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Electric Charge

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Start with charge

Before we can make sense of volts, amps, ohms, or watts, we need one idea underneath all of them: electric charge.

A useful first picture

Many of us were first shown an atom a little like a tiny solar system: a nucleus in the middle with electrons moving around it. That is the familiar Bohr-style picture, and it is useful for learning which particles are present and which ones carry charge.

+ + − − nucleus electron electron

What is really happening?

Electrons are not actually little planets traveling on neat circular tracks. Modern quantum physics describes electrons with probability distributions — regions where an electron is more or less likely to be found.

For beginning electrical work, we do not need quantum mechanics. We only need to remember that electrons carry negative charge, protons carry positive charge, and in ordinary metal wire some electrons can move through the material.

Another way to think of this

Think of the copper atoms as seats bolted to the floor. The positively charged nuclei stay with the seats. Some electrons are more like people able to pass from seat to seat. Electrical conduction involves those mobile electrons.

The picture is not literal, but it helps us separate the fixed structure of the metal from the mobile charge.

Symbols used on this page

C after a number means coulomb, the unit of electric charge. For example, 2 C means two coulombs of charge.

What is a coulomb?

The coulomb, abbreviated C, is the SI unit used to measure an amount of electric charge.

1 C ≈ 6.24 × 1018 elementary charges

One electron carries only about −1.602 × 10−19 coulomb, so one coulomb represents an enormous number of electrons.

Where did the name come from?

The coulomb is named for French physicist Charles-Augustin de Coulomb (1736–1806). He used a very sensitive torsion balance to study the force between electric charges. His experiments helped put electrostatics on a quantitative footing.

That is why the amount of charge is measured in coulombs: the unit honors the scientist whose measurements helped describe how charges interact.

Worked example

1 coulomb ≈ 6.24 × 10¹⁸ e

Question: Which has more electric charge: one electron, or one coulomb?

Step 1: One electron carries one elementary charge.

Step 2: One coulomb contains about 6.24 × 1018 elementary charges.

Answer: One coulomb is vastly more charge than the charge on a single electron.

Try these yourself

These are intentionally simple. The goal is to make the basic ideas automatic before we build on them.

Which particle carries a negative electric charge? − electron
The electron carries negative elementary charge. A proton carries an equal-magnitude positive charge, while a neutron has no net charge.
About how many electron charges make one coulomb? 1 coulomb ≈ 6.24 × 10¹⁸ e
One coulomb is about 6.24 × 1018 elementary charges — roughly 6.24 quintillion.
In an ordinary copper wire, which charged particle is free to move through the metal? − − − mobile electrons
In a metal conductor, some electrons are mobile. The positively charged nuclei remain bound in the crystal structure.

A little deeper

Electric charge is a fundamental property of matter. Charge can be positive or negative, and like charges repel while unlike charges attract. In ordinary electrical circuits we usually track the collective motion of tremendous numbers of electrons rather than individual particles.

The next step is natural: once we know what charge is, we can ask how fast charge is moving. That quantity is electric current.

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