Introduction
Tension forces act on liquid surfaces, the thread in them is pulled into circular arcs. The pull is referred to as surface tension of the liquid. Surface tension exists uniformly in all directions. The forces affect the shape and motion of liquids that have open surfaces. Tension forces can be understood regarding forces that occur between the molecules. Repulsion occurs in molecules when they are close but attract when they are far from each other. Inside the liquid, molecules are close to each other. Thus, there are slight repulsions. Molecules at the surface of the liquid are some distance far from the neutral distance thus they attract each other. Molecules at the surface possess a higher energy that molecules at the interior of the liquid. The free-energy per unit area of the surface is the surface tension (T) of that liquid measured with Newton per meter or dynes per centimeter, N/m, and dynes/cm respectively. Considering figure 1. Below, assuming that only a single surface exists, the work, W, done to move the rod can be represented as:
.. (For).1
Figure 1. Change in surface area
The surface area of the liquid increases by ld when the rod is moved. Since each surface has the energy of T-Joules, the increase in free surface energy becomes T-ld, the work done can be equated by the force and increase in free surface energy.
Or 2
Contracting the surface of the liquid, the surface layer heats up. Some heat flows to the interior of the liquid. Some potential energy of the surface molecules is changed into heat, and the remaining energy is available to do the work on the rod. The energy in the system available for doing the work is referred as the free surface energy.
When a ring show in figure 2 below is used to determine the surface tension, the net force needed to pull this ring out of the liquid can be represented as;
3
That is, the total force is given by the surface tension area plus the weight of the ring. The total force acting downwards is due to the surface tension;
4
Where L is the mean of outer and inner circumferences, that is .
Figure 2. The ring experiment
In this experiment, errors arise because forces due to surface tension are not always directed vertically downwards as shown in figure 3. The true value of F is less than the expected F in equation 4. To correct this, a correction factor, G is introduced. G, a non-dimensional parameter depends on the circumference and ring size. Equation 4 becomes,
5
Figure 3. The actual forces acting on the ring.
Aims and objectives
The purpose of this experiment was to determine the value of surface tension in water and paraffin oil from a downward pull on a horizontal ring that was fastened to spring. Factor G is a ratio of F and the density of the liquid.
Apparatus and materials
A piece of platinum-iridium ring equipped with a stirrup, L = 4cm
A jolly balance with an adjustable platform
A spring
Distilled water
Paraffin oil
A scale to find the density of the paraffin oil density
A petri dish
A Bunsen burner
A level
Tweezers
A striker
Procedure
Familiarization with the set up was the first step of the experiment – the level of the setup and the scale on the jolly balance was checked to understand the take measurements with the equipment.
Figure 4. A jolly balance
The ring was placed over a Bunsen burner to remove any dirt or grease from it. The petri dish was filled with distilled water and placed on the support P as shown in figure 4 above. The ring was hung from the spring of the apparatus, and the dish with the distilled water moved up towards it until the ring touched the water. The readings of the balance position were taken, and the balance raised using the wheel W until the ring broke free from the pull of distilled water. The readings on the balanced position were taken.
Next, the petri dish was filled with paraffin oil and placed on the support P as shown in figure 4 above. The ring was hung from the spring of the apparatus, and the dish with the paraffin oil moved up towards it until the ring touched the oil. The readings of the balance position were taken, and the balance raised using the wheel W until the ring broke free from the pull of the oil. The readings on the balanced position were taken.
Data Analysis
Results
Experiment 4
Reference reading = 1.89cm
Weights
Distilled water
Paraffin
Calculations
Distilled water
Using equation 2. The average length of the immersion can be found as; (0.41+0.41)/2 = 0.42 cm.
. L =0.00042 meters, F = 3.064*10-4
=0.00072 *100
0.072
For paraffin oil
The average distance of immersion is (0.98 cm+0.28 cm+0.50 cm+0.48 cm+0.58 cm)/5 = 0.564 cm. Using equation 2, the solution becomes;
. L =0.000567 meters, F = 3.5*10-4
Discussion
Theoretically, fresh (distilled) water at room temperature has a surface tension of about 0.073 dynes/cm. From the experiment, the calculated surface tension of distilled water is 0.072 dynes/cm. The experiment is not the expected value of surface tension in water. This is as a result of uncertainties and error that occur during the experiment. From the experiment, the surface tension of paraffin is 0.062 dynes/cm. The theoretical value of surface tension of paraffin at room temperature is 0.064 dynes/cm. The error is caused by experimental uncertainties (Winn 201).
The introduction of factor G in the expression of surface tension is important because the pull does not occur vertically downwards but at an angle.
Conclusion
The experiment was straightforward. The results from the experiment were genuine, and no difficulties were experienced in the experiment. The calculated values of surface tension in water and paraffin oil were almost the theoretical values. In future, to achieve perfect values, elimination of the uncertainties will give perfect results.
Work cited
Kontogeorgis, Georgios M, and Sören Kiil. Introduction to Applied Colloid and Surface Chemistry. , 2016. Print.
Rothstein, Aser, and E N. Harvey. The Enzymology of the Cell Surface Tension at the Cell Surface. Vienna: Springer Vienna, 1954. Internet resource.
Troy, David B. Remington: The Science and Practice of Pharmacy. Philadelphia, PA: Lippincott, Williams & Wilkins, 2005. Print.
Winn, Will. Introduction to Understandable Physics. Bloomington, IN: AuthorHouse, 2010.Print.
