Prelab

1. Print this datasheet and bring at least one copy per group to the lab.

2. Define the terms expansion work and compression work.

3. Explain what is doing work and what has work done on it in each case, and where each process takes place in the lab procedure below.

Theory

In the study of changes in pressure from an engineering context, we often refer to expansion work, and compression work. 

Expansion work occurs when a gas expands and exerts a force that does work on the surroundings. The work is done by increasing the volume of the gas, and thus it's container. There are many examples of this, but a common one is seen in a car engine. Gasoline vapor is ignited, causing rapid expansion within the combustion chamber. The hot gas exerts pressure on the pistons above the chamber which in turn turn a shaft that powers the vehicle. The gas starts out as occupying a small volume, and ends by occupying a larger volume. The sign for expansion work is positive, since work is done by the gas on the surroundings. 

Compression work occurs when work is done by the surroundings on a gas. An example of compression work will be investigated in the lab below. Work is done by a compressor to reduce the volume of a gas stored in a tank. Since the tank is rigid (constant volume) no work can be done by the gas while it is contained. The gas in the tank therefore stores potential energy that can later be used to perform expansion work when allowed to expand upon release. In compression work, a gas starts out at a large volume and ends at a smaller volume. The sign for compression work is negative, since work is done on the gas by the surroundings.

The easy way to remember the sign convention is to always consider the work from the perspective of the gas. If the gas does work, the sign is positive. If the gas has work done on it, the sign is negative.

As a gas is compressed its temperature rises, and as a gas is decompressed its temperature drops. This is, in the simplest sense, because air contains a certain amount of energy. By adding small amounts of air into the system, or removing small amounts of air from the system also a adds or removes the energy it contains. If the tank contains X amount of energy and we add a small amount of air that contains Y energy, the tank now contains X+Y total energy.

A process is polytropic when the ratio of heat transferred to work transferred is constant throughout the process. Stated more simply, we expect that over the whole experiment, repeatedly applying small amounts of work (compression in this case) will cause continuous small temperature increases. If we consider the heat gained by the system (the compressor's tank) to result from the added air, then this is intuitive. The compressor is just repeatedly adding small puffs of air, and the energy it contains over the course of the test. Mathematically a polytropic process is one where ΔQ/ΔW = C . The amount of heat gained for each amount of work done can be used to classify the type of process occurring, and will be found empirically as the dimensionless "n" in the results of the lab.

In the course of this experiment we will observe the internal temperature of the tank increase as work is done on the gas, and the energy of air from the room is added to the tank. We will then vent the air, releasing the energy in that volume expelled back into the room. The final tanks temperature will reflect the energy remaining after the expansion work done by the gas. The compressor tank is not insulated so the cycle will not begin and end at the same point as perfect theoretical adiabatic process would.

Problem

AP Engineering wishes to model the behavior of a high pressure vessel in which various chemical processes will be conducted. The supplier for the reaction vessel has again provided a scale model to characterize the behavior to ensure customers are happy with the much larger, more expensive version once received.

You are tasked to investigate a few key properties and report back with your findings. The most important parameters are the heat gained by the compressed air by action of the compressor bringing it from the environment into a confined space, the amount of air, and the amount of work required to do so. From these two finding several other important can be derived directly without further testing.

Procedure

1. Read the whole procedure before beginning, several steps must be done quickly to get good results, make a mental note of them.

2. Start your computer and be sure Multimeter is in (max) Temperature mode to monitor the tanks temperature.

3. Open the check valve and bleed away any pre-existing pressure from inside the tank. 

   • This is the skinny red plastic level shown in the image below.

   • The lever is closed in the image below, it is open when pointing away from the pressure gauge.