As freshly laundered clothes tumble about in a hot dryer, they rub against one another. Sliding friction transfers electric charges from one piece of clothing to the other so that some items become positively charged and others negatively charged. These charge accumulations produce static cling. Fabric softener is a soap-like chemical that binds to wet fabric fibers and lubricates them. This lubrication is what softens the clothes. But fabric softener also attracts moisture, which in turn reduces static cling.
1. As you unload the dryer you find several socks clinging to a shirt. How do the charges on the socks and shirt compare to one another?
Answer: The charges are opposite one another.
Why: Since the socks cling to the shirt, you know that they are experiencing attractive forces. For an electrostatic force to be attractive, it must be between opposite charges.
2. As you pull the socks off the shirt, you do work on the socks. What form is your energy being transformed into?
Answer: Electrostatic potential energy.
Why: As you pull the items apart, they are clearly storing energy because they will snap back together if you let go. The potential energy between charged particles is electrostatic potential energy.
3. Suppose that your socks are covered with positive charge. As you pull them away from the shirt, what happens to the voltage of the charges on the socks?
Answer: The voltage increases.
Why: Voltage measures energy per charge. As you pull the items apart, the electrostatic potential energy increases. Since the number of charges involved remains constant, the energy per charge is increasing so the voltage is increasing.
4. The farther each sock is from the shirt, the less that sock and shirt attract one another. Explain.
Answer: The forces between charges decrease with distance.
Why: The electrostatic force or Coulomb force between two charges is inversely proportional to the square of the distance separating them. As the socks move away from the shirt, the distances between charges increase and the forces between them decrease.
5. After removing them from the shirt, you find that the socks repel one another. Explain.
Answer: The socks have like charges.
Why: What type of charge accumulates on a particular piece of clothing depends on its fabric. Since the two socks are made of the same fabric, they tend to accumulate similar charges. Once they are away from the shirt, their like charges repel.
6. If you put one of the clean socks on the wall, it sticks. The wall is made of an insulating material that was not initially charged, so why does the charged sock stick to it?
Answer: The sock's charge causes the wall to become electrically polarized.
Why: Although the wall is insulating and has no net charge, the proximity of the sock's charge causes a rearrangement in the wall's charges. Charges oppose those on the sock are pulled toward the sock while charges like those on the sock are pushed away. The polarization of the wall leads to an attraction between the wall and the sock.
7. If you hold a metal needle so that its point is very close to the charged sock, a faint blue glow will appear at the needle's tip and the sock will lose some of its charge. What is happening at the needle's tip that is reducing the charge on the sock?
Answer: A corona discharge from the needle's tip is spraying opposite charges toward the sock.
Why: The sock's charge attracts opposite charges out to the needle's tip, where they become very closely packed. Their mutual repulsions push them out into the air, producing a corona discharge and allowing them to flow toward the sock. They then neutralize some of the sock's charge.
8. The next time you do your clothes you decide to add fabric softener. Your clothes emerge from the dryer with a thin layer of moisture on them and this moisture permits charge to move freely about the clothes. Why does this charge mobility prevent the build-up of separated charge?
Answer: Separated charges (opposite charges) attract one another and will flow toward one another whenever possible. Because softened clothes conduct electricity, charges will move through them until they are electrically neutral throughout. [The lubricating effect of the softener may also reduce the charging effects of sliding friction]
Why: To sustain a charge separation, you must have an electric insulator. Electric conductors permit charge to move so that any charge separation is quickly neutralized. If you were to fill the drier with conducting aluminum foil instead of non-conducting clothes, it would not build up any static charge. The fabric softener helps to make the clothes more conducting so that they don't build up much static charge.