The force will be calculated at the end of the experiment. Measure the length of the needle in meters using a ruler before starting the experiment.
Mark the center of the material to be used for your beam (straw, plastic ruler) and drill or poke a hole through it; this will be the fulcrum point (the point that allows the beam to rotate freely). If you are using a plastic straw you can just poke a pin or nail right through it. Drill or poke a hole at each end of the beam ensuring that they are the same distance from the middle. Thread a string through each hole to serve as holders for the balance dishes. Make sure that there is 1 string for each hole at either end. Rest the nail horizontally between two stacks of books so that the center beam can rotate freely.
Hang the box or dish from one end of the beam. Poke small holes in the sides of the dish and thread the string through to hold up the dish.
This is called counterbalancing. The clay does not affect the calculations because it is balancing out the beam.
Make sure the string holding the needle in place remains taut once the needle is on top of the water.
Count out a number of pins or drops of water and weigh them. Determine the individual weight of each drop or pin by dividing the total weight by the number of pins or water drops. For example, let’s say 30 pins weigh 15 grams: 15/30 = 0. 5. Each pin weighs 0. 5 grams.
Count the number of pins or drops of water needed to remove the counterweight from the water’s surface. Record each reading. Repeat the exercise several times (5 or 6) for more accurate readings. Calculate an average of the results by adding the total number of pins needed in each trial and dividing that by the total number of trials.
Multiply the number of pins added to the dish by the weight of each pin. For example, 5 pins at 0. 5 g/pin = 5 x 0. 5 = 2. 5 g. Multiply the amount of grams by the conversion factor 0. 00981 N/g: 2. 5 x 0. 00981 = 0. 025 N.
Continuing our example, let’s say the needle was 0. 025 m long. Plugging the variables into the equation yields: S = F/2d = 0. 025 N/(2 x 0. 025) = 0. 05 N/m. The surface tension of the liquid is 0. 05 N/m.
The height the liquid rises can be used to calculate the surface tension of that liquid. Cohesion causes water to form bubbles or droplets on a surface. When a liquid is in contact with air, the molecules feel attractive forces towards each other and make a bubble on the surface. Adhesion causes the meniscus that is seen in liquids when they cling to the sides of a glass. It is the concave shape at the top of the liquid seen at eye level. [7] X Research source An example of capillary action is watching water rise in a straw placed in a cup of water.
When working through this equation, make sure all of your units are in the proper metric form: density in kg/m3, height and radius in meters, and gravity in m/s2. If the density of the liquid is not given, you can look it up in a reference book or calculate it using the equation density = mass/volume. The unit for surface tension is one newton per meter (N/m). A Newton is equal to 1 kg-m/s2. To work out the units on your own, simply solve the equation with just units. S = kg/m3 * m * m/s2 * m. Two of the meter units cancel out two of the per meter units and you are left with 1 kg-m/s2/m or 1 N/m.
If you repeat this with different liquids, make sure the dish is thoroughly cleaned and dried before adding the next liquid. Alternatively, just use separate dishes for each liquid.
To measure the radius, simply place a ruler across the top of the tube and determine the diameter. Divide the diameter by 2 and you have the radius. You can buy these tubes online or from a hardware store. It can be difficult to accurately measure small changes in the height the liquid will rise in a straw or wide tube. As the height to which the water will rise is inversely proportional to the diameter of the tube (narrower tube = higher rise) this experiment is much easier to do with a narrow transparent capillary tube. These can be purchased at low cost online, but confirm the inside diameter is provided (typically around 1mm-1. 2mm) and both ends are open. As these are fragile and made of glass, ensure care when handling them.
For example, let’s say we are measuring the surface tension of water. Water has a density around 1000 kg/m3 (we will use approximate values in this example). [12] X Research source The variable g is always 9. 8 m/s2. The radius of the tube is . 029 m and the water rises 0. 0005 m. What is the surface tension of the water? Plugging the variables into the equation yields: S = (ρhga/2) = (1000 x 9. 8 x 0. 029 x 0. 0005)/2 = 0. 1421/2 = 0. 071 J/m2.
Make sure the penny is completely clean and dry before beginning the experiment. If there are other liquids on the penny, the experiment will not be accurate. This experiment does not allow you to calculate surface tension, but just determine surface tensions of different liquids relative to each other.
Write down how many drops it takes for the liquid to flow over the side of the penny.
Try mixing a little bit of dish soap to the water and dropping again to see if the surface tension changes.
Substances with a higher surface tension will have more drops on the penny than substances with a lower surface tension. The dish soap lowers the surface tension of the water, using fewer drops to fill the penny.
