Static electricity experiment with two metal balls and a charged rod.

Problem 1: Static Electricity and Charged Balls

#### Given:
- Two lightweight metal balls, P and Q, are suspended using nylon threads.
- A positively charged rod is placed between the two balls.
- Ball P is attracted towards the positively charged rod.
- Ball Q is repelled away from the positively charged rod.

#### Task:
Determine the correct conclusion for the charge on the balls P and Q.

#### Solution:
1. Understanding Attraction and Repulsion:
- Opposite charges attract each other.
- Like charges repel each other.
- A neutral object can also be attracted to a charged object due to induced polarization (but it does not repel).

2. Analysis of Ball P:
- Ball P is attracted towards the positively charged rod.
- This attraction can occur in two scenarios:
- P has a negative charge (opposite to the positive charge of the rod).
- P is neutral (a neutral object can be attracted to a charged object due to induced polarization).

3. Analysis of Ball Q:
- Ball Q is repelled by the positively charged rod.
- Repulsion occurs only when like charges interact.
- Therefore, Q must have a positive charge (like the positive charge of the rod).

4. Conclusion:
- Ball P can either have a negative charge or be neutral.
- Ball Q must have a positive charge.

#### Correct Answer:
The correct option is:
C. negative or neutral (Ball P), positive only (Ball Q)

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Problem 2: Charging by Friction

#### Given:
- A Perspex rod is rubbed with a dry woollen cloth.
- The rod becomes positively charged.

#### Task:
Explain why the rod becomes positively charged.

#### Solution:
1. Charging by Friction:
- When two different materials are rubbed together, electrons are transferred from one material to the other.
- The material that loses electrons becomes positively charged, while the material that gains electrons becomes negatively charged.

2. Perspex Rod and Woollen Cloth:
- Perspex (polymethyl methacrylate) tends to lose electrons when rubbed with a woollen cloth.
- The woollen cloth, being more electronegative, gains these electrons.

3. Resulting Charges:
- The Perspex rod loses electrons and becomes positively charged.
- The woollen cloth gains electrons and becomes negatively charged.

4. Explanation of Options:
- A. the cloth has lost some of its electrons.
- Incorrect. The cloth gains electrons, not loses them.
- B. the rod has gained some protons from the rod.
- Incorrect. Protons are not transferred during charging by friction; only electrons are transferred.
- C. the rod has changed some of its electrons into protons.
- Incorrect. Electrons cannot be converted into protons during this process.
- D. the cloth has gained some electrons from the rod.
- Correct. The woollen cloth gains electrons from the Perspex rod, leaving the rod positively charged.

#### Correct Answer:
The correct option is:
D. the cloth has gained some electrons from the rod.

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Final Answers:

1. C. negative or neutral (Ball P), positive only (Ball Q)
2. D. the cloth has gained some electrons from the rod.

$$
\boxed{C \text{ and } D}
$$