Direction of the 2nd Law

Model Description

This is not a hands on demonstration, but a visual aid designed to generate discussion about the Clausius statement of the 2nd Law. This activity should take 5-8 minutes.

Engineering Principle

The Clausius statement of the 2nd Law states “It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower temperature body to a higher temperature body.”  Explicitly, this statement specifies the direction of heat transfer that will occur spontaneously.  However, the larger implication of this statement and the 2nd law is that all processes must occur in a specific direction leading toward greater disorder.

How It’s Done

Before Class: Prepare the pictures below in a slide show of some type.

In Class: Display the two pictures of the wine glass pictured above.  Explain to the students that both pictures are of the same glass at different times.  Ask which picture was taken first in a timeline.  This question will generate discussion about order and disorder.  Most students will intuitively guess that the intact glass is first in the sequence and the broken glass is next.  This is because we have all been subject to the 2nd Law our entire lives without expressly understanding it.  The instructor can then use this question to begin discussion of the Clausius statement and the implications of the 2nd Law.

Additional Application: Inevitably, a student will comment that the broken glass could have been the first picture in the sequence and that if someone put it together perfectly, the intact glass could be the subsequent picture.  This statement is partially true and will help generate discussion on the 2nd Law internally, externally, and as a total system. Ask the student to define the system boundaries if you choose to fix the broken glass.  The system in that case will include the glue, the broken glass and the “mechanic”.  Explain that the entropy of the glass may have decreased but the entropy of the entire system has increased.

Cite this work as:

Richard Melnyk (2019), "Direction of the 2nd Law," https://www.handsonmechanics.org/thermal/604.