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Lab 6: Waves Lab
There are two principal types of currents that occur in our oceans: surface currents, which usually have a depth of several hundred meters and are driven by global wind patterns, and deep currents, which move slowly at greater depths beneath the ocean surface and are driven primarily by differences in water density.
Part 1: Shallow Water Waves
Objective: Students will understand the behavior of shallow water waves in a simulated laboratory environment, at various water depths.
Aside from the currents driven by seismic activity, a majority of the wave activity we see on the surface of lakes and oceans are driven by wind. Wind-generated waves represent mechanical energy that has been transferred to the surface of water. The size of a wind-generated wave is determined by a few major factors – the velocity of the wind, the length of time the wind is blowing and the distance over which the wind has been in contact with the surface of the water (referred to as fetch).
Regardless of wave type, all waves have some basic features in common. The “anatomy” of a wave consists of the crest, which is the “peak” or highest point of a wave, the trough, which is the “valley” or lowest point of a wave, the wavelength, which is the distance between two successive crests or troughs, the wave height, which is the vertical distance between the crest and trough, the wave period, which is the time required for two successive points of waves (two crests or two troughs) to pass a fixed point, and the amplitude, which is the vertical distance between the baseline (sea level) and a crest or a trough.
Waves change behavior are they approach the shore. Waves rotate in a circular motion and can be interrupted by the floor. This is referred to as ‘feeling bottom,’ and it occurs when the water is at a depth that is approximately half of the wavelength of the circular water movement of the wave. When waves feel bottom, they increase in wave height and decrease in wavelength (similar to the compression of an accordion). As waves enter environments with this depth, then they are referred to as shallow water waves.
In this environment, wave speed is dependent solely on depth using the formula:
C = where:
C = wave speed
g = gravitational acceleration (9.8 m/s2 or 980 cm/s2)
d = depth
Part 1 Materials: Wave tank, water, meter stick, ruler, stopwatch, calculator
Procedure:
- Measure the inside length of the wave tank to the nearest centimeter (it should be close to 70 centimeters in length).
- Multiply this value by 2 (Length x 2) to get the total measurement length, as we will be measuring the wave as it travels to the end and back again.
- Using a ruler, pour a measured height of .5 cm of water into the tank. Measure with the ruler in the center of the tank to improve accuracy.
- Lift the tank by two inches on one end from the surface of the table. Allow the water to settle and then firmly place the tank down on the table. This will generate a wave.
- Wait for the wave to reach one end of the tank BEFORE starting measurement, then start the timer and stop it once the wave has hit two more walls (one direction and back again after the initial waiting period).
- Repeat this process five times and record your data in the five trial lines. Find the average of your dataset.
- Increase the water height to 1.0 cm, 1.5 cm, 2.0 cm, and 2.5 cm for each set of trials and repeat steps 4-6.
- Calculate the average speed for each average using the formula: speed = distance / time (where distance was Length x 2).
- Calculate the percent error for each average speed measurement using the theoretical formula value (Percent error = theoretical value – measured value / theoretical value x100).
Data Tables
| Depth (d) in Centimeters | |||||
| Time (s) | .5 cm | 1 cm | 1.5 cm | 2 cm | 2.5 cm |
| 1 | |||||
| 2 | |||||
| 3 | |||||
| 4 | |||||
| 5 | |||||
| Average Time | |||||
| Average Speed | |||||
| Percent Error | |||||
Analysis:
What are the impacts of water depth on the speed of waves?
How did your results compare to the theoretical results? What factors might have contributed to this variation?
Part 2: Ocean Density Currents
Objective: Students will understand the way that water moves at depth, beneath the surface of the ocean.
Water density is directly impacted by changes in temperature and salinity. Large amounts of dissolved solids within ocean water make it denser than pure freshwater, as dissolved solids add mass to the water. Salinity is the term used to describe the solids that are dissolved in seawater and are expressed as units of ‘parts per thousand.’ This represents the number of grams of dissolved solids in one kilogram of seawater. As salinity of seawater increases, so does the density
Most of the surface water in the world ocean ranges from 33-36 ppt, though salinity can be locally impacted by the amount of freshwater and additional salts that are contributed to the sea by runoff, by the freezing and melting of icebergs and pack ice, and by changes in the rate of evaporation.
Temperature also plays a role in the density of ocean water, as the density increases as the water grows colder. The temperature of ocean water is primarily determined by exposure to infrared radiation received from sunlight. The densest ocean water is found in the polar regions, which experience little to no direct sunlight from season to season, and the least dense ocean water is found around equatorial regions.
When large masses of water that have different densities meet, they typically do not mix. At these locations, deep ocean currents are produced as cold, dense, polar water sinks and flows beneath the warm, less dense ocean waters that originate near the equator. Deep ocean currents move slowly and in a diffuse manner.
Part 2 Materials: Clear plastic box, beaker (50 ml), wood block, pipettes, food coloring, water (hot and cold), ice cubes, salt, teaspoon
Procedure:
- Set up the clear plastic box – place the block of wood under one end, so that it is tilted. Fill the opposite end with about 800ml of room temperature water. Let the water become calm before proceeding.
- Place 25ml of room temperature water into a beaker. Add one teaspoon of salt and one drop of yellow food coloring to the water and stir until the salt dissolves. Slowly pour the solution into the raised end of the box and then observe what happens to the solution.
- Place 25ml of ice water into a beaker and stir in blue food color. Slowly pour the solution into the raised end of the box and then observe what happens to the solution.
- Rinse your beaker and place 25ml of hot water into the beaker. Stir in one drop of red food coloring. Slowly pour the solution into the raised end of the box and then observe what happens to the solution.
Write your observations so far. What happened during steps 2-4?
- Add a spoonful of salt and a drop of green food coloring to 25ml of ice water. Stir until the salt dissolves. Slowly pour the solution into the raised end of the box and then observe what happens to the solution.
- Complete the diagram below, by coloring it based on your observations (look at the tank from the side).
Analysis:
- Describe the materials used in terms of their density.
- How would an increase in evaporation impact the density of ocean water?
- The salinity of the Mediterranean Sea is about 40 ppt, while the salinity of the Atlantic Ocean is about 35 ppt. Why do you think this is the case? How would this impact your ability to float in each setting?