With the Electric Whirl Electrostatics Demonstration, illustrate an equal and opposite force reaction. It can only spin the direction opposite to the points.
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When brought near the dome of a Van de Graaff generator, the whirl spins rapidly in a direction that is away from the sharp points of the whirl arms. The electrons escaping from the Van de Graaff dome to the spokes ionize the air at the sharp points. Since the spokes are continuously charged with the generator running, the ionized air particles are repelled and the spinning continues. This illustrates an equal and opposite force reaction. Even if the whirl is given an initial spin in the opposite direction, it will eventually stop, reverse and spin in the direction opposite to the points.