Posted: April 6th, 2015

TEST 03 / ELECTRICITY / Part 2 / Name:

TEST 03 / ELECTRICITY / Part 2 / Name:

This question refers to the last demonstration in the PBS video watched in class this Thursday, where a plastic cup containing several Styrofoam peanuts was placed on a Van der Graaf generator.

1.    Consider first a scenario A where just one Styrofoam peanut is placed directly on top of the sphere in the Van der Graaf generator. The Styrofoam peanut is initially electrically neutral overall.  The generator can be operated so that either type of charge, positive or negative, can be accumulated gradually in its metal sphere. Let’s assume that the sphere becomes progressively depleted of electrons. Predict what will happen to the Styrofoam after a long enough charging time has elapsed. Explain what happens to the electrons, the sphere, and the Styrofoam. Justify your prediction.

2.    Under the above assumption, is the sphere of the generator being positively or negatively charged? Is the Styrofoam being positively or negatively charged? Explain your answers. In this question, your explanation counts more than your answers.

3.    Consider now a scenario B where there is an empty plastic cup on top of the generator’s sphere, and that the generator is still operated as described in question 1, where the number of electrons in its sphere is depleted gradually as time passes by. What do you expect for the net electric charge of the cup?

4.    How does the plastic cup become electrically charged? The metallic surface of the sphere in a Van der Graaf generator is capable of holding enough electric charge to produce an electric field capable of ionizing the air in its surroundings. The typical ionization potential of some polymers is about 5 – 6 eV.  Research the ionization potential of the elements present in air. Quote your source.  What is your conclusion regarding the Van der Graaf generator ability to ionize the plastic cup?  Give a reasoned answer in terms of said ionization potentials.

5.    Will the cup be lifted by the electric field emanating from the generator’s sphere? Why or why not? Same question phrased differently: Will the cup fly due to the electrical interaction between the electric charge in the generator’s sphere and the cup’s electric charge? Why or why not?

6.    Now, consider the scenario C that matches the actual demonstration in the video. In this scenario, the peanuts are inside the plastic cup, which is in direct contact with the electrically charged generator’s metal sphere.  Explain from beginning to end, in a coherent manner, using appropriate physics concepts and reasoning, why the peanuts fly apart and the cup does not.