Losses and the 2nd Law


Model Description

This is a demonstration designed to generate a conceptual discussion about the 2nd Law of Thermodynamics and the Kelvin-Planck statement of the law in particular.  It utilizes a pendulum to introduce the concept of irreversibilities due to friction. This demonstration should take 10-15 minutes.

Engineering Principle

This demonstration is based on the assumption that the course has already spent time covering the 1st Law of Thermodynamics (Conservation of Energy) and is about to transition to the 2nd Law.  By now, students should be familiar with the principles of conservation of energy from Thermodynamics and 2-D motion from Physics: E=KE+PE+U.

What You Need

Item Quantity Description/Clarification
String 1 A length of 2-3 ft will do in most classroom environments
Weight 1 Any object can be used as a weight provided it will not damage the classroom ceiling.
Chair 1 The demonstrator will need something to stand on during the demo.

How It’s Done

Before Class: Obtain the materials above and suspend the weight from the ceiling in a location where it will not impede the rest of the class, but is visible to students.  Ensure there is sufficient room for the weighted object to swing without striking a wall or other object.

In Class: After a discussion on energy and how energy is conserved, discuss the pendulum.  Based on simply analyzing the kinetic and potential energy of the pendulum, the object should return to its original height if displaced and then allowed to oscillate.  Ask if the students believe that premise and then offer to demonstrate the principle by standing on the chair and placing the weight on the tip of your nose or chin (figure below, left). Allow the weight to drop (figure below, right). Do NOT push it during the release, or you may get smacked in the nose! Then ensure you do not flinch as the weight rises, falls and then rises again just missing your face.  At this point, a discussion can begin about losses, friction and irreversibilities.  This will act as a bridge into discussion of the 2nd Law and the Kelvin-Planck statement of the law.

Observations:  Students should be able to eventually understand how the 1st Law can still be satisfied while conforming to the 2nd law.  The irreversibilities that prevent full recovery of the original potential energy manifest themselves as internal energy or the heat produced by friction between the string and ceiling and ambient air.  While this is too negligible to accurately measure in the classroom it can be demonstrated by having the students rub their hands along the desks.

Additional Application: The demonstration has the potential to inject some drama into the classroom.  Ask the students if they think the weight will return and smack you in the face.  If people disagree you can step off the chair and discuss the problem once or twice before actually accomplishing the demonstration to add suspense.

Did you try this? Comment below to let us know how it went.

Cite this work as:

Richard Melnyk (2019), "Losses and the 2nd Law," https://www.handsonmechanics.org/thermal/607.

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