A model of molecular motion and pressure using practice golf balls.
What it shows:
The kinetic energy of gas molecules bouncing off a surface causes pressure.
Increasing the molecules' speeds increases the pressure and the volume of the gas.
How it works:
Plastic practice golf balls represent monatomic gas molecules. They are provided with kinetic energy by a rapidly spinning plate with raised ridges that knock the balls about. The container has four walls of 1/4" glass, with a floor that is the spinning disc and a roof that is a freely moving metal mesh screen. The screen weighs about 1.2kg, so is just short of being counter-balanced by a 1kg mass that is linked by wire over a high mounted pulley. As the golf balls start hitting the screen, the additional force pushes it upwards. This is a qualitative demonstration of Charles' Law showing temperature proportional to volume.
The spinning plate is made of 2mm sheet aluminum, 40cm in diameter, backed with 1/4" plywood. The ridges are two aluminum ribs, rising 5mm above the plane of the disc. The housing for the motor is not shown in figure 1; it is a wooden box (52 cm of side, 30cm high) upon which the atmosphere sits. The motor for the spinning plate is a Bodine series 500 1 with a maximum rotation speed of 173 r.p.m., mounted directly below the spinning disc. Speed control is provided by a potentiometer. The spinning plate has a braking mechanism (otherwise it takes a long time to slow, and this demo is noisy!) provided by a second switching circuit that shorts the windings of the motor's armature, causing the motor to slow by eddy current damping.
figure 1. The Golf Ball Atmosphere.
Setting it up:
For ease of transport the pulley pole can be unscrewed and lowered into the body of the atmosphere (its nearly been knocked off a couple of times when being moved!). The golf ball atmosphere is also on legs (60cm) and wheels (omitted from the diagram).
We originally experimented with Ping-pong balls, but they were destroyed almost instantaneously! Furthermore, if you plan to build a golf ball atmosphere of your own, don't be temped to use a large plastic (or glass) cylinder for the side walls. You will create one mean golf ball tornado! (There is a reason why blenders have square walls).
The motor has low starting torque, and can stall if the spinning plate binds up on the edges. If turning on and up gives a hum but no motion, stop, unplug, and make sure the plate is free to spin.
1 Bodine Series 500 high torque 1/8 h.p. 115V DC control motor. Bodine Motors, Chicago, Ill.