SUBJECT: The Electrostatic Force Experiment
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DESCRIPTION: This applet is a simulation of an experiment in which an aluminum sphere sitting on the bottom plate of a capacitor is lifted to the top plate by the electrostatic attraction generated as the capacitor is charged. While the sphere is in contact with the lower plate, their surface charge densities are approximately equal. Thus, as the capacitor is charged, the charge density on the sphere increases proportional to the potential difference between the plates. In addition, energy flows into the region between the plates as the electric field builds up. This can be seen in the motion of the electric field lines as they move from the edge to the center of the capacitor.
As the potential difference between the plates increases, the sphere feels an increasing attraction towards the top plate, indicated by the increasing tension in the field as more field lines "attach" to it. Eventually this tension is enough to overcome the downward force of gravity, and the sphere is "levitated". Once separated from the lower plate, the sphere's charge density no longer increases, and it feels both an attractive force towards the upper plate and a repulsive force from the lower one. The result is a net force upwards.
In this simulation we have placed a non-conducting barrier just below the upper plate to prevent the sphere from touching it and discharging.