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Writer's picturekeshprad

PhysicsBowl 2018 Q2


2) This question asks us to describe what will happen when a grounded person touches an initially positively charged metal electroscope. First, let's take a minute to visualize the problem and the current state of the electroscope.

Where red is for protons, and orange is for the electroscope's leaves

Now, we need to figure out what is going to happen when a grounded person touches the electroscope.


The concept regarding grounding is that the Earth is considered essentially an infinite bank of electrons. When a grounded person touches an electroscope, the electrons will either travel into the ground or out of the ground.


In our case, since the electroscope is positively charged, electrons are attracted to it. Thus, electrons will flow from the ground, through the person, and into the electroscope until it comes to equilibrium.

Where red is for protons, blue is for electrons, and orange is for the electroscopes's leaves

As for the leaves of the electroscope, the only forces acting on them is the gravitational force, and the electric force from their charges. So when the charge is unbalanced, the leaves start to repel from each other, but when the charge is neutral, the gravitational force pulls the leave together.


We have concluded that electrons will flow into the electroscope which will cause the leaves of the electroscope to come together.


Answer: B

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