(JUNE 1998 416): Spheres A, B,C, D, and E illustrated below are electrically charged. The following diagrams show what happens to these spheres when they are suspended in pairs close to each other.
The spheres are then rearranged so that the following pairs are close to each other: A and E, B and C, B and E, C and D.
Which of the following is TRUE?
Spheres A and E will repel each other.
Spheres B and C will repel each other.
Spheres B and E will repel each other.
Spheres C and D will attract each other.
(JUNE 1997 416/436): A student has been given a wool cloth and a vinyl ruler. Two styrofoam balls, A, and B, have been suspended from ring stands.
She records the following observations:
STEP
RESULT
1. Rub the ruler with the wool cloth.
1. The wool cloth and the ruler acquire opposite charges.
2. Touch ball A with the ruler.
2. Ball A and the ruler have the same charge.
3. Touch ball B with the wool cloth.
3. Ball B and the wool cloth have the same charge.
4. Bring the ruler close to ball A, but without touching it.
4. ???
5. Bring the ruler close to ball B, but without touching it.
5. ???
What were the results of steps 4 and 5?
A. result of step 4: the ruler and ball A repelled each other.
Result of step 5: the ruler and ball B attracted each other.
B. result of step 4: the ruler and ball A repelled each other.
Result of step 5: the ruler and ball B repelled each other.
C. result of step 4: the ruler and ball A attracted each other.
Result of step 5: the ruler and ball B repelled each other.
D. result of step 4: the ruler and ball A attracted each other.
Result of step 5: the ruler and ball B attracted each other.
(JUNE 1999 416): A balloon rubbed against human hair becomes negatively charged with static electricity.
How does this happen, and what charge, if any, will the hair be left with?
Answer:
Friction (from rubbing) causes some electrons to leave the hair and settle on the surface of the balloon. This causes the balloon to have too many electrons, so it becomes negatively charged. Meanwhile, since the hair has lost negatives, it now has an excess of positive particles(protons), so we say that the hair is
positively charged. If it's a very dry day, electrons will not be quickly restored to the hair. As a result the individual hairs repel one another and they stand on end, converting an attractive teenager into a temporary monster.
(JUNE 2000 416): In a lab, a student was given a piece of fur, a plastic rod and a suspended balloon. Using these materials, the student performed the following steps in the order shown below.
STEP
1. The plastic rod was rubbed with the piece of fur.
2. The suspended balloon was touched with the plastic rod.
STEP
RESULT
3. The rod was brought closer to the balloon.
The balloon and the rod repelled each other.
4. The fur was brought close to the balloon.
??????????
Predict the result for step 4 and justify your answer.
Answer:
1.The plastic and the fur will assume opposite charges when rubbed. Assume that the plastic was (+) and that the fur was(-). (The reverse assumption will lead to the same conclusion if followed consistently).
2. Touching the balloon with the plastic rod will give it the same charge, so if we assumed that the plastic was (+), it will cause the balloon to be (+).
3. Result: The (+) balloon will be attracted to the negative fur. Opposites attract.
Notes:
- Static results from an excess or shortage of electrons
- An object with too many electrons is (-)
- An object with too few electrons is (+)
- Rubbing causes electrons to leave one object and enter the other material that is being rubbed. The loser of electrons becomes (+). The gainer becomes (-).
- Temporarily touching a neutral object with a charged object can cause that object to assume the same charge as the charged object. So, for example, if you touch a positive object, some electrons will leave your body and enter that charged object. With less electrons, you are now (+). In cases where materials hang on to their electrons stubbornly, like in a wall, the electrons from the wall will move to the surface if you place a (+) balloon there. Since the electrons don't actually leave the wall, the opposite charges cause the balloon to stick.
- Identical charges repel (move away from each other if they are attached to a freely swinging thread)
- Opposite charges attract (move towards each other if they are attached to a freely swinging thread)
