2 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. Tyouchapter his reviews the fundamental con- cepts of electricity. This review will help master the material in this book. The material covered in this chapter is intended as a refresher for those already familiar with the basic concepts and fundamental electrical rela- tionships. If you have no prior training or expe- rience with electrical theory, this chapter will provide the basics and encourage you to pursue further study. 1.1 Basic Electrical Circuit Units The fl ow of electricity in a circuit can be com- pared to the fl ow of water through pipes. There are several measurable quantities associated with water fl ow. There is the rate of fl ow, the force pushing the water through the pipe, and the opposition to the fl ow caused by friction with the pipe. An electrical circuit has similar characteristics. The rate of fl ow of electricity is called current, the force pushing the electricity is called voltage, and an opposition to the fl ow of electricity is called resistance. These are the basic units of electricity. 1.1.1 Current All matter is made up of tiny particles called atoms. Each atom has a central nucleus around which electrons orbit in much the same way the planets orbit around the sun, Figure 1-1. Depending on the type of atom and its struc- ture, some electrons are free to move to other nearby atoms. Under normal conditions, this occurs randomly without any specifi c direction or net gain or loss within the substance. If an electron force is applied to the sub- stance, the electrons can be “pushed” in one direction. This directional movement is current. Thus, current is to electricity as fl ow is to water, Figure 1-2. Certain substances, particularly metals, are composed of atoms that have many easily moved electrons. These substances—such as silver, copper, and aluminum—are called conductors. Other substances—such as rubber, plastic, and wood—are composed of atoms with few easily moved electrons. These substances, which resist electron fl ow, are called insulators. They are unable to carry a current. Current is a measurable quantity. The num- ber of electrons that fl ow through a conductor Goodheart-Willcox Publisher Figure 1-1. In an atom, electrons orbit the nucleus the same way planets in the solar system orbit the sun. Electron force causes movement Electron Orbital path Nucleus Water flow Goodheart-Willcox Publisher Figure 1-2. Current flow through a wire is similar to water flow through a pipe.