Engines and the Four Thermodynamic Processes

1. Isobaric Processes result in a linear relationship between volume and temperature.
2. Isochoric processes result in a linear relationship between pressure and temperature.
3. Isothermal processes create an inverse relationship between pressure and volume.

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This is what we predicted when asked what will happen to a rubber band when heated with a blow dryer. We were wrong, and the rubber band actually contracts.

Here is a 4 step process for an engine, which is really a rubber band, that lifts cans of spinach to place them on a conveyor belt. Heating up the rubber band requires heat flow into the rubber band, and work is performed on the rubber band, causing it to contract. Heat then flows out of the rubber band, and the rubber band performs work, allowing it to elongate.

A system that turns all of the energy entering it into work in impossible. There will always be some heat lost.

The greek letter eta represents the efficiency of an engine. All efficiency ratios compare what is produced to what is required to produce. The closer to 1, the more efficient the engine is.

From 1 to 2, an ideal gas expands at constant pressure. From 2 to 3, the system decreases in pressure at constant volume. From 3 to 4, the system condenses at constant pressure again. From 4 to 1, the system returns to where it started.

The total energy of a system is threes halves the product of the systems pressure and volume. Since these state variables are known for each point, the internal energy at that point can be calculated, as well as the changes in internal energy in each transition.

For the isochoric processes, no work is performed, and the change in internal energy is due only to the heat flow. For isobaric processes, W = p(V,final - V,initial). Using the first law of thermodynamics, it is is possible to find the heat flow for each process.

A flask filled with air is placed into a cold water bath. The flask is connected to a syringe. The syringe will undergo volume changes as the air is heated. Initially, an erasure is placed on the syringe, pushing it down increasing the volume. The air is then heated, and the syringe increases its volume and witnesses an increase in pressure. Then the erasure is removed, and the syringe shoots upward briefly, but the pressure drops.

A table representing work, heat, and changes in internal energy for each of the processes described above. Temperatures were know, and the air was approximately entirely nitrogen gas. Once the number of moles present were found, nitrogren gases molar mass was used to convert this into a mass of nitrogen gas. This mass was used in Q = mc(T,final - T,initial).