Francis J. Degnan
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I. Topic Area
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Some Of The Properties Of Gases
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II. Starting Off Statement
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What is this stuff we call air and what are some of its properties?
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III. Science Processes
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a. observation
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b. prediction
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IV. Materials
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sheets of paper, propane torch, plastic laundry clothes bag, hair dryer or hot air paint stripper, tape, seltzer
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V. Questions To Be Asked
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a. Which object will fall faster? They are both sheets of paper what is different about them?
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b. What has been done to the gas in the metal container?
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c. Why does the balloon rise? What is the difference between the air in the balloon and outside the balloon? What effect does the heat have?
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d. Where do the bubbles in the seltzer come from?
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VI. Behind The Events
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Sometimes it is as if we do not pay attention to the ocean of air we live at the bottom of. These activities are meant to illustrate some of the properties of gases.
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a. The Paper Drop
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The paper in the crumpled ball offers less resistance to the air than the flat sheet of paper. It should be noted that the weight of the sheets is the same, but the surface area has changed.
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b. The Propane Torch
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The gas in the metal container has been compressed or squeezed. Could you do this to a stone or a desk top. You do this when you inflate a tire, basketball or balloon, your outdoor gas grill uses compressed gas and some homes are heated this way.
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c. The Hot Air Balloon.
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The heated gas expands, weighs less and therefore is lighter, the balloon rises.
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d. The Seltzer Bottle
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The carbon dioxide has been pressurized into the liquid. Some liquids make their own bubbles, champagne and beer for example. Again the bubbles come out of the liquid.
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VII. Procedure
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a. Completely a teacher demonstration
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b. Be sure each student has a Now What sheet. Be sure the student fills in the column for their predictions.
IV. Fluids
The two initial activity sheets have included investigations of gravity, pressure and gases. The analogy was drawn between the ocean and the atmosphere. The effects of gravity and pressure on both were discussed. Scientists unite the matter that occurs in gaseous and liquid states in a field called fluid mechanics. In this context a fluid is a material that flows. Fluids can be studied at rest or in motion. Fluid mechanics is the basis for aerodynamics. Even though water is nearly eight hundred times as dense as air, the compressibility of gases does not have to be considered until supersonic speeds are approached. It is now time to consider in what ways liquids and gases are alike. One important aspect of these fluids is that they change shape easily. A parallel could be drawn between objects that are placed in an aquarium full of water, the water immediately surrounds every facet of what is submerged, and our being ‘submerged’ in a room full of air. Is there any place air isn’t?—the inside of light bulbs! Perhaps the most important similarity between fluids of different states for those who study aerodynamics is that the streamlines, lines that indicate local velocity about an object, are the same. Usually streamlines in a wind tunnel may be shown by lines of smoke injected at equal intervals in the flow field. Another experiment would have children raise their hands as the scent of a cologne reaches them. This should be followed with the more concrete demonstration using a clear gallon container full of water to which is added a drop of dye. Both fluids permit diffusion. Children may come up with many other ways in which gases and liquids are similar.
V. Flight
Flight takes place when a machine carrying one or more people under a pilot’s control raises itself into the air, goes forward without a loss in speed and eventually lands at an equal or higher elevation. This is as true today as it was in 1903. These were the conditions that the Wright Brothers were able to achieve and document, in order to proclaim that they were the first to fly. They had studied the works of two other scientists, Otto Lilienthal and Octave Chanute, and united ideas and designs to create their Wright Flyer I. Their craft was almost totally wings and they had calculated that it was the wing and its shape that is primarily responsible for lift.
An airplane in order to achieve and maintain flight must both exert and overcome certain forces. These are the aerodynamic forces of weight, lift, drag and thrust. (see figure 1) Weight is again the force of gravitational pull on the mass of the plane. Thrust and lift are forces that put a plane in motion and drag and weight are the forces that attempt to bring the plane to a stop. A unioue situation arises when a plane is at a particular altitude at a fixed speed. In this case the thrust and drag are equal and the lift and weight are equal, there are no aerodynamic forces working on the plane.
The four aerodynamic forces
Figure 1
(figure available in print form)
The key to flight is lift. It was Daniel Bernoulli’s investigations in the eighteen hundreds that led to an important part of the explanation of lift. He found that observation of one dimensional motion of a steady, inccmpressible flow where density is constant provided a model from which to derive a mathematical and physical description of fluid motion. His calculations linked the laws of conservation of mass and energy. Pressure, velocity and cross sectional area were the variables when considerating calculations of mass, while pressure, kinetic and potential energies were the variables in the energy calculations. While the total of the variables remained constant, Bernoulli demonstrated that proportional shifts in variables were allowable. If the velocity of a fluid is increased, the pressure is decreased and if the area is decreased, velocity will be increased and pressure lowered. Now the wing and how its shape effects the flow of air must be considered.
The wing and streamlines showing the air flow over and under it
Figure 2
(figure available in print form)
The major parts of a wing are; A the leading edge, B the trailing edge and C the upper surface with its curve given the special name camber. Figure 2 has indicated two positions with numerals 1 and 2. In a wind tunnel the velocity would be the same at those two points. Lift occurs because of what happens over and under the wing. The camber forces the air passing up over the leading edge to l) travel further and 2) be forced into a smaller area. Bernoulli calculated the fluid must move faster to maintain its velocity at point 2 and as a result the pressure above the wing becomes less, we have lift! Meantime the air under the wing remains for the most the same and then there is higher pressure under the wing. The activities on the student sheet that follows this section
What
Now?
are all examples of applications of Bernoulli’s law. The task of the students is to explain what happens using velocity and pressure relationships.
Not all lift is explained by Bernoulli. Isaac Newton’s third law of motion states that for every action there must be another action in the opposite direction of equal force. A wing deflects air especially at take off. The amount of deflected air depends upon its angle of attack, or tilt into the wind. When this air is deflected downward there is an opposite reaction that forces the wing upward.
A final, seldom mentioned, amazing factor that creates lift is the Magnus effect. Magnus studied the ideal flow around a cylinder. Then he theorized a second flow of concentric streamlines around a rotating cylinder. Employing Bernoulli’s law he had a low pressure on the top and a high pressure below that created lift on a cylinder. see Figure 3
Flows that are imposed for the Magnus effect
Figure 3
(figure available in print form)
The lift was evident in illustration C of figure 3. Illustrations A and B are superimposed to create C. This interesting phenomenon has more immediacy in our lives than you might at first realize. Any spinning sphere in a wind tunnel would exhibit the same cross sectional streamlines. Therefore we can actually account for the curve of a pitched baseball by understanding the Magnus effect. Hooks and slices of both tennis and golf balls also be explained this way. An explanation of how to demonstrate the Magnus effect is at the Institute office. In summation, “The whole theory of lift is an excellent example of the mixture of ideal and real flows, steady and unsteady motion, all combining to explain the remarkably complex circumstances in which heavier-than-air machines can fly.” Wegener, 8.9
What Now?
(figure available in print form)
