What makes plastic wrap cling to a container? Why do clothes sometimes stick together after tumbling in a dryer? Why can a tiny spark jump from your finger to a metal doorknob after you walk across carpet? All of these are everyday examples of static electricity.

Static electricity is not a modern discovery. Ancient Greek writers noted more than 2500 years ago that rubbing amber could temporarily make it attract lightweight objects such as straw or feathers (Figure 2.1). The word electric itself traces back to the Greek word for amber, electron.

You can explore many features of static electricity by rubbing common materials together. Rubbing can create:

• the spark you feel after walking across a dry carpet,
• static cling in laundry,
• the attraction of small objects to recently rubbed amber or plastic, and
• the ability of a balloon rubbed on hair to cling to a wall.

Static electricity is also important in health, safety, and industry. In dry environments, charge can build up easily. When fueling a car, for example, a spark from your body can ignite gasoline vapors—one reason it is safer to touch a metal surface before handling the nozzle. In clinical environments, static sparks are also undesirable: in operating rooms where oxygen concentration may be elevated, even small sparks can create serious risk, so grounding and special footwear are used to reduce charge buildup.

Several basic characteristics summarize what we observe about static electricity:

• Static electricity is explained using a physical quantity called electric charge.
• There are two types of charge, called positive and negative.
• Like charges repel, while unlike charges attract.
• The force between charges decreases with increasing distance.

How do we know there are exactly two types of charge? When different materials are rubbed together under controlled conditions, particular combinations reliably produce one type of charge on one material and the opposite type on the other. By historical convention, we label one type of charge “positive” and the other “negative.”

For example, when glass is rubbed with silk, the glass becomes positively charged and the silk becomes negatively charged. Because the charges are opposite, the glass and silk attract. If you rub two glass rods with silk, both rods become positively charged and they repel each other. Similarly, two silk cloths rubbed in the same way become negatively charged and repel. Figure 2.2 illustrates these interactions.

Once we can describe what happens, deeper questions naturally follow: Where do these charges come from? Can charge be created or destroyed? Is there a smallest unit of charge? And how does the force between charges depend on distance and the amount of charge? These questions shaped the development of electromagnetism and remain central to modern physics and biomedical technology.

Charge Carried by Electrons and Protons

Early researchers such as Benjamin Franklin could describe the effects of charge but did not know the microscopic origin. Today we have the advantage of atomic theory: matter is composed of atoms, and atoms contain positive and negative charges—usually in equal amounts.

In the simplified “planetary model” of the atom, negatively charged electrons surround a nucleus that contains positively charged protons (Figure 2.3). The third major particle in ordinary atoms is the neutron, which carries no net charge.

Electrons and protons have charges that are equal in magnitude but opposite in sign. Nearly all charge encountered in everyday life—static electricity, electric circuits, the electrical behavior of cells—ultimately comes from electrons and protons.

All observed macroscopic charges are built from a basic unit of charge. The magnitude of the charge on a single electron (or proton) is:

The SI unit of charge is the coulomb (C). A coulomb is a large amount of charge in terms of individual particles. The number of elementary charges in 1.00 C is:

The same number of electrons would carry a charge of −1.00 C. Just as there is a smallest unit of an element (an atom), there is a smallest unit of charge. No isolated charge smaller than [latex]\left|q_e\right|[/latex] has been directly observed in ordinary matter, and macroscopic charges occur in integer multiples of this fundamental amount.

A familiar demonstration of charge is the Van de Graaff generator. When a person touches it, charge is transferred to their body; the repulsion of like charges can cause hair strands to spread apart and stand up (Figure 2.4). This “macroscopic” effect is produced by microscopic charge imbalance.

At even smaller scales, the proton is not fundamental in the same way the electron appears to be. Experiments show that protons contain smaller constituents called quarks. Quarks carry fractional charges (such as [latex]-\frac{1}{3}[/latex] and [latex]+\frac{2}{3}[/latex]), but isolated fractional charges have never been observed—quarks are confined inside particles such as protons and neutrons. Figure 2.5 illustrates this idea.

Separation of Charge in Atoms

Many static electricity effects occur because electrons move from one material to another. Different materials hold onto electrons with different “strengths.” When two materials are rubbed together, electrons may transfer, leaving one object with an excess of electrons (net negative charge) and the other with a deficit (net positive charge). Figure 2.6 shows this process schematically.

Rubbing is not the only way to separate charge. Batteries separate charge through chemical reactions. In biology, charge separation across membranes is essential: nerve impulses, muscle contractions, and many transport processes rely on charge imbalance and electric potential differences created by ions moving through channels and pumps.

In all of these examples, charge is not created from nothing. Instead, existing charges are rearranged. The total amount of charge remains constant in any process. This is called the law of conservation of charge.

Even in high-energy physics, where particles can be created from energy, charge conservation still holds. Mass can be created from energy according to:

If a charged particle is created, an oppositely charged particle is created at the same time, so the total charge created is zero. For example, electron–positron pair production creates one negative electron and one positive positron. When matter and antimatter meet, they can annihilate, converting mass back into energy—but total charge remains conserved throughout (Figure 2.7).

The law of conservation of charge has never been observed to be violated. That makes charge a fundamental physical quantity, comparable in importance to energy and momentum, and essential for understanding both technology (electronics, imaging, sensors) and biology (nerve signaling, ion transport, and bioelectric effects).

Why does a balloon stick to your sweater, and why can it also stick to a wall afterward? Use this simulation to visualize charge transfer and polarization. Observe the charges in the sweater, balloon, and wall as you rub and reposition the balloon.