1 Introduction to Electric Charge and Electric Field

Many students first encounter electricity through everyday experiences like a small spark when touching a metal doorknob, clothes clinging together after drying, or hair standing up after sliding down playground equipment. These familiar effects are examples of static electricity, a buildup of electric charge on the surface of an object.

One of the earliest scientific investigations of electricity is associated with Benjamin Franklin (1706–1790), who demonstrated that lightning and static electricity are related phenomena. As shown in Figure 1.2, Franklin used a kite and a metal key to collect electric charge from a thunderstorm, showing that lightning is an electrical process. Although dangerous and not recommended today, this experiment revealed an important scientific principle: phenomena that appear very different in scale may arise from the same physical laws.

Electricity soon became important in the life sciences as well. In the late 1700s, Luigi Galvani discovered that electrical stimulation could cause frog leg muscles to contract. This work helped establish the connection between electricity and biological systems—an idea that remains fundamental in modern physiology, neuroscience, and cardiology.

Alessandro Volta expanded on Galvani’s work and developed the first electric battery, providing a steady source of electric current. Today, controlled electrical signals are essential in technologies such as pacemakers, electrocardiograms (ECGs), nerve conduction studies, and many types of medical imaging.

At the same time that scientists were learning about electricity, they were also developing atomic theory and organizing the elements in the periodic table. These advances revealed that many everyday forces—such as friction, cohesion, and adhesion—arise from interactions between atoms and molecules. These interactions are governed by the electromagnetic force.

In fact, nearly all of the forces we experience directly in daily life—touch, tension in muscles and tendons, pressure in fluids, and the structural strength of bones and tissues—ultimately arise from electromagnetic interactions between atoms. Gravity also plays an important role in biology, but even the sensation of weight is transmitted through electromagnetic interactions between molecules in the body and surrounding surfaces.

This chapter begins our study of electricity and magnetism, collectively known as electromagnetism. We start with situations in which electric charges are stationary, a field known as electrostatics. Understanding electrostatics provides the foundation for later topics, including electric circuits, nerve signaling, and modern medical technologies.