Coulomb: Definition & Interactions

Instructor: Aaron Miller

Aaron teaches physics and holds a doctorate in physics.

This article is about electric charges that make up the matter around us. Electric charge is measured in units called coulombs and comes in two types, positive and negative charge. We discuss where charge originates in a model of an atom and how an imbalance of charge leads to observable electrical interactions in our daily lives.

Introduction: Static Electricity

Have you ever been shocked when you touch a metal object? Most of us have. Even objects that aren't electrically-powered appliances can still produce a shock. For example, after driving on the highway for a few hours, I usually pull into a roadside rest area or arrive at my destination. When I reach for the door handle to get out of the car, I may feel a shock as I make contact with the handle. You might have a similar experience when you're wearing socks on carpet, then suddenly touch a doorknob. We attribute this mildly surprising experience to static electricity.

The kind of shock you experience in everyday life is a result of excess electric charge buildup, either on you or on another object you touch. Most objects around us, including our bodies, are electrically neutral most of the time, which is why electrical interactions are rare and surprising. However, your body or an object may become electrically charged when it is rubbing against a rough or sticky material, like your socks and the carpet. In this article, we will discuss different aspects of static electricity, how electric charge is measured and where it comes from.

Lightning is a common phenomenon resulting from build-up of excess static charge.
lightning

Coulomb: The Unit of Measurement of Charge

The movement of charge is well understood by scientists, who have developed instruments and methods that precisely measure the excess charge on an object. One of the first scientists to study charge transfer using scientific methods was Charles-Augustin de Coulomb in the mid-1700s. Because of his early contributions to the scientific theory of electrostatics, the study of interactions between charged objects, the standard unit of measurement of electric charge is named after him: the coulomb, abbreviated by the letter C.

Charles-Augustin de Coulomb conducted early scientific studies in the field of electrostatics.
coulomb

A coulomb is related to other units of measurement commonly associated with electricity. For example, an electrical appliance that runs on 1 amp of electric current has exactly 1 coulomb of charge passing through its circuitry each second. Electric charge is also related to electric force, but this is the subject of another article on Coulomb's force law.

Where Does Electric Charge Come From?

Objects that are electrically charged are volatile, readily interacting with nearby objects. Why are some objects electrically charged while others are not? Furthermore, why does rubbing and other motion cause matter to become charged? Answers to these questions provide strong evidence for our modern scientific understanding of subatomic particles that make up the matter around us.

Modern science has adopted a model of individual atoms consisting of a central particle, the atomic nucleus, which is orbited by one or more smaller particles called electrons. The electrons and nucleus are described as subatomic because they are parts of atoms and are, therefore, smaller than atoms. The atomic nucleus has been shown to be made of yet smaller subatomic particles, protons and neutrons. Charge originates with subatomic particles.

A schematic depiction of a beryllium atom showing the atomic nucleus and electrons.
example_atom

A long progression of experiments, beginning in the 1800s, provides evidence that subatomic particles are characterized by two different types of electric charge, which we label positive charge and negative charge. As scientists discovered how to manipulate matter to separate the negative charges from the positive charges inside an atom, they consequently learned that electrons, by nature, are negatively charged. On the other hand, the protons within the atomic nucleus are positively charged.

Fundamental Charge

As far as scientists know, all electrons carry the same electric charge, and it is impossible to 'discharge' an electron. In other words, an electron's negative charge is a natural characteristic. No known process can remove or destroy an electron's charge. The negative charge of a single electron has been meticulously measured in experiments and it is found to be 1.602x10^-19 C. That is an unimaginably small number with 18 zeros after the decimal before the leading '1' (0.0000000000000000001).

Similar results have been measured for protons: they are electrically charged by nature, and every proton has precisely the same amount of charge. Experiments have shown the amount of charge carried by a proton is 1.602x10^-19 C. Somewhat inexplicably, a proton carries same amount of electric charge as an electron, but the type of charge it carries is different. Protons carry positive charge instead of negative charge.

The magnitude of charge carried by an electron or proton is called the fundamental charge and is often represented with a lowercase e.


Definition of fundamental charge.
fundamental charge


Sometimes an electron's charge is represented as -e, where the minus sign indicates the type of electric charge. Likewise, a proton's charge is sometimes written as +e.

Net Charge

No known process can remove the electric charge from an electron or proton; they are inseparable in the same way that an electron is inseparable from its mass. However, the mathematical theory of electrostatics says that electrical effects of a charged particle -- say, a proton -- can be essentially negated if a particle with the same magnitude of charge, but opposite type, is near the particle. This brings us to the important idea of net charge.

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