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Electric Forces
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| Solutions |
| Q1: The
original source of charge for the electrophorus is the charged Styrofoam
plate. How does this charge come about? What is the sign of this charge? (Hint: here is a list of triboelectronegativities that will help determine the sign of the charge on the Styrofoam plate.) The Styrofoam plate is initially charged due to the friction with another surface (presumably someone's hair). If the plate is charged by rubbing it on someone's hair it is negatively charged. Q2: Describe the flow of electrons to or from the Al plate when it is DOWN on top of the Styrofoam plate and the bulb is touched. Draw a diagram indicating charge distributions before and after the hand contacts the bulb. When the free end of the bulb is touched while the pan was on the Styrofoam plate, electrons flow from the edge of the pan, through the bulb, and onto your finger. NOTE: The amount of charge transferred from the plate to the
pan is insignificant. Styrofoam is an insulator and therefore charge
is generally bound in place. |
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| Q3: Again
describe what happens to electrons when the Al plate is raised in
the air and again touched. Again diagram the charge distribution before
and after contact. How is the up-in-the-air situation different?
When the bulb it touched while the pan is raised in the air, electrons flow from your finger, through the bulb, and onto the plate. |
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| When the pan is on the Styrofoam
plate, charge is separated: negative around the rim, positive toward
the center. This separation is induced by the negative charge of the
Styrofoam plate. The pan has a positive and negative "side", but the
net charge is zero. After the plate has been discharged (electrons
have been allowed to move to ground through your hand) it has a net
positive charge.
Q4: Closely examine the Ne bulb during the down-touch, up-touch cycle. Only one side of the NE bulb SHOULD glow at a time, and this should change depending on whether the Al plate is up or down. Which side (most positive or most negative) glows? The most negative side glows. When the pan is down this is the side toward the pan. When the pan is up this is the side toward your finger. Q5: The down-touch, up-touch cycle can be repeated almost indefinitely or until the Styrofoam charge leaks away. Since the charge on the Styrofoam is not "used up," where does the energy to light the bulb repeatedly come from in this device? You provide the energy. You do work moving the charges on the pan through the electric field produced by the negative charges on the plate. Q6: Write a complete description of how the electrophorus works. Use your description to provide examples to define and explain charge transfer by friction, contact and induction. The Styrofoam plate is initially charged due to the friction between it and someone's hair (friction). Because it has a higher affinity for electrons than hair, it becomes negatively charged. Placing the pan in the styrofoam electric field produced by the negative charge causes the free charges on the pan to separate, the electrons fleeing to the rim (induction). Touching the loose end of the bulb provides the electrons a path to leave the pan, lighting the bulb as they move through it. The pan is then left with a positive charge. When the pan is removed from the electric field of the plate and the loose end of the bulb touched, electrons from your hand move through the bulb onto the pan (contact). The pan is then neutral and the process can be repeated. Advanced Question: Arizona gas pumps have a sticker on them as shown. Explain why. |
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