lecdem.physics.umd.edu - Kinetic and Diffusive Processes

Kinetic and Diffusive Processes

I6-25: DIFFUSION - DISTRIBUTION OF PING PONG BALLS

I6-25

Demonstrate on a macroscopic scale using ping pong balls how random molecular motion causes substances to diffuse.

This model consists of a wooden frame with clear plastic covers, about one ping pong ball in width, ten bins at the top and bottom for setting up initial and analyzing final distributions, with several rows of pegs in between. When the horizontal plastic baffle holding the balls at the top is pulled away, balls will drop through the peg array, become randomly scattered, and drop into bins at the bottom.
Engagement Suggestions
• Put four orange balls into bin 5 and four white balls into each of bins 3, 4, 6, and 7. Challenge students to predict whether they will keep this same arrangement as they fall. (When the balls reach the bottom, the four orange balls will have become distributed into the white balls.)

Background
This shows on a larger scale how random molecular motion causes substances to diffuse. The array of fallen balls will approximate a probability curve; this is an opportunity to introduce statistical concepts in a physical, measurable manner.
I6-31: MOLECULAR MOTION DEMO - BROWNIAN MOTION

I6-31

Model Brownian motion.

A set of small balls models the air. The balls are set into motion by vibration of the walls. A large mass models a smoke particle which is moved about randomly by collisions with the smaller air molecules. The device must be tilted so that the balls will not stop moving.
I6-32: MOLECULAR MOTION DEMO - RANDOM MOTION IN GASES

I6-32

Model random molecular motion.

A set of small balls of the same mass models the air. Random motion of any ball can be observed.
I6-33: MOLECULAR MOTION DEMO - GAS PRESSURE

I6-33

Model gas pressure.

A set of about 20 steel balls models the air. A bar is positioned in the center of the device so that it will be continuously struck by the moving balls. The balls are set into motion by vibration of the walls with the device tilted. Collisions of the balls with the bar push the bar upward to model the force of a gas on a surface.

I6, PW1
I6-34: MOLECULAR MOTION DEMO - TEMPERATURE OF A GAS

I6-34

Model gas pressure.

Two sets of small balls (larger green and smaller blue) are used to model the molecules in the air. The balls are set into motion by vibration of the walls. Increasing the vibration speed of the walls imparts more energy to the balls, simulating higher temperature.
Using a single set of balls, the distribution of velocities can be observed. Using two sets of balls with different mass, the average velocity of the smaller balls is seen to be greater than that of the larger balls.

I6, PW1
I6-35: MOLECULAR MOTION DEMO - DIFFUSION

I6-35

Model gas diffusion.

Balls with two different masses are placed into the demonstrator, one type on each side of a barrier. A diffusion barrier with a small slot is positioned in the center of the device, and the balls are set into motion by vibration of the walls. The balls will mix, representing diffusion. Differences between smaller and larger balls can be observed, and fluctuations can be discussed.
I6-36: MOLECULAR MOTION DEMO - AVOGADRO'S HYPOTHESIS

I6-36

Help justify Avogadro's hypothesis using a model.

A set of small balls of varying mass models the air. The balls are set into motion by vibration of the walls. Using two sets of six balls with differing mass, start the device in motion. Both sets of balls move around and occupy the entire space. The small balls move faster than the large balls, so they "occupy" the same amount of space overall.
I6-37: MOLECULAR MOTION DEMO - VAN DER WALLS FORCES

I6-37

Introduce the concept of attractive force between molecules.

A set of 9/32" steel balls models the air. The balls are set into motion by vibration of the walls with the device tilted. A weak magnet is placed at one end of the volume. If the velocity of the steel ball molecules is small enough some of the balls will stick to the magnet and to each other, representing condensation.
I6-38: MOLECULAR MOTION DEMO - BOYLE'S LAW

I6-38

Model Boyle's law.

A set of small balls of equal mass models the air. The balls are set into motion by vibration of the walls with the device level. A bar is positioned in the device to divide the volume into two parts, with all of the balls on one side. A rough observation of the rate at which balls hit the wall is then made. The bar is removed, keeping the motion the same. Note that fewer balls hit the same section of wall in the same time, indicating that when the volume increased the pressure decreased.
I6-39: MOLECULAR MOTION DEMO - CHARLES' LAW

I6-39

Model Charles' law.

A set of about 20 small steel balls of equal mass models the air. The balls are set into motion by vibration of the walls with the device tilted. A moveable bar positioned in the device is pushed upward by collisions with the balls. As the vibration rate of the walls is raised, raising the temperature and thus increasing the average molecular speed, the bar is pushed further upward, representing increased volume at a constant pressure (the weight of the bar). Use of a 140 Volt Variac extends the temperature range upward to make the trend more clear.

I6, PW1
I6-40: MOLECULAR MOTION DEMO - SOLIDS

I6-40

Model the behavior of solids.

A set of small balls of equal mass is placed in the vibrator in a curved glass dish with the concave side up. The gravitational force caused by the curvature of the dish causes them to form an array, corresponding to a crystal lattice. As the vibrator motion is increased, the molecules do not move about randomly, but rather vibrate about their original positions, representing vibratory motion of atoms in a crystal lattice.
I6-41: MOLECULAR MOTION DEMO - LIQUIDS

I6-41

Model the behavior of liquids.

Using the arrangement of balls from the solid (Demonstation I6-40), the vibration speed is further increased. The balls remain in a small clump, with a few "boiling off" but most free to migrate within the tight clump, representing molecular motion within the liquid state. Use of a Variac operating at 140 VAC makes this effect more clear.
I6-51 ENTROPY - SORTING MARBLES

I6-51

Demonstrates that increasing entropy requires less energy than decreasing entropy

Shaking the system with the larger holes on the top causes the marbles to separate by size (yellow, green, pink, and blue). Simply inverting allows them to fall under the influence of gravity to their lowest level and mix. It apparently takes more energy to unmix the marbles than to mix them.

I6