Title: Floating Styrofoam Cup
Objective: A Styrofoam Cup placed upside down on a van der Graaf generator lifts itself
into the air.
Purpose: To show the tendency for electric charges to transfer from the surface of the
van der Graaf generator onto nearby objects and to show that like charges repel.
Materials:
Find a van der Graaf static generator to borrow (unless you have one), a styrofoam cup,
A grounding ball, stick, and wire
Procedure: Turn on the van der Graaf generator and ground the sphere of the van der
Graaf generator (we use a metal ball on a long insulating stick, with a wire that connects
the ball to earth ground) to make it safe (or less painful) to touch. Put an inverted
Styrofoam cup on top of the ball and remove the grounding ball. As charge accumulates
on the van der Graaf generator's sphere, some of it will transfer to the nearby cup. Soon
the sphere and cup will repel one another strongly enough for the cup to lift up into the
air.
Science Behind It: An electric charge on the surface of the van der Graaf generator can
lower its total energy by moving to the Styrofoam cup. It does so with the help of passing
air molecules, which serve as ferries for the charges. Once the cup and the sphere are
each sufficiently charged, the upward Coulomb force on the cup exceeds its weight and
the cup accelerates upward.