Chapter 20, Problem 1CQ

To determine

The state of the charge on balls $A$ , $B$ , $C$ and $D$.

Answer to Problem 1CQ

The ball $A$ is negatively charged, ball $B$ is neutral, ball $C$ is positively charged and ball $D$ is neutral.

Explanation of Solution

When a plastic rod is rubbed with wool, the plastic rod acquires negative charge on it because from charge model, the rubbing charges the objects by transferring charge from one to the other.

The ball $A$ is negatively charged because the negatively charged plastic rod is touched to the ball and also according to charge model; the charge can be transferred from one object to another when they are placed in contact with each other.

Case 1: Balls $B$, $C$ and $D$ are attracted to $A$.

The charge model states that two objects with same charge exert repulsive force on each other and two objects with different charge exert attractive force on each other.

The neutral objects have an equal mixture of positive and negative charge and they can be attracted by positively or negatively charged body due to charge polarization.

The balls $B$, $C$ and $D$ can be either positively charged or neutral because they are attracted to ball $A$. This attractive force between ball $A$ and other balls is either due to positive charge on $B$, $C$ and $D$, due to the fact that opposite charges attract, or due to $B$, $C$ and $D$ being neutral.

Case 2 : Balls $B$ and $D$ have no effect on each other.

Both balls, $B$ and $D$, are neutral because if either one of them have certain amount of charge, the other ball could have experienced attractive or repulsive force. But they have no effect on each other.

Case 3 : Ball $B$ is attracted to ball $C$.

Ball $B$ is neutral and it is attracted to ball $C$. This implies ball $C$ has some charge on it due to which is attracted to ball $A$ earlier. Ball $C$ is positively charged due to which it get attracted towards ball $A$ and it attracted ball $B$ which is neutral.

Conclusion:

Thus, the ball $A$ is negatively charged, ball $B$ is neutral, ball $C$ is positively charged and ball $D$ is neutral.