Do the metals in the electrophorus and electroscope have a charge and store a charge?

Question Answered by Ben Jenkins , Systems Engineer, Idaho Falls Power

The metal in an electrophorus polarizes when in presence of a nearby static charge. When one side of the polarized metal plate is discharged then the metal plate is no longer in balance and contains a net positive charge.

An electroscope is based on the attracting or repelling forces of electrons. An nearby static charge causes the atoms in the metal (gold leaf) or pith ball of the electroscope to polarize. This polarization creates a force that moves the metal or pith. This measures the voltage of the nearby static source. The electroscope does not hold a charge unless discharge one side of the device while in presence of the static charge. Then it will act just like the electrophorus.

In short, yes the electrophorus stores a charge. The electroscope is not intended to, but it can if misused.

What is an Electrophorus?
An electrophorus is a device that uses static electricity to make small sparks. This experiment works best on very dry days.

Materials: Styrofoam plate, Styrofoam cup, aluminum pie pan, tape, wool (or just your hair)

  1. Place the pie pan face-up on the table.
  2. Tape the Styrofoam cup upside down to the middle of the pan. The cup will be an insulating handle.
  3. Build up static electricity (unbalance the charge) by rubbing wool on the Styrofoam plate.
  4. Once the Styrofoam plate is charged, place in on the table.
  5. Holding the pie pan by the handle, lower it onto the plate. After a moment, bring your finger close to the metal pie pan and you will get a small shock.
  6. Once the pie pan stops giving you any more shocks, lift it up and away from the Styrofoam plate using the handle. Touch the pie pan to get another shock.

You can keep repeating Steps 3 – 6. Each time you should get a shock.

When you rub wool against the Styrofoam, the wool loses some of its negative charges to the plate. It now has extra negative charges. Since the foam is an insulator, the charges are stuck in place.

When you put the pie pan on top, you charged the aluminum by induction. You didn’t really put a negative charge on the pie pan. The negative charges on the Styrofoam plate stayed there. They simply made the repelling force that rearranged the charges on the pan.

By touching the metal pan, you provided a path for the repelled negative charges to flow away from the pan. You felt a shock.

Since the pie pan just lost some negative charge, it now has extra positive charge. Lifting the pan up, away from the Styrofoam plate, the net positive charge can spread out evenly on the pan. Like charges repel and you felt a shock when you touched the pan again. That is the effect of negative charges flowing from you and into the positive pie pan just like it was in the beginning.

What is an Electroscope?
An electroscope is a device that can be used to test for the presence of charge, or that can be charged. Let’s make your electrophorus into an electroscope.

Materials: electrophorus, aluminum foil, mylar balloon cut into small strips

  1. Make a small person out of the aluminum foil. It should be about 4 inches tall. Give it a sturdy base so it doesn’t fall over. A cone shape with a round head works well. You can use tape, if necessary.
  2. Put it into the aluminum pan, making sure your aluminum foil person is touching the metal of the pie pan.
  3. Take three 6-in strips of Mylar and tape them down the middle of figure’s head to give it hair.
  4. Repeat Steps 3-6 of the Electrophorus Experiment. What happens to the figure’s hair when you put the pie pan onto the Styrofoam? What happens when you get a shock?

The Mylar strips are also conductors. When the strips are charged with negative charges, like charges repel and the hair strips move away from each other. When you touch the pan and the charges move to your hand, the hair strips move back together.

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