Question

We’ve explored the behavior of charge in the context of the Van de Graaff (VDG) generator. In one experiment, we tried to predict the behavior of a grounded metal wand in relation to the VDG dome. We saw the wand experience moments of attraction to the dome, interrupted by brief arcs between them. We came to some understanding of what was happening, and in the process, students asked several great questions. One question was about how the behavior of the wand would be different if it was not grounded. I was able to make the quick adjustment, and there was indeed a behavioral difference across the two situations (grounded vs. ungrounded wand). For this problem, I’m asking you to describe the difference and why it arises. Even if you didn’t observe the difference in the two experiments (so that from that you could posit an explanation), you can instead think about the nature of a grounded vs. ungrounded metal wand, how that would impact the interaction between the wand and the dome, and propose the behavioral differences.

Answer 1

Let us consider that in this experiment the Van de Graaff (VDG) generator is the type which accumulates positive charges in its outer shell.

A grounded metal wand is brought near the generator. As stated in the question, moments of attraction were observed, with brief moments of interruptions between them.

Now the outer dome of the VDG generator being highly positively charged, attracts the negative charges of the metal rod towards it, and repels the positive charges. As a result the end of the rod nearer to the VDG is negatively charged, and the end further gets positively charged. Now when the rod is close enough to the VDG, positive charges break the air and jump to the metal rod. As a result, momentarily, the rod has excess positive charges, which leads to the brief moments of interruption observed. This is in fact the result of momentary repulsion between the VDG and the metal rod.But since the metal rod is grounded, the excess positive charges can flow out, making the rod electrically neutral once again. The rod once again can attract charges from the VDG.

If the rod is not grounded, the excess positive charges have nowhere to escape. As a result the rod remains positively charged, and is repelled by the VDG from then onwards.