Thermodynamic Processes (Dan Perry)

Using various items to demonstrate basic thermodynamic processes.

Principle(s) Investigated: First Law of Thermodynamics

Standards : PH3. a. Students know heat flow and work are two forms of energy transfer between systems.

Quickwrite

Materials:

Film Canister

Fire Piston

String

Balloons

Ice Water Bath

Extremely Hot Water Bath

Tongs

Procedure:

Isobaric Process

1. Prepare an ice water bath and a hot water bath in containers that will accommodate your balloon.
2. Inflate the balloon and place it in the ice water bath making sure to submerge it as much as possible. Let it remain in the ice water bath for 1 minute.
3. Use the string to measure the circumference of the balloon and record in the form below.
4. Place the balloon in the hot water bath fully submerged for 1 minute.
5. Use the string to measure the circumference of the balloon and record in the form below.

Isochoric Process

1. Prepare a boiling bath.
2. Using tongs, submerge the film canister about 3/4 of the way leaving the lid exposed to the atmosphere.
3. After some time, a noticeable event will occur. Record your observations in the form below.

Adiabatic Process

1. Place a small piece of cotton inside the fire piston.
2. Quickly slam the piston down compressing the contents.
3. Record your observations in the chart below.

Student prior knowledge:

Ideal gases:

PV = nRT, monatomic vs diatomic gasses, properties of an ideal gas

Work and Energy:

W = Fd

Energy is the capacity to do work and work can be done to store energy

Temperature:

Temperature is a measure of the average kinetic energy of gas particles

Kelvin scale

Pressure

P = F/A

The exploratory nature of the demonstration leaves a lot of flexibility in prior student knowledge.

Explanation: These are all thermodynamic processes. As such, that means that heat and work affect the energy contained in the systems. According to the first law of thermodynamics, the change in internal energy (E_int) of a gas is equal to the heat (Q) put into the gas minus the work (W) done by the gas.

ΔE_int = Q - W

The internal energy of a system is directly proportional to the temperature. The work done by the gas is dependent on fluctuations in pressure and volume and is affected by the manner in which the gas changes its state.

Each process above has unique properties that manifest in various ways.

The isobaric process is a constant pressure process. A balloon very closely approximates this. If the pressure inside the balloon increases, the balloons volume will increase in an effort to eliminate any pressure differential. By placing the balloon in the hot water bath, we increase the temperature of the gas and thus the number of collisions with the balloon. The increased collisions force the walls of the balloon outward and give it a larger circumference.

The isochoric process is a constant volume process. By using a rigid container the added energy from immersing in hot water results in a drastic increase in the amount pressure exerted on the walls of the canister. As a result the lid of the cannister should pop open.

The adiabatic process is a process where no heat is gained or lost. Only work is done. By working on the gas quickly before heat can flow, all the energy added through work is contained in the gas. This results in a drastic increase in the temperature and pressure of the gas. A combustible material confined in a space with a denser amount of oxygen at a higher temperature will ignite.

Questions & Answers:

Propose an example of a process that undergoes an isothermal process. Remember that internal energy is directly proportional to temperature.

If the temperature does not change, then the internal energy does not change. This means that the work done by the gas and the heat added to the system must be the same. In order to do this it would require work being done at the same rate as heat would be added. This can be approximated by extremely slow changes in volume and allowing the system to equilibrate in temperature.

For an isochoric process, the volume does not change. What conclusions can you make about the work done by the gas in this situation?

Since the volume does not change the force of the gas exerted on the walls of the containers do not act through any distance. As a result, the total work in an isochoric process is zero.

Consider the special case of a gas that expands freely in a vacuum. What would the heat transferred, the work done by the gas, and the change of the internal energy be in this case?

Although the gas is expanding, the fact that it expands into a vacuum means that it does not push against anything and thus the force is equal to zero. So the work done must be zero. In a vacuum, there is no energy to transfer as heat so there is also no heat added. This means that the change in the internal energy is also 0 due to the first law of thermodynamics.

Applications to Everyday Life:

Thermodynamic processes apply to everyday life in profound ways. The most blatant and common of which is the gasoline engine. The gasoline engine uses a series of thermodynamic processes combined into a cycle. This cycle is known as the Otto cycle and is made of 2 adiabatic processes and 2 isochoric processes.

More information can be found here: Otto Cycle

Other thermodynamic processes can be found in many other types of engines including the Diesel engine, the Stirling Engine, and more. However, thermodynamic cycles also govern refrigerators which is much like the reverse of an engine. Engines must use heat to do work, but refrigerators use work to move heat from a low temperature to a high temperature.

More information can be found here: Refrigerator

One of the most important applications to our daily lives is in power plants. Most power plants use heat as a source of energy to power our homes. The heat causes steam to turn turbines that generate electricity. The added heat increases the temperature and pressure of the steam forcing to do work on the turbine. There are several different kinds and several ways to generate the heat.

A good starting place to learn about steam generators is wikipedia: Thermal Power Stations, Steam Turbines

Different kinds of thermal power plants: Nuclear, Fossil Fuel

Photographs:

Videos: Below are some helpful videos. This is just a tiny sample. There are so many videos out there about thermodynamic processes.

Fire Piston Explained

Drinking bird is a thermodynamic cycle

Adiabatic Process