Charge Is Neither Created Nor Destroyed — It Only Moves 🔄

Here comes the first major law:

Principle of Conservation of Charge: The algebraic sum of all electric charges in an isolated system remains constant.

This means charge can be transferred from one object to another, but it is never created or destroyed. To this day, no experiment has violated this law — making it one of the most reliable laws in physics. 🎯

Second Law: Quantization of Charge 🧱

The second interesting discovery is that charge is not continuous — it’s not like water, where you can take any amount. Charge comes in discrete packets, and the smallest packet is the charge of a single electron (the elementary charge, $e$). Therefore, any observed charge is always an integer multiple of $e$:

$$ q = pm n e quad , quad n = 0, 1, 2, … tag{1-1} $$

A good analogy: the number of students in a class is always a whole number. You can’t say “24.3 students are in the class” or “12.4 eggs are in the box” 🥚. Charge works the same way — it’s always a whole-number multiple of $e$.

💡 Example 1-1 (from the book): You walk across a carpet, become charged, and receive a small shock when shaking hands with a friend — as if about $1 text{nC}$ of charge has been transferred. How many electrons were transferred?
From $q = ne$, we have: $n = dfrac{q}{e} = dfrac{1times10^{-9}}{1.6times10^{-19}} approx 6times10^{9}$ electrons!
That means six billion electrons were transferred with a simple handshake 🤯.

🧮 Calculate it yourself (Exercise 1-1): The atomic number of uranium is $Z = 92$. What is the electric charge of the uranium nucleus? What is the total charge of the electrons in a neutral uranium atom? So, what is the total charge of the neutral atom? (Hint: The nucleus has 92 protons, each with a charge of $+e$.)

“Did You Know?” Box 💡 — Do Quarks Break the Rules?

The electron is a fundamental particle; that is, it is not made of anything else. But protons and neutrons are different — they are made of smaller particles called quarks. And here is something strange: quarks have charges of $-frac{1}{3}e$ and $+frac{2}{3}e$! That means fractional charge! 😵

Doesn’t this contradict the quantization of charge? No — because a single quark has never been observed. Quarks cannot be separated. For example, a proton = two up quarks ($+frac{2}{3}$ each) + one down quark ($-frac{1}{3}$), which sums exactly to $+e$. Nature always delivers its packaging correctly 😎.

🔗 For the Curious — Deeper Study

• HyperPhysics — Georgia State University (Level: Intermediate, English): The electricity section, explaining charge and electric force with diagrams → hyperphysics.phy-astr.gsu.edu
• CERN — European Organization for Nuclear Research (Level: Beginner to Intermediate): If quarks and fundamental particles have caught your interest, read here in simple terms about the building blocks of matter → home.cern

📌 The experiment that first measured the fundamental charge of the electron was the Millikan oil-drop experiment (1913). Robert Millikan suspended tiny charged oil droplets between two plates and showed that their charge was always a multiple of a constant value. Such a simple idea won a Nobel Prize!

Test Yourself 📝

Click on each question to reveal its answer 👇

In the next section (1-3), we’ll explore a fundamental law: how strong is the force between two charges? Coulomb has a beautiful formula for us! 👋