Section 21.2 Charge
Subsubsection Key Ideas
Definition 21.2.2. Charge.
Electric charge, commonly represented with the letter \(q\text{,}\) is a fundamental physical property of matter with the following properties:
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Charge governs interactions with the electric field. Charged objects create electric fields, and they also feel a force due to other electric fields.
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Charge can be positive or negative. In an atom, protons are positively charged, electrons are negatively charged, and neutrons are electrically neutral (zero charge). Unlike charges tend to attract while like charges tend to repel.
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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.
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Charge is quantized. Electric charge, which is measured in coulombs, occurs in discrete, integer multiples of the fundamental unit of charge\begin{equation*} e \equiv 1.602 \times 10^{-19} \mathrm{~C} \end{equation*}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.
| 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*}
|
Analogy with Gravity.
Gravity and electricity share a mathematical structure, so you can often use gravity as a strong analogy for the behavior of electric charge. For example, just like mass generates gravitational fields, electric charges generate electric (and magnetic) fields. A charged particle also experiences an electric force due to an electric field, in the same way that a massive object experiences a gravitational force due to a gravitational field.
Subsubsection Activities
Activity 21.2.1. Charge on an Atom.
An atom has the same number of protons as electrons. What is the total charge on any atom?
Answer.
Since electrons and protons have equal charges with opposite sign, and atoms have the same number of each, the net charge on an atom is always zero, making them neutral! An atom that has an unequal number of protons and electrons is typically called an ion.
Although you might speak of the positive and negative charges “canceling”, the charges are still present, typically so close together that they cannot be distinguished.
Activity 21.2.2. Charged Object.
Suppose you have an object with a net charge of exactly \(1.0 \mathrm{~C}\text{.}\) How many more protons than electrons are in this object? Based on this, do you think a net charge of \(1.0 \mathrm{~C}\) is typical for an everyday object, or is it much larger or smaller than you would expect for an everyday object?
