have played a role in the discrepancy. As you mentioned, the thermometer you used was not digital and may not have been accurate. It is also possible that the temperature of the water bath was not as constant as you thought, which could affect the viscosity of the glycerol.

Lastly, the density values you obtained for the steel spheres and glycerol may not be entirely accurate. The density of a material can vary depending on its purity and how it was measured. It may be worth double-checking these values and using more precise measurements if possible.

In conclusion, while your effort to troubleshoot potential sources of error is commendable, it is possible that the use of a homemade viscosimeter and potential inaccuracies in temperature and density measurements may have affected your results. I suggest repeating the experiment using more precise equipment and carefully controlling all relevant parameters to obtain more accurate results.

Best of luck with your research.