Questions: Elementary Charge and Charge Conservation

Question 1

You rub a glass rod with a silk cloth. The rod becomes positively charged. What happened at the microscopic level?

• A. Positive charge was created on the glass surface by the friction energy
• B. Electrons transferred from the glass rod to the silk cloth, leaving the rod with a deficit of negative charge
• C. Protons were knocked free from the silk and deposited on the glass
• D. Both positive and negative charges were created in equal amounts on the two objects

Answer: (B)

Charge conservation forbids creation or destruction of charge. What actually happens is electron transfer: electrons (the mobile charge carriers in most materials) move from the glass to the silk. The glass loses electrons and becomes positively charged; the silk gains electrons and becomes negatively charged. The total charge of the glass+silk system remains zero throughout. Protons are tightly bound in atomic nuclei and do not transfer during everyday friction.

Question 2

An isolated system initially contains particles with charges +5e, −3e, and +2e. After a series of interactions, collisions, and charge transfers within the system, what must be true?

• A. The total charge could be anything — interactions can redistribute charge freely
• B. The total charge remains +4e — charge is conserved in isolated systems
• C. The total charge approaches zero as positive and negative charges neutralize each other
• D. The total charge is +4e only if no particles left the system

Answer: (B)

Charge conservation is absolute in isolated systems: the algebraic sum of all charges never changes. Starting with +5e − 3e + 2e = +4e, the total must remain +4e regardless of what interactions occur internally. Option C is a common misconception: positive and negative charges can neutralize each other locally (like an electron and positron annihilating), but in any such event the total charge is preserved — in annihilation, the photons produced carry zero charge, and the −e of the electron exactly cancels the +e of the positron.

Question 3

In an isolated system, negative charge can be destroyed if an equal positive charge is brought nearby — the two charges annihilate and cancel.

• T. True
• F. False

Answer: False

Charge conservation is absolute: charge cannot be created or destroyed in any process. When an electron (−e) and positron (+e) annihilate, they produce photons with zero charge — the total charge before (+e − e = 0) equals the total charge after (0 from photons). The charges do not 'cancel out and disappear'; rather, the charged particles transform into uncharged photons while the total charge of the system is preserved. No experiment has ever observed a net change in total charge.

Question 4

A photon has zero electric charge. When a high-energy photon produces an electron-positron pair, charge conservation is satisfied because the positron carries positive charge that offsets the electron's negative charge.

• T. True
• F. False

Answer: True

This is a clean example of charge conservation at the particle physics level. The photon has charge 0; the electron has charge −e; the positron has charge +e. Total charge before: 0. Total charge after: −e + e = 0. Charge is conserved exactly. This is why pair production always creates a particle and its antiparticle together — charge conservation demands it. You cannot create just an electron from a photon without violating charge conservation.

Question 5

Why is it physically incorrect to say that 'rubbing creates charge'? What actually happens, and what law of physics does this illustrate?

Model answer: Rubbing does not create charge — it transfers existing charge between objects. When two materials are rubbed together, electrons move from one object to the other based on the materials' relative electron affinities. The object that gains electrons becomes negatively charged; the object that loses electrons becomes positively charged by an equal amount. The total charge of the system (both objects combined) remains exactly what it was before rubbing — typically zero if both started neutral. This illustrates charge conservation: the total electric charge in an isolated system is constant and cannot be created or destroyed.

The language 'creates charge' is misleading because it implies charge came from nothing, which violates conservation. A better description is 'separates charge' — rubbing moves already-existing charge from one place to another. The conservation law is not a guideline but an exact empirical fact: it holds in every known physical process, from static electricity to nuclear reactions to particle annihilation, always with zero exceptions.