Investigation into the effect of temperature on viscosity

Introduction:

The task is to investigate into the effect of temperature on viscosity based on two main variables, the temperature of the honey (the fluid used) and the time it takes for a ball bearing to travel through a distance at its terminal velocity. Using Stoke’s law and knowledge of the flow of fluids investigate how the fluid temperature and the terminal velocities of a ball bearing are related to viscosity.

Basic setup:

First thoughts:

The above rough experimental design has much room for improvement. A greater level of accuracy can be achieved by using a combination of more sensitive equipment and a better overall setup. A formula linking viscosity and time will be derived and a graph of viscosity against temperature will be produced.

Derivation:

Diagram of variables and constants:

F= viscous drag force, N/force, N

r= radius, m

η= co-efficient of viscosity, Nsm-2

v= velocity, ms-1

W= weight, kg

g= gravitational field strength, 9.81ms-2

a= acceleration, ms-2

ρ= density, kg m-3

s= displacement, m

t = time, s

A formula linking η (viscosity) and t (time) must be produced.

The following consists of rules and principals that could be applied to this situation.

Archimedes’ Principle: When a body is partially or totally immersed in a fluid the upthrust is equal to the weight of fluid displaced.

Viscosity can be defined as a measure of resistance to flow that a fluid offers when subjected to shear stress. The viscosity is due to the friction between layers of the fluid.

If an object is dropped into a fluid it experiences two primary forces, weight, which acts in the downward direction and also upthrust (equal to the weight of fluid displaced). If these values are equal then there will be no resultant force.

Cooling the honey

Ice is added to the water surrounding the measuring cylinder to cool the honey, again the honey is stirred to produce a constant temperature throughout the honey.

Apparatus:

Plan:

1. Before beginning the experiment all equipment will be checked, i.e. functionality of the stopwatch and heating equipment.
2. Next the apparatus will be setup as in diagram 3.
3. Stability of the apparatus will now be checked.
4. The markings will be added using a pen and ruler, the distance markings are made with a gap of 11cm. The volume marking will be made at the 100cm³ mark. This level of honey allows the ball enough time to reach terminal velocity (as found in the preliminary investigation) and also allows the ball to start at its rest position. If the ball was dropped through the air and into the honey it could accelerate past its terminal velocity (in honey), meaning it would be travelling faster than it should as it moved through the honey. Therefore the ball is dropped from the surface of the honey with its initial velocity equalling 0ms-1
5. A micrometer screw gauge will be used to accurately measure the diameter of the ball bearing in use.
6. The stop watch will be setup and readied while the correct temperature is being met. The hot temperatures will be met by heating the honey to the desired temperature (through the transfer of heat energy from the hot water). The colder temperatures are met in a similar way, by reducing the temperature of the water. Once the thermometer has reached the desired temperature the experiment may begin.

Ball bearings with different radii could be used. This would allow a greater spread of results giving a better average selection of viscosities.

The main problem with the investigation was human error with the timing. This problem can be completely eliminated with the use of light gates. The light gates would have to be able to measure across a large distance due to the specific path the ball bearing takes through the honey. But once setup correctly they could time with far more precision.

The other problem with the investigation is the temperature. The way this could be solved is by heating the whole room to the desired temperature. This would require large industrial machinery, but would allow the honey to be accurately heated/cooled to the desired temperature, maintaining equilibrium of temperature throughout the honey. A more likely solution would be the use of a water bath, although this would not be as effective.

A more suitable release mechanism could be implemented so that the ball is dropped directly onto the honeys surface and is dropped centrally as well. This would help prevent the problems caused when the ball moves at angles and is affected by the extra friction imposed on it by the walls of the cylinder.

A complete change of the basic experimental setup could be used. A viscous drag viscometer could be used to measure the viscosity. The apparatus could be enclosed in the same temperature controlled room, this would allow the viscosities to be accurately measured and the results from the two investigations could be compared. One problem that may occur with this is that a metal is used to control the needle for the display. A suitable metal that is not affected by heat (through expansion) would have to be used.