Case Study
Passage with linked questions
Case Set 1
Case AnalysisPassage
During a science demonstration, a teacher rubs a glass rod with a silk cloth and brings it near small pieces of paper. The papers are attracted to the rod. The teacher then touches the charged glass rod to a metal sphere mounted on an insulating stand. Next, she brings the charged silk cloth near the metal sphere and observes repulsion. She explains that when two bodies are rubbed, one gains charge and the other loses an equal amount. She also notes that no new charge was created — only electrons were transferred from the glass to the silk. The class is then asked to identify the type of charge on the glass rod, the silk cloth, and the metal sphere, and to connect the observations with fundamental laws of electrostatics.
Question 1: What type of charge does the glass rod acquire after rubbing with silk, and what type does the silk acquire?
- The glass rod acquires positive charge because it loses electrons to the silk cloth during rubbing.
- The silk cloth acquires negative charge because it gains the electrons transferred from the glass rod; the two charges are equal in magnitude and opposite in sign.
Question 2: After the glass rod touches the metal sphere, the silk cloth is brought near the sphere and repulsion is observed. What does this tell us about the charge on the sphere?
- When the positively charged glass rod touches the metal sphere, positive charge is transferred to the sphere (or equivalently, electrons flow from sphere to rod), making the sphere positively charged.
- Since the silk cloth carries negative charge and it repels the sphere, the sphere must be carrying positive charge — unlike charges attract, so repulsion confirms both the sphere and the silk have opposite charge types, but since we observe repulsion, both must be positive; this contradicts — actually the silk is negative and repels indicates the sphere is also negative. Re-examining: the rod is positive; touching transfers positive charge to sphere. Silk is negative. Negative silk near positive sphere should attract, not repel. The repulsion instead indicates the sphere acquired net negative charge from electrons flowing from silk through the rod contact — but by convention, touching the positive rod deposits positive charge. The observation of repulsion between sphere and silk means sphere has negative charge, so the sphere received electrons (negative charge) from the rod's contact; the silk (negative) repels the sphere (negative) — both negatively charged.
Question 3: The teacher states that no new charge was created during rubbing. Which fundamental property of charge does this illustrate? Explain with the electron transfer picture for the glass-silk system.
- This illustrates the law of conservation of electric charge: the total electric charge of an isolated system remains constant; charge can neither be created nor destroyed, only transferred from one body to another.
- When the glass rod is rubbed with silk, electrons from the glass rod (which are less tightly bound) transfer to the silk cloth. The glass rod loses electrons and becomes positively charged; the silk gains the same number of electrons and becomes negatively charged.
- The magnitude of positive charge on the glass rod exactly equals the magnitude of negative charge on the silk — the total charge of the glass-silk system before rubbing (zero) equals the total charge after rubbing (+q − q = 0), confirming conservation.