Q.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.

The key idea is that charge is neither created nor destroyed — it is only transferred. Rubbing transfers electrons from one body to the other, leaving one positively charged and the other negatively charged.

1. Before rubbing, both the glass rod and the silk cloth are electrically neutral — the total charge is zero.
2. When rubbed, electrons (negative charge) move from the glass rod to the silk cloth.
3. The glass rod loses electrons and becomes positively charged; the silk cloth gains those electrons and becomes negatively charged.
4. The magnitude of charge on each is exactly equal, so the net charge of the system (rod + cloth) remains zero — unchanged from the initial state. …

When you rub a glass rod with silk, the rod becomes positively charged and the silk negatively charged. This isn’t magic — it’s a transfer of electrons. The glass rod loses electrons to the silk, so the rod ends up with a net positive charge (more protons than electrons), and the silk gains those electrons, giving it a net negative charge.

The key point is that charge is never created or destroyed — it’s only moved from one place to another. Before rubbing, both the rod and the silk were electrically neutral. After rubbing, the total charge of the system (rod + silk) is still zero: the positive charge on the rod is exactly equal in magnitude to the negative charge on the silk.

This is exactly what the law of conservation of charge states: the net electric charge of an isolated system remains constant. Here, the system is the rod and silk together — no charge enters or leaves the pair.

Let’s walk through it step by step.

1. Initial state: Both the glass rod and the silk cloth are neutral. That means each has an equal number of protons and electrons. The total charge of the system is

2. During rubbing: The friction causes electrons to be transferred from the glass rod to the silk cloth. Glass holds its electrons less tightly than silk does, so electrons move from glass to silk.

   • The glass rod loses, say, $n$ electrons. Each electron has charge $-e$, so the rod’s charge becomes $+ne$ (since it now has $n$ more protons than electrons).
   • The silk gains those $n$ electrons, so its charge becomes $-ne$.

3. Final state: The rod has charge $+ne$, the silk has charge $-ne$. The total charge of the system is

   $Q_{\text{final}} = (+ne) + (-ne) = 0.$ …

Method: Law of Conservation of Charge — Explanation via Charge Transfer

This is not a numerical problem but a conceptual explanation. The method below shows how to reason step-by-step using the principle of charge conservation.

Step 1: Identify the initial state

• Before rubbing, both the glass rod and the silk cloth are electrically neutral.
• This means the total charge on each is zero:

• Therefore, the total charge of the system (rod + silk) is also zero.

Step 2: Describe what happens during rubbing

• Rubbing transfers electrons from one material to the other.
• Glass loses electrons → becomes positively charged.
• Silk gains those electrons → becomes negatively charged.

Step 3: Apply the law of conservation of charge

• The total charge of an isolated system remains constant.
• Initially:

• After rubbing: Let charge on glass rod = $+q$ …

Here’s a breakdown of the common mistakes students make when connecting the rubbing of a glass rod with silk to the law of conservation of charge, along with how to avoid each.

Mistake 1: Thinking charge is created during rubbing

What students do wrong:

They see that the rod becomes positively charged and the cloth becomes negatively charged, and assume that rubbing “produces” charge out of nowhere.

Why it’s wrong:

Charge is never created or destroyed — it is only transferred. The total charge before rubbing is zero, and after rubbing it is still zero.

How to avoid:

Always remember: Rubbing does not create charge; it separates charge.

• Before rubbing: rod (neutral) + cloth (neutral) → total charge = $0$
• After rubbing: rod ($+Q$) + cloth ($-Q$) → total charge = $0$

Key idea: The net charge of an isolated system remains constant.

Mistake 2: Ignoring the sign of the charges

What students do wrong:

They say “the rod gains positive charge and the cloth gains negative charge” without checking that the magnitudes are equal.

Why it’s wrong:

Conservation of charge requires that the algebraic sum stays zero. If the rod gets $+5 \, \mu\text{C}$, the cloth must get exactly $-5 \, \mu\text{C}$ — not $-4 \, \mu\text{C}$ or $-6 \, \mu\text{C}$.

How to avoid:

Always write the equation:

So if $Q_{\text{rod}} = +q$, then $Q_{\text{cloth}} = -q$.

Mistake 3: Confusing “conservation of charge” with “charge is constant on each body”

What students do wrong:

They think that because the rod ends up positive, it “keeps” that charge forever, and that this is the conservation law.

Why it’s wrong:

Conservation of charge applies to the total charge of an isolated system, not to individual objects. The rod can later lose its charge by touching a conductor — that’s fine, as long as the total charge in the system remains unchanged.

How to avoid:

• Conservation of charge: $\sum Q_{\text{initial}} = \sum Q_{\text{final}}$ for the whole system.
• Individual objects can gain or lose charge — that’s transfer, not violation.

Mistake 4: Not linking the observation to the law explicitly

What students do wrong:

They describe the experiment but never state: “This shows that charge is conserved.”

Why it’s wrong:

The question asks how the observation is consistent — you must explicitly connect the observation to the law.

How to avoid:

End your explanation with a clear sentence:

“Thus, the total charge before and after rubbing is zero, which is consistent with the law of conservation of charge.”

Mistake 5: Using Gauss’s law incorrectly in this context

What students do wrong: …