# School of PhysicsLecture Demonstration Manual

## He-2 The Maxwell Demon Bottle

Published: Monday 13 March 2006 - Updated: Monday 28 March 2011

### Aim

To demonstrate the statistical nature of irreversible processes.

### Apparatus

The Maxwell Demon bottle

### Diagrams

Click on pictures to enlarge.

 Maxwell Demon Bottle Pic Maxwell Demon Bottle Diagram

### Description

The Maxwell Demon demonstrator consists of a sealed flask containing 5 white and 5 black spheres. The demonstration begins with the balls in the stem of the flask in the unmixed condition. Inverting the flask mixes the balls and a discussion of the probability of the balls unmixing on inverting the flask again can follow. With 5 black and 5 white balls the odds against reproducing the initial array are 252 to 1.

Maxwell's Demon can be introduced and his task illustrated in the following manner; rotate the flask in a stem up position to impart a swirling motion to the spheres. Turn the flask to the stem down position and slowly reduce the rotation rate and the balls will spiral back into the neck. With a little practice the initial array is restored.

The "demonability" resides in the fact that the black spheres are constructed so as to rapidly lose momentum (see diagram).