Question

When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge.

Hint:

Rubbing only transfers electrons; it does not create charge. The glass gains \(+q\) and the silk gains \(-q\), so the total stays zero, obeying conservation of charge.

Answer 1

This question links the rubbing of bodies to the law of conservation of charge.

Step 1: State the law of conservation of charge. The total electric charge of an isolated system remains constant. Charge can be neither created nor destroyed; it can only be transferred from one body to another.

Step 2: Describe what happens on rubbing. Before rubbing, both the glass rod and the silk cloth are electrically neutral. Their equal amounts of positive and negative charge cancel out, so each has zero net charge.

Step 3: Mechanism of charging. When the glass rod is rubbed against silk, electrons are transferred from one body to the other (from glass to silk in this case). The glass rod loses electrons and becomes positively charged; the silk gains the same number of electrons and becomes negatively charged.

Step 4: Show consistency with conservation. The amount of positive charge appearing on the glass is exactly equal in magnitude to the negative charge appearing on the silk. No new charge is created; the charges simply separated and moved. Therefore the total charge of the glass-plus-silk system is
\[q_{\text{glass}}+q_{\text{silk}}=(+q)+(-q)=0\]
which is the same as before rubbing. Equal and opposite charges always appear together.

Step 5: Conclusion. Since charges always appear in equal and opposite pairs and the net charge stays zero, the phenomenon fully obeys the law of conservation of charge.

Answer 2

Expert framing: a redistribution argument with a book-keeping (balance-sheet) check.

Step 1: Treat the rod and cloth together as one closed (isolated) system. No charge enters or leaves this system from outside, so whatever conservation demands must show up as an internal balance.

Step 2: Friction does not manufacture charge; it merely supplies the small energy needed to dislodge loosely bound electrons from one surface and deposit them on the other. The act is a transfer, not a creation.

Step 3: Let \(N\) electrons cross from glass to silk, each carrying \(-e\). The glass is left with a deficit \(+Ne\); the silk acquires \(-Ne\). Write the balance sheet before and after:
before: \(0+0=0\); after: \((+Ne)+(-Ne)=0\).

Step 4: The total is invariant at zero, so the bodies become charged but the system's net charge is conserved. The unfailing pairing of equal-and-opposite charges in every such friction experiment (and indeed in pair creation in particle physics) is the experimental fingerprint of charge conservation.

Step 5: Hence the appearance of opposite charges on the two rubbed bodies is not a violation but an illustration of the conservation law: the books always balance.