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MIT Physics 8.02 - Electricity & Magnetism

SECTION : Electrostatics      

SUBJECT: Charging and Discharging Two Van de Graff Generators  

In this animation we show the charging of two Van de Graff generators sitting side by side to equal and opposite charges Q. As we charge them, there is a Poynting flux outward from the region where we are creating electromagnetic energy. This region is where the charge is being moved against the electric field on the conveyer belts of the two generators (inside the cylindrical shafts of the generators). This is where the creation rate per unit volume for electromagnetic energy (-J dot E) is positive. Energy is created in those regions and flows out to fill the space around the generators. That flow can be seen in the motion of the electric field lines, which is always in the direction of the Poynting flux.

After charging the generators, we turn off the conveyer belts. We then ionize the air between the two spheres so that they discharge along the line joining them. We see the spark discharge and the inflow of energy from the electrostatic field powers that spark. Again we see the energy drain out of the electrostatic field as indicated by field line motion, which is in the direction of the Poynting flux.





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