Water Pressure at Depth
You might have heard of the difficulties divers face when they descend into the ocean depths. Before a person can scuba dive, she needs to have many hours of instruction and training. Divers can experience joint pain, light headedness, loss of coordination, and paralysis if they do not carefully limit the depth and length of their dives. They also need to be especially cautious during their ascents and descents. Why?
Pressure, or the amount of force per unit of area, is the key. The Earth is covered with a layer of air 80 miles thick. Like any type of matter, the gases in air have mass, and since Earth’s gravity pulls this mass down onto the Earth’s surface, the air produces a force called atmospheric pressure.
Water is much denser than air. A column of water 10 meters thick exerts the same amount of pressure as the entire Earth’s atmosphere (which is 80 miles thick!). That means if a diver descends just 10 meters underwater, he would experience the pressure of two atmospheres pressing down on every part of his body. If he were to descend to 20 meters, he would experience three atmospheres pressing down, and so on. The increased pressure changes the way gases behave in the diver’s body. You might think you need a large ocean tank to see how depth affects water pressure, but believe it or not, you can observe how pressure changes with depth just by using a plastic soda bottle.
- 2-liter soda bottle with cap
- Permanent marker
- Grown-up helper
- 2-3” inch nail, with reasonably sharp point
- Duct or masking tape
- Use the scissors to remove the label from the soda bottle.
- Use the ruler to mark a point three inches from the bottom of the bottle.
- Mark a second point directly above the first, about eight inches from the bottom of the bottle.
- Put the cap on the soda bottle and lay it horizontally on a flat surface.
- Have your grown-up poke the nail through each of the holes you marked.
- Cover the holes you made with tape.
- Fill the bottle to the top with water, and set the bottle near the side of the sink, with the holes facing the sink.
- Remove the tape from the holes and watch!
Water will stream out both the holes, but the water will shoot out farther and more forcefully from the bottom hole.
The water coming out of the bottom hole is under greater pressure than the water coming out of the top hole, because it has more weight pressing down on it. Therefore, it exits the hole with more force.
Try poking another hole in the middle of the bottle and compare the size of its stream. Does pressure increase at a steady rate with depth? What happens when you use a wider cup? Does more water change the pressure, or is depth the only thing that matters?
You could also try using different liquids, like salt water or corn syrup. If you want to do some research, look into how diving animals like seals adapt to the ocean depths.
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