Answers to p74


1.         Study the situation below. What would cause the electrons to flow out of one sphere and into the other? Draw in the answer in the first picture.


If a (-) balloon comes close to the left sphere, electrons will be repelled into the sphere on the right, leaving the ist sphere with an electron-deficit,  meaning that it will be (+)’ly charged. The sphere on the right gets the electrons refugees and becomes negatively charged.




2.         Turntables, those ancient record-players that consisted of a needle at the tip of a mechanical arm which made contact with a turning vinyl record, had to be grounded to the amplifier. In other words, aside from the wires that brought in the current and sent the sound signals, there was a third ground wire connecting the back of the two components. Why?


The contact between the spinning record and metallic needle causes electrons to move out of the metal and into the plastic. The ground wire allows this excess charge to flow out of the sound system and into ground.


3.         You are asked to see if two electric charges have the same sign, using the following materials:


-  2 plastic rulers

-  1 piece of string

-  1 ring stand

-  1 piece of wool


Explain the produce you would use to do this.


Ideally, you could hang each charged piece from the ring stand using strings to see if they repel one another, but they’re only giving you one piece of string and no scissors. So charge one plastic ruler by rubbing it wool, and hang each charge from the ring stand, one at a time. If the ruler has the same effect on each charge, then the two charges are identical. Otherwise they have opposite signs.


4.         Two spheres are in contact with one another. Each originally has the same charge. But one of the spheres is smaller (area = 12 cm2), so it loses 4 units of charge to the less crowded and larger sphere (area = 36 cm2).

            a.         How many units of charge did each sphere begin with?

            b.         What is each sphere's charge density after the transfer?



a.         (x - 4)/12 = (x + 4)/36

36[(x-4)/12 = (x+4)/36]

3(x-4) = x + 4

3x – 12 = x + 4

2x = 16

x = 8

b.         (8-4)/12 = 1/3 = 1 unit of charge per 3 cm2 of area



5.         Silk and glass are two electrically neutral materials.  Silk can be represented by


 and glass by  .


After these materials are rubbed together, silk becomes negatively charged and glass becomes positively charged.


Which of the following models may represent silk and glass after these materials have been rubbed together?
















6.         (430 only)


a.         What happens to the force between two charges if the distance separating them triples?

It becomes 32 = 9 times weaker.


b.         If the repulsive force between two objects is to remain the same, and if one object's charge becomes sixteen times bigger, what distance should separate the charges?

            It should become 4 times greater.

7.         Explain why it's safer to hook up a static generator to the sink before touching it for the hair-raising trick.

            It prevents excess charge from building up.