Just inside the front of a television screen there is a positively charges surface. (We'll learn about the details of how a television works later this semester.)
1. The front of a television screen tends to get dusty over time. Briefly explain how a neutral (no net charge) dust particle can be attracted to the front of the positively charged screen.
Answer/Why: The dust particles become polarized. Negative charges move to the side nearest the screen and positive charges move to the side farthest from the screen. Since the negative charges are closer to the screen than the positive charges the attractive force will be larger than the repulsive force causing the dust to stick.
2. If you rub a balloon in your hair it will become negatively charged. If you then dangle the balloon from a string in front of the television screen, what will happen to the balloon?
Answer: It will accelerate toward the screen.
Why: The attractive force between two oppositely charged objects will cause the objects to accelerate toward one another.
3. For the above question, how does the force on the balloon vary with its distance from the television screen? Please give a descriptive answer, not a formula or equation.
Answer: The force gets stronger as the balloon gets closer to the screen.
Why: The electrostatic force is inversely proportional to the square of the distance separating the two charged objects, so as the distance becomes smaller, the force becomes much larger.
4. After rubbing the balloon in your hair, your hair may stand on end as separated strands. Briefly explain this behavior. This works particularly well if you hair is very dry so that it acts as a good insulator.
Answer: After rubbing the balloon in your hair, each strand of hair will be positively charged. Since positive charges repel, each strand of hair will be repelled from its neighbors.
Why: For your hair to stand straight up, it must accumulate enough charge so that the electrostatic forces between strands of hair are large enough to overcome the force of gravity pulling down on them. Also, some conditioners cause your hair to conduct charge. This helps keep your hair from becoming frizzy since the charge can now leave your hair and spread out on the surface of your whole body.
5. Suppose you have two televisions and are curious which one has a larger charge on the front of its screen. To find out you place the two televisions with their screens facing each other and just a few inches apart. You then rub a piece of yarn against a balloon to give the yarn a positive charge. You stand beside the two televisions so that one is on your left and the other is on your right and dangle the charged yarn in between the two. The yarn is attracted toward the screen on your right. Which television screen has more positive charge on its front face?
Answer: The screen on the left has more positive charge.
Why: The screen on the left produces an electrostatic force on the string to the right with an amount proportional to the charge on this screen. The screen on the right produces an electrostatic force on the string to the left with and amount proportional to the charge on its screen. If the charge on the two screens were equal, these forces would cancel, but the yarn is pushed to the right indicating that the rightward force from the left screen is larger than the leftward force from the right screen.
6. For the above situation, in which direction does the electric field point?
Answer: To the right.
Why: The electric field points in the direction that a positive charge would accelerate due to the electrostatic force.
A xerographic copier can be used to copy onto clear plastic sheets such as those used with an overhead transparency projector. In many copiers, the plastic comes out with lots of positive charge on one side and lots of negative charge on the other.
7. Plastic is a good insulator. At what energy is the Fermi level located in plastic?
Answer: The Fermi level is located at the top of a band of energy levels that is just below a large energy gap.
Why: This energy gap prevents electrons from rearranging themselves among the levels so the plastic doesn't conduct charge.
8. If you hold a piece of plastic that has just come out of the copier under an incandescent lamp which produces lots of visible light, will the charges migrate from one side of the plastic to the other leaving the page unpolarized? Explain why.
Answer: No. Visible light does not have enough energy to rearrange the electrons in the plastic and allow it to conduct charge.
Why: If you took the plastic outside in the sunlight where there is ultraviolet light the plastic may begin to conduct enough to unpolarize the page since ultraviolet light carries much more energy than visible light.