College of Liberal Arts & Sciences
8B50.45 - Hydrostatic Equilibrium
Code Number:
References:
8B50.45
Demo Title:
Hydrostatic Equilibrium
Condition:
Good
Principle:
Hydrostatic Equilibrium
Area of Study:
Astronomy
Equipment:
Galilean Thermometers, Tall glass cylinder, Saturated Salt Water, Pure Water, Hard Boiled Egg, Helium, Balloons or Mylar Balloons, String.
Procedure:
The Galilean thermometers are an example of hydrostatic equilibrium. If you wish to have one of the indicator balls floating mid-way in the thermometer you may warm the thermometer with your hands for several minutes or use a warm towel until you get it to the desired level. It has also been reported that a bit of clay can achieve neutral buoyancy in water and is very sensitive to temperature and barometric pressure changes.
Fill the tall glass cylinder 1/2 way with saturated salt water solution. Then tilt the cylinder and gently pour pure or tap water into the cylinder until it is several inches from the top. Some mixing will take place at the salt water/pure water interface, but if you are gentle enough this mixing will not reach all the way to the bottom, thus preserving a layer of saturated salt water. Drop in one or even two hard boiled eggs.
The Three Helium Balloons demo.
One balloon is more dense than air and sinks (blow this one up with your breath or use CO2 to inflate. One balloon full of helium is lighter than air and will float upward. The third balloon has a mixture of air and helium so that it is neutrally buoyant (it will not sink or rise).
DESCRIPTION Three mylar balloons are filled with different amounts of helium and experience buoyant forces of different magnitudes as a result. The balloon with the most helium experiences a very strong buoyant force and floats up to the ceiling. The balloon with the least helium experiences a very weak buoyant force and sinks to the ground under the force of gravity. The third balloon, however, has exactly the right amount of air and helium for the buoyant force to equal the force of the gravity, and this balloon rests in midair.
SUGGESTED TECHNIQUE
Attach strings to all three balloons so you can tape them to the table. Untape the string of the balloon with the most helium. Show that the balloon floats up to the ceiling. Drop the balloon with the least helium off the side of the lecture bench. Show that it falls to the floor.
Hold the third balloon in midair and release it. Show that the balloon floats in hydrostatic equilibrium. Note: a long string will be attached to the stem of this balloon. If the balloon floats down, cut a bit off of the string; if the balloon floats up, tie on an extra piece of string (string and scissors will be provided).
Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist. The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose. The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources. Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks. These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage. Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.