For a science fair project, I will be using the power of Dry Ice sublimation to power a Hero's engine. What energy is used to power this engine, and how exactly am I obtaining this energy from simply removing a piece of dry ice from a freezer?
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A Hero's engine is really an application of Newton's third law of motion - forces always come in pairs. These forces are equal in size but act in opposite directions and on different objects. Its also the principle behind rockets. When the spent fuel exhaust leaves the nozzle of the rocket, the rocket is propelled in the opposite direction. Physics will say that the mass of the exhaust particles times their velocity backward equals the mass of the rocket times its velocity forward.
When dry ice sublimes, it changes directly from a solid (at -78.5 oC) to a gas. If the gas is then allowed to escape through a very small tube (as in the design of a Hero's engine) , the escaping carbon dioxide gas can propel the Hero's engine in the opposite direction. My suggestion is to add some warm water to the Hero's engine vessel prior to dropping in the dry ice and then immediately stopper the opening allowing only the small tubes to vent the gas. The heat from the warm water will cause the sublimate (the gaseous CO2) to expand building up some pressure. This pressure increase will add to the net force that spins the engine. Fluids always move from an area of high pressure (inside of the Hero's engine) to an area of lower pressure (outside the engine). The rate of movement of the fluid (the carbon dioxide gas) depends on the difference between the two pressures.
Mathematically it would be nice to be able to measure the internal pressure of the engine. Since pressure = force / area, you could calculate the force pushing the gas out through the tubes by dividing the internal pressure by the surface area inside of the engine. And since Newton's action/reaction forces are the source of the push, the force of the gas exiting the engine is the same size as the force pushing the engine in the opposite direction.
When dry ice sublimes, it changes directly from a solid (at -78.5 oC) to a gas. If the gas is then allowed to escape through a very small tube (as in the design of a Hero's engine) , the escaping carbon dioxide gas can propel the Hero's engine in the opposite direction. My suggestion is to add some warm water to the Hero's engine vessel prior to dropping in the dry ice and then immediately stopper the opening allowing only the small tubes to vent the gas. The heat from the warm water will cause the sublimate (the gaseous CO2) to expand building up some pressure. This pressure increase will add to the net force that spins the engine. Fluids always move from an area of high pressure (inside of the Hero's engine) to an area of lower pressure (outside the engine). The rate of movement of the fluid (the carbon dioxide gas) depends on the difference between the two pressures.
Mathematically it would be nice to be able to measure the internal pressure of the engine. Since pressure = force / area, you could calculate the force pushing the gas out through the tubes by dividing the internal pressure by the surface area inside of the engine. And since Newton's action/reaction forces are the source of the push, the force of the gas exiting the engine is the same size as the force pushing the engine in the opposite direction.
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