Electric Charge: Properties, Conductors, and Insulators

What is Electric Charge?

At its heart, electricity is all about the behavior of electric charges. These are not just abstract concepts; they are a fundamental property of the matter that makes up everything around us, including ourselves.

To get a bird's-eye view of the topics we'll cover today, take a look at the following infographic. It neatly summarizes the key ideas about electric charge, its properties, its carriers, and how it behaves in different materials.

As the infographic shows, there are two types of electric charge: positive and negative. The most basic rule governing their interaction is that like charges repel and unlike charges attract. This simple principle is the starting point for all of electrostatics.

To see these principles in action and build some intuition, let's watch a short video with some engaging demonstrations.

The video introduces the idea that rubbing objects together can transfer charge. An object becomes "charged" when it has an imbalance of positive and negative charge carriers. In most materials, the positive charges (protons) are locked within the atomic nuclei, while the much lighter negative charges (electrons) can sometimes move. An object with more electrons than protons has an excess negative charge, while one with fewer electrons than protons has an excess positive charge.

To delve deeper into the historical discovery of these charge carriers and formalize our understanding, the following reading is helpful.

The Two Pillars: Conservation and Quantization

Now that we have a feel for what charge is, let's examine two of its most fundamental and unwavering properties: charge is conserved, and charge is quantized.

1. Conservation of Charge

The law of conservation of charge is one of the bedrock principles of physics. It states that electric charge cannot be created or destroyed, only transferred from one object to another. In any closed system, the total amount of electric charge remains constant.

You can think of this like accounting for money. You can move money from your savings account to your checking account, but the total amount of money you have doesn't change (unless you make an external transaction). Similarly, rubbing a balloon on your hair doesn't create new charge; it just moves electrons from your hair to the balloon.

The following video provides an excellent, clear explanation of this law and works through a practical example.

The total charge before the interaction was $74\mu C+(-36\mu C)=+38\mu C$. After the interaction, the total charge must be the same: $25\mu C+13\mu C=+38\mu C$. The charge is conserved.

2. Quantization of Charge

The second pillar is the principle of quantization. This means that electric charge exists in discrete packets, rather than in continuous amounts. The smallest unit of free charge ever observed is the charge of a single electron or proton. This fundamental amount of charge is called the elementary charge, denoted by the symbol $e$.

Its value is approximately $e\approx 1.602\times 10^{-19}$ Coulombs (C), where the coulomb is the SI unit of charge.

The charge on any object, $Q$, must be an integer multiple of this elementary charge:

where $n$ is an integer (..., -2, -1, 0, 1, 2, ...). You can have an object with a charge of $+2e$ or $-10e$, but you can never have an object with a charge of $0.5e$ or $\pi e$. Charge doesn't come in fractions of $e$.

The following video does an excellent job of explaining this concept.

As the video makes clear, because matter is made of an integer number of protons and electrons, the total charge must be an integer multiple of $e$.

A Conceptual Challenge

Here is a question to test your understanding of quantization, a concept you'll frequently encounter in exam questions.

An experimenter claims to have measured a net charge of $-2.40\times 10^{-19}$ C on a small oil drop. Based on your knowledge of charge quantization, is this claim credible? Explain your reasoning.

Think about this for a moment. To check the claim, you would calculate the number of elementary charges this corresponds to:

Since $n$ is not an integer, the claim is not credible. An object can have one excess electron or two, but it cannot have one and a half.

Conductors and Insulators

Finally, let's connect these ideas to the familiar concepts of conductors and insulators. You already know that metals conduct electricity and materials like rubber or glass do not. The reason lies in the mobility of their charge carriers.

• Conductors: These are materials, like copper and aluminum, in which electrons are loosely bound to their atoms and are free to move throughout the material. These "free electrons" act like a fluid, able to flow when an electric force is applied. This is why excess charge placed on a conductor will spread out over its entire surface, as the like charges repel each other as far as possible.

• Insulators: These are materials, like glass, wood, and plastic, in which electrons are tightly bound to their atoms. There are very few, if any, free electrons. Because the charges cannot move freely, any excess charge placed on an insulator tends to stay in one spot.

To solidify these definitions, let's read a concise explanation.

The idea that conductors possess mobile electrons is the microscopic explanation for their ability to conduct electricity.

Conclusion

In this introductory lesson, we have laid the groundwork for our study of electricity and magnetism. We established that electric charge is a fundamental property of matter and reviewed the core principles that govern its behavior.

Here are the key takeaways:

• Electric charge is a fundamental property of matter, carried by particles like protons (positive) and electrons (negative). Like charges repel, and unlike charges attract.
• The Law of Conservation of Charge states that the total electric charge in an isolated system remains constant. Charge is transferred, not created or destroyed.
• Charge is quantized, meaning it exists in discrete units of the elementary charge, $e$. The net charge on any object must be an integer multiple of $e$.
• Conductors are materials with mobile charge carriers (free electrons) that allow charge to move easily. In insulators, charges are tightly bound and cannot move freely.

In our next lesson, we will build directly on this foundation. We will explore the specific mechanisms by which charge is transferred between objects—namely, charging by friction, conduction, and induction. This will allow us to predict and explain how objects become charged in a variety of situations.