Figure 2.1 shows a piece of amber, the material that gave electricity its name after ancient observers noticed that it could attract lightweight objects when rubbed.

Why does plastic wrap cling tightly to a container? Why do clothes sometimes stick together after coming out of a clothes dryer? Why can you feel a small spark when you touch a metal doorknob after walking across a carpet? Although these situations appear unrelated, they are all examples of the same phenomenon: static electricity, the accumulation of electric charge on the surface of an object.

The underlying property responsible for these effects is electric charge. Electric charge is a fundamental property of matter, much like mass. Objects become electrically charged when electrons are transferred from one material to another, creating an imbalance between positive and negative charge. These imbalances produce forces that can attract or repel other charged objects, giving rise to the familiar effects of static electricity.

People have been fascinated by these phenomena for thousands of years. More than 2500 years ago, the ancient Greeks observed that amber, after being rubbed with fur or cloth, could attract lightweight objects such as feathers or bits of straw, as shown in Figure 2.1. In fact, the word electric comes from the Greek word electron, meaning amber. Although the mechanism behind this attraction would remain a mystery for centuries, these simple observations marked the beginning of the scientific study of electricity.

Common examples of static electricity include:

• a spark produced after walking across a dry carpet,
• clothes sticking together after tumbling in a dryer,
• small pieces of paper being attracted to a rubbed plastic comb or amber rod, and
• a balloon rubbed on hair sticking to a wall.

Static electricity is much more than a scientific curiosity. It has important applications in medicine, industry, and everyday life. Electrostatic forces are used in laser printers and photocopiers to position toner on paper, and they help remove pollutants from industrial smokestacks. In healthcare settings, however, unwanted static electricity can present hazards. A small electrostatic spark can ignite flammable gases or vapors, so operating rooms and areas where oxygen is used often include grounding systems, conductive flooring, and specialized footwear to reduce charge buildup.

Experimental observations show several important properties of electric charge:

• Static electricity is explained by the presence of electric charge.
• There are two types of electric charge, called positive and negative.
• Like charges repel, while unlike charges attract.
• The electric force becomes weaker as the distance between charges increases.

How do we know that there are exactly two kinds of charge? When different materials are rubbed together under controlled conditions, one material consistently acquires one type of charge while the other acquires the opposite type. By historical convention, these two kinds of charge are called positive and negative. The names themselves are arbitrary, but the distinction between the two types is fundamental.

For example, when a glass rod is rubbed with silk, the glass becomes positively charged while the silk becomes negatively charged. Because opposite charges attract, the glass rod and silk cloth pull toward one another. If two glass rods are rubbed in the same way, both become positively charged and repel. Likewise, two silk cloths rubbed under the same conditions both become negatively charged and also repel. Figure 2.2 summarizes these two basic interactions between electric charges.

Understanding these observations raises several important questions. Where does electric charge come from? Can charge be created or destroyed? Is there a smallest possible amount of charge? And how can we predict the force between charged objects? Answering these questions will allow us to develop a quantitative understanding of electricity and build the foundation for topics ranging from electric circuits and nerve impulses to medical imaging and modern biomedical devices.

Charge Carried by Electrons and Protons

Early investigators such as Benjamin Franklin could observe and describe the behavior of electric charge, but they had no knowledge of its microscopic origin. Today, thanks to atomic theory, we know that all matter is made of atoms, and that atoms themselves contain electrically charged particles. Understanding these particles allows us to explain everything from static electricity to the electrical signals that travel through the human nervous system.

In the simplified "planetary model" of the atom, negatively charged electrons move around a small, dense nucleus that contains positively charged protons together with electrically neutral neutrons. Although modern quantum mechanics provides a more accurate description of atoms, this simplified model is very useful for understanding the origin of electric charge and many of the concepts discussed in introductory physics. Figure 2.3 illustrates this simplified model.

Electrons and protons carry charges that are exactly equal in magnitude but opposite in sign. By convention, the charge on a proton is defined as positive, while the charge on an electron is negative. In a neutral atom, the numbers of protons and electrons are equal, so their charges cancel. Whenever an object becomes electrically charged, it is almost always because electrons have been transferred from one object to another. The protons remain tightly bound inside the atomic nucleus and do not move under ordinary circumstances.

Nearly all of the electrical phenomena encountered in everyday life—including static electricity, electric circuits, batteries, and the electrical activity of nerve and muscle cells—ultimately result from the behavior of electrons. The same electromagnetic interactions that hold atoms together also allow neurons to transmit signals, muscles to contract, and medical devices such as electrocardiographs (ECGs) and pacemakers to function.

Experiments show that electric charge is not continuous but comes in discrete packets. The smallest amount of charge that exists independently in ordinary matter is called the elementary charge, which is the magnitude of the charge carried by a single electron or proton.

The magnitude of the elementary charge is given by

The SI unit of electric charge is the coulomb (C). Although a coulomb may seem like a small quantity, it actually represents an enormous number of elementary charges.

The number of individual proton charges contained in 1.00 C can be calculated as follows:

An equal number of electrons would carry a charge of −1.00 C. Because the elementary charge is so small, even a tiny static spark involves the movement of trillions of electrons.

