The top part of the globe of the Van de Graaff generator becomes negatively charged as excess electrons are transferred from a connection to the ground. This is done as a fast-moving belt rushes by metal combs at the top and bottom of the machine.  Because the globe of the Van de Graaff generator is a conductor, the excess electrons redistribute themselves evenly over the surface of the globe. These excess electrons cannot return to the base because the rubber belt and the plastic column supporting the globe are insulators.

It takes work to force electrons to move from the ground connection at the base to the globe at the top. This is why input energy must be supplied by the motor of the generator to rotate the rubber belt on its two pulleys. You'll learn more about the energy required to do work on moving charges in later lessons. 

Now that you have more information about transferring charges, reconsider the behaviour of the soap bubbles and the Van de Graaff generator.


The behaviour of the soap bubbles is much more complex than that of the animal fur, the pie plates, or the confetti because the soap bubbles are subject to a sequence of interconnected events.

These events have been organized into the four steps listed below.  In each step, describe what is happening and then explain.  You can check your answers by clicking on the Description and Explanation buttons.

 


Neutral soap bubbles are attracted to the negative globe.


 

A stream of neutral soap bubbles is blown toward the negative globe of the Van de Graaff generator. As the bubbles get closer to the Van de Graaff generator, a charge shift occurs within the soap molecules. The side of the molecules closest to the negative globe becomes positively charged. The closest bubbles are attracted to the negative globe.

 

This is an example of induction because the electrons within the soap molecules shift due to the presence of the negative globe. Molecules of the soap solution are attracted to the Van de Graaff generator because the positive sides of these molecules are closer to the Van de Graaff generator than the negative sides.




The first soap bubble collides with the globe and becomes negatively charged.


This step happens in an instant. Upon contact with the Van de Graaff generator, the soap bubble picks up excess electrons from the Van de Graaff generator. Then the negatively charged soap bubble bursts, sending tiny droplets of negatively charge soap into the air.

This is an example of charging by conduction because electrons are transferred to the molecules of the soap solution as they touch the negatively charged globe.



The repelled tiny negative droplets collide with incoming soap bubbles.


The tiny droplets from the burst soap bubble are now repelled from the negatively charged Van de Graaff generator. A spray of negatively charged droplets fills the air around the globe. These negative droplets are attracted to the bubbles' positive side (see step 1). Some of these droplets collide with incoming bubbles, adding excess electrons to the bubbles, making them negatively charged by conduction.

This is an example of charging by conduction because electrons are transferred to the incoming soap bubbles as they collide with the negatively charged droplets.



Incoming bubbles are repelled and move away from the negatively charged globe.


The incoming soap bubble is now negatively charged, so the negatively charged globe repels it.

The incoming negatively charged soap bubble is now pushed away from the negatively charged globe because like charges repel.