1. **Problem 16.19** When a Perspex rod is rubbed on a woollen cloth, the rod acquires a negative electric charge. a) What type of electric charge does the cloth acquire? b) What can you say about the amounts of charge on the two charged items? c) If you had two Perspex rods charged up in this way, how could you show that they both have electric charges of the same sign? --- **Step 1: Understanding the problem** When two objects are rubbed together, electrons can transfer from one to the other. The object gaining electrons becomes negatively charged, and the other becomes positively charged. **Step 2: Answering part (a)** Since the Perspex rod acquires a negative charge, it has gained electrons. Therefore, the woollen cloth loses electrons and acquires a positive charge. **Step 3: Answering part (b)** The law of conservation of charge states that the total charge before and after rubbing remains the same. Therefore, the amount of negative charge gained by the Perspex rod equals the amount of positive charge gained by the woollen cloth. Mathematically: $$|Q_{rod}| = |Q_{cloth}|$$ where $Q$ denotes the charge. **Step 4: Answering part (c)** If two Perspex rods are charged by rubbing with woollen cloth, both rods acquire negative charges. To show they have charges of the same sign, bring the two rods close together. Since like charges repel, the rods will repel each other. --- 2. **Problem 16.20** a) A girl has two metal bars, 1 and 2. i. Bar 1 attracts both the N and S poles of a magnet. ii. Bar 2 attracts the N pole but repels the S pole. b) Two table-tennis balls A and B coated with conducting material. Ball A is negatively charged and moved towards ball B until they touch. --- **Step 1: Understanding part (a)(i)** Bar 1 attracts both poles of the magnet. This means bar 1 is a magnetic material (like iron) and is magnetized or ferromagnetic. **Step 2: Understanding part (a)(ii)** Bar 2 attracts the N pole but repels the S pole. This behavior is typical of a magnet with a specific pole. The bar itself is magnetized and has a definite magnetic pole. **Step 3: Understanding part (b)** Ball A is negatively charged and touches ball B. Since both balls are conductors, electrons will flow from ball A to ball B until they reach the same electric potential. This means ball B will acquire some negative charge. When ball A is moved away, both balls will repel each other because they have like charges. --- **Final answers:** **16.19** a) The woollen cloth acquires a positive electric charge. b) The amounts of charge on the rod and cloth are equal in magnitude but opposite in sign. c) Two negatively charged Perspex rods repel each other, showing they have charges of the same sign. **16.20** a)(i) Bar 1 is magnetic and attracts both poles, indicating it is ferromagnetic. a)(ii) Bar 2 is magnetized with a specific pole, attracting one pole and repelling the other. b) Ball B acquires a negative charge after contact and both balls repel each other.