The existence of this smallest unit means that electric charge is quantized. Just as matter is built from individual atoms, all observable electric charge is built from integer multiples of the elementary charge. No isolated charge smaller than the elementary charge has ever been observed in ordinary matter, making it one of the fundamental constants of nature.

Figure 2.4 shows a familiar classroom demonstration using a Van de Graaff generator.

At an even deeper level, physicists have discovered that the proton is not an indivisible particle. Instead, it is made of smaller constituents called quarks, elementary particles that combine to form protons and neutrons. Unlike electrons and protons, quarks carry fractional electric charges, such as [latex]-\frac{1}{3}e[/latex] and [latex]+\frac{2}{3}e[/latex]. However, quarks are never found in isolation. They are permanently confined inside larger particles, such as protons and neutrons, and cannot normally be separated.

A proton consists of three quarks whose fractional charges combine to produce the familiar positive elementary charge. Figure 2.5 illustrates how these fractional charges add together. Two quarks each contribute a charge of [latex]+\frac{2}{3}e[/latex], while the third contributes [latex]-\frac{1}{3}e[/latex]. Together they give the proton its total charge of [latex]+e[/latex].

Separation of Charge in Atoms

We have seen that objects become electrically charged when electrons are transferred from one material to another. But why are electrons able to move while protons remain fixed inside the atom? The answer lies in the structure of matter. Electrons occupy the outer regions of atoms and are often only loosely bound to them, whereas protons are tightly confined within the atomic nucleus. Under ordinary conditions, protons do not move from one object to another, but electrons can be transferred relatively easily.

Different materials hold onto their electrons with different strengths. When two materials come into close contact and are then separated—especially if they are rubbed together—electrons may move from one surface to the other. The material that gains electrons acquires an excess of negative charge, while the material that loses electrons is left with a net positive charge. Figure 2.6 illustrates this process.

Although rubbing is one of the most familiar ways to separate charge, it is far from the only one. Batteries use chemical reactions to separate positive and negative charges, creating the potential difference that drives electric current through a circuit. In living organisms, specialized proteins embedded in cell membranes continually move charged ions across the membrane. This separation of charge creates the electrical potential differences responsible for nerve impulses, muscle contraction, and many other biological processes.

One important feature is common to all of these examples: electric charge is never created or destroyed. Electrons simply move from one place to another, producing an excess of charge in one region and an equal deficit elsewhere. If one object gains negative charge, another object must lose the same amount of negative charge. The total amount of electric charge before and after any process remains exactly the same.

This fundamental principle is known as the law of conservation of charge. It states that the total electric charge of an isolated system remains constant. Charge may be transferred between objects, but it cannot be created from nothing or disappear.

The law of conservation of charge is one of the fundamental conservation laws in physics. Every experiment, from simple demonstrations of static electricity to studies of subatomic particles, has confirmed that the total electric charge before and after a process is always the same.

Even in high-energy physics, where matter can be created directly from energy, electric charge is still conserved.

Einstein's mass-energy relationship is

However, whenever new particles are created, the total electric charge must remain unchanged. For example, a high-energy photon can produce an electron–positron pair. The electron carries one negative elementary charge, while the positron—the electron's antimatter counterpart—carries one positive elementary charge. Because the two charges are equal in magnitude and opposite in sign, the total charge created is zero.

Likewise, when an electron and a positron meet, they can annihilate one another, converting their mass back into electromagnetic radiation. Although matter disappears during this process, the total electric charge remains zero before and after the interaction. These examples illustrate that charge conservation is a universal principle that applies across every scale of physics, from everyday electrostatic phenomena to the highest-energy particle interactions. Figure 2.7 illustrates both processes.

The law of conservation of charge has never been observed to fail. Every experiment—from simple demonstrations with rubbed balloons to the highest-energy particle collisions—has confirmed that the total electric charge of an isolated system remains constant. This remarkable consistency makes electric charge one of the most fundamental quantities in physics.

Charge conservation is also essential for understanding modern technology and the life sciences. Electronic circuits function because charge is transferred rather than created. Medical devices such as electrocardiographs (ECGs), pacemakers, and neural stimulators rely on the controlled movement of electric charge, while living cells continuously transport charged ions across their membranes to generate the electrical signals required for nerve conduction, muscle contraction, and many other physiological processes.

Interactive Exploration: Balloons and Static Electricity

Static electricity is a familiar phenomenon that results from the transfer of electric charge between objects. In this simulation, you'll investigate how rubbing a balloon on a sweater causes electrons to move from one object to another, leaving the balloon and sweater with opposite charges. You'll also discover how a charged object can attract a neutral object through a process called polarization.

Experiment by rubbing the balloon on the sweater and then moving it near the sweater and the wall. Observe how the distribution of charges changes during each interaction and relate these microscopic charge movements to the electrostatic forces discussed in this chapter.

Accessibility note: If you are unable to use the interactive simulation, read the guided exploration questions first and compare your predictions with the discussion that follows. Your instructor may also provide screenshots or a demonstration of the simulation.