Golf Ball Atmosphere

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.


golf ball

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.