09 First Law of Thermodynamics

Idealized Stirling Cycle

I created a new GeoGebra app based on an ideal Stirling Cycle (A. Romanelli Alternative thermodynamic cycle for the Stirling machine, American Journal of Physics 85, 926 (2017)) which includes two isothermal and two isochoric processes. The Stirling engine is a very good example to apply the First Law of Thermodynamics to, as the amount of gas is fixed so the macro-variables are only pressure, temperature and volume. Simplifying the cycle makes it even easier for first time learners to understand how the engine works.

For those who prefer to be impressed by an actual working model, it can be bought for less than S$30 on Lazada. All you need for it to run is a little hot water or some ice. Here’s a video of the one I bought:

The parts of the Stirling engine are labelled here:
How a Stirling Engine works

My simulation may not look identical to the engine shown but it does have the same power piston (to do work on the flywheel) and displacer piston (to shunt the air to and fro for more efficient heat exchange).

Geogebra link: https://www.geogebra.org/m/pbnw2yas

PV Diagram for an Ideal Gas

GeoGebra link: https://www.geogebra.org/m/xyqhfvyw

Applying the 1st Law of Thermodynamics to 4 simple changes on an ideal gas, students can check their understanding using this Geogebra app. When is work done positive? Which processes bring about an increase in internal energy or temperature? Which processes require heat input?

Stirling Engine

I bought a simple beta Stirling engine online at dx.com recently and it came in the mail today. It works well with a cup of hot water placed under it, although it might take a little push to get it started due to the initial static friction. However, once it starts spinning, the wheel goes on and on for a very long time.

From the video, you can observe the expansion of the air within the main piston cylinder as the heat below raises the temperature and pressure. This forms the power stroke. When the piston rises, it pushes air into a secondary piston, which also helps to provide torque to the wheel. When the air in both pistons expand, it cools down. An understanding of the 1st law of Thermodynamics (JC syllabus) is necessary to appreciate why that happens. Upon cooling, pressure decreases and the pistons fall. The cycle repeats itself.


Adiabatic Process Demonstrations

Here are some interesting lecture demonstrations on adiabatic thermodynamic processes you can carry out. In an adiabatic process, there is no heat transfer between the system and other systems (including its environment.) According to the First Law of Thermodynamics ($$\Delta U=Q+W$$), where Q = 0, a compression of a gas which is associated with work being done on the gas will cause the internal energy and hence, the temperature of the gas to rise. On the other hand, when an expansion of a gas takes place, the gas will cool down.

1. Adiabatic compression using a fire syringe

(available from Funlearners for $30 in Singapore)

2. Adiabatic expansion using a fire extinguisher