Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (nylon against silk, glass against polyester, etc.) and each of the pithballs is touched by one or more of these objects (transferring some charge with each touch). It is found that pithballs 1 and 2 attract each other and that pithballs 2 and 3 repel each other. From this we can conclude

a - At least one of the pithballs must have negative charge. b - Pithballs 1 and 3 carry charges of opposite signc - We need to do more experiments to determine the sign of the charges. d - One of the pithballs carries no charge. e - All three pithballs carry charges of equal sign.

the correct statements are a and b

Explanation:

To solve this problem we must use the fact that charges of the same sign repel and charges of the opposite sign attract.

Let's analyze the facts that give us

* Spheres 1 and 2 attract, so they must have different sign, positive charge and negative charge

* Spheres 2 and 3 repel, so they must have the same sign

Let's build a table with the sign possibilities that meet the previous relationships. We set the sign of the first sphere and place the possibility of the other two, now we fix the sign of the second sphere we place the possibility of the others and we fix the sign of the third sphere and we place the sign of the other

Table charge

1 2 3

+ - -

- + +

- + +

+ - -

- + +

+ - -

Now let's analyze the relationships they give

a) true, some of the spheres have a negative charge

b) true, in all cases they have different signs

c) false, all charges are determined

d) false, would not move, the force would be zero

e) false, one of the spheres has a different sign

From the previous analysis we see that the correct statements are a and b