Chapter 1 Electrical Fundamentals Review 3 Copyright Goodheart-Willcox Co., Inc. in a given amount of time is measured in units called amperes (A). One ampere is equivalent to 6.25 × 1018 electrons passing a given point in one second. Amperes are usually abbreviated as amps or A. The symbol used in formulas to rep- resent current is I. 1.1.2 Voltage For a current to fl ow in a conductor, a force must be applied at one end. This electron-pushing force is called the electromotive force (emf). This force can be produced in various ways— chemically, mechanically, or by other forms of energy (such as sunlight). Specifi c examples are batteries (chemical), generators (mechanical), and photovoltaic cells (sunlight). Voltage is a measurement of the emf produced by a source. The volt (V) is the basic unit. It is defi ned as the potential difference between two points in an electric circuit when the energy needed to move one ampere between the points is one joule. 1.1.3 Resistance The opposition to the fl ow of electrons through a circuit is called resistance and is measured in units called ohms (Ω). The symbol for ohms is the Greek letter omega. One ohm is defi ned as the electrical resistance that allows one ampere to fl ow when one volt is applied. For an electric circuit having a constant resistance, the current and voltage are directly related. That is, if the voltage is doubled, the cur- rent will double if the voltage is increased ten times, the current will increase ten times. Thus, for any given circuit (with a constant resistance), the ratio of voltage to current is constant. This relationship is called Ohm’s law and can be expressed mathematically as: R = E _ I NOTE In formulas, voltage is represented by either E or V. E will be used in this text. where R = Resistance (ohms, Ω) E = Voltage (volts, V) I = Current (amps, A) As we go on, we will see how Ohm’s law is applicable to many types of problems involving electrical circuits. 1.1.4 Power, Energy, and Work The basic purpose of an electric circuit is to perform work. Work is the process by which energy is transformed from one type to another. For example, an electric motor performs work by changing electrical energy into mechani- cal energy. Any form of energy has the ability to do work. Both work and energy are practically identical and can be thought of interchangeably. Power is the rate at which energy is trans- formed it is the rate of doing work. Electric power is measured in watts (W), and is the product of voltage times current. Since a watt and a volt-ampere are the same, power is often expressed in volt-amperes (VA). Power is expressed mathematically as: P = E × I or P = I2 × R where P = Power E = Voltage I = Current R = Resistance Figure 1-3 shows the mathematical relation- ships of power, voltage, current, and resistance. 1.2 Basic Circuits Simple and complex circuits are composed of the same components. All circuits must have a source, a device, and a conductor. The source provides the electromotive force. The device, resistance, or element (these terms are equivalent) uses the energy to perform some