Definition 20.2.1. Charge.
Electric charge is a physical property of subatomic particles, measured in coulombs (C), that governs how matter generates (and interacts with) electric and magnetic fields.
Section 20.2 Charge
Electric charge is a fundamental property of matter. Objects that are “charged” can display interesting behaviors when interacting with electric fields that we will explore in this chapter.
Electric charge is often represented with the letter \(q\text{.}\) Charge, like mass, is a physical property of subatomic particles. Electric charge can be positive \((+q)\) or negative \((-q)\text{.}\) In an atom, protons are positively charged and electrons are negatively charged while neutrons are neutral. We represent positive charges with the symbol “+” and negative charges with the symbol “-”.
In examining the behavior of positive and negative charge, we can find that unlike charges tend to attract while like charges repel. Therefore, two electrons or two protons will repel each other while a proton and an electron will tend to attract.
Note 20.2.3. Properties of Charge.
In addition to the existence of two types of charge, several other properties of charge have been discovered:
- Charge is quantized. This means that electric charge comes in discrete amounts, and there is a smallest possible amount of charge that any object can have. In the SI system, this smallest amount is called the fundamental unit of charge and is equal to\begin{equation*} e \equiv 1.602 \times 10^{-19} \, \mathrm{C}\text{.} \end{equation*}No free particle can have less charge than this. Therefore, the charge on any object must be an integer multiple of this amount. The charge on a single proton is \(+e \) and the charge on a single electron is \(-e \text{.}\) All macroscopic, charged objects have charge because electrons have either been added or taken away from them, resulting in a net positive or negative overall charge.
- Charge is conserved. Charge can neither be created nor destroyed; it can only be transferred from one location in space to another, or from one object to another. The net charge of the universe, or of any closed, isolated system is constant. This is referred to as the law of conservation of charge. We often speak of two charges “canceling”, however, this is verbal shorthand. It means that if two objects have equal and opposite charges, and are physically very close to each other, then the (oppositely directed) forces they apply on each other cancel as vectors. However, the charges on the objects still remain intact. The net charge of the universe remains constant in time according to conservation laws.
- Charges generate electric fields. Just like mass can generate a gravitational field, electric charge can generate an electric field. Mass and charge physically alter the space around them such that any other mass or charge that comes into their field of influence feels a gravitational or electric force. Understanding how charges generate electric fields and how other charges interact with electric fields will be our primary focus for understanding electric interactions.
- Moving charges generate magnetic fields. Just like stationary charges can create electric fields, charges that move with some velocity generate magnetic fields. Magnetic fields, like electric fields, can interact with charges in interesting ways that we will explore in further chapters.
| Particle | Charge (C) | Mass (kg) |
| Electron |
\begin{equation*}
-1.602 \times 10^{-19}
\end{equation*}
|
\begin{equation*}
9.11 \times 10^{-31}
\end{equation*}
|
| Proton |
\begin{equation*}
+1.602 \times 10^{-19}
\end{equation*}
|
\begin{equation*}
1.67 \times 10^{-27}
\end{equation*}
|
Exercises Activities
1. Neutral Atom.
A neutral atom always has the same number of protons as electrons. What is the total charge on such an atom?
2. Charged Object.
Suppose you have an object with a net charge of exactly 1.0 C. How many more protons than electrons are in this object? Based on this, do you think a net charge of 1.0 C is typical for an everyday object, or is it much larger or smaller than you would expect for an everyday object?
