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3 Electromagnetism (continued) • Electromagnetic waves consist of rapidly changing electric and magnetic fields that travel as waves. Electromagnetic waves are emitted any time an electrically charged object or magnet accelerates. These waves are generally referred to as light. All frequencies of light consist of electromag- netic waves, from radio waves on the low-frequency (long wavelengths) end of the electromagnetic spectrum to gamma rays on the high-frequency (short wavelengths) end (Fig. 2). Physical Laws of Electromagnetism Behavior of the Electromagnetic Field The manner in which electromagnetic fields are created and behave are summarized by four laws known collectively as Maxwell's equations: 1. An electrically charged particle creates an electric field. This principle, known as Gauss's law, is the operating principle behind simple electric circuits, capacitors, and spark plugs. 2. Magnetically charged particles do not exist and therefore certain types of magnetic fields do not exist. This principle is known as Gauss's law for magnetism. 3. A changing magnetic field creates an electric field. This law is the operating principle behind electric generators, microphones, and transformers. This principle is known as Faraday's law of induction. 4. A moving or spinning electric charge creates a magnetic field. This is the operating principle behind electromagnets and permanent magnets. This principle is known as Am- père's law. Also, a changing electric field creates a magnetic field. When both principles are combined, it is known as the Ampère-Maxwell law. As a description of the fields, Maxwell's equations are complete and self-consistent. Additionally, Maxwell's equations implicitly contain other physical laws, including conservation of charge, conservation of energy, conservation of momentum, conser- vation of angular momentum, and the electromagnetic wave equation. Furthermore, the electromagnetic wave equation implicitly contains the basic laws of optics. These laws are the operating principles behind optical devices such as lenses, mir- rors, and prisms. The Electromagnetic Force While Maxwell's equations describe how electromagnetic fields are generated and behave, the Lorentz force law describes how the fields interact with charged particles. This law states that an electric field exerts a forward or backward force on a charged particle and a magnetic field exerts a sideways force on a mov- ing charged particle. Charged objects, electric currents, and magnets exert forces on each other through the electromag- netic field according to Maxwell's equations and the Lorentz force law. This principle is the operating principle behind chemical bonds, loudspeakers, and electric motors. Quantum Properties Electric charge, magnetic moment, and the electromagnetic field each come as a collection of discrete, indivisible, quan- tum packets. The quantum of the electromagnetic field is the photon. The quantum of electric charge is the charge held by a fundamental particle such as an electron or positron. The quantum of intrinsic magnetic moment is the moment carried by a charged fundamental particle with quantum spin such as, again, an electron or positron. The most accurate description of electromagnetism is the quantum version of all of the physical laws mentioned above, which are known collectively as quan- tum electrodynamics (QED). QED unifies the quantum laws of electromagnetism and the special theory of relativity. Material Effects In general, charged particles can be either free or bound inside molecules. Materials that act strongly like collections of free charged particles are known as conductors. Important conduc- tors include metals, doped semiconductors, water, and plasma. Materials that act strongly like collections of bound charged particles are known as insulators or dielectrics. Important insu- lators include plastic, glass, oil, and air. Similarly, electric currents can be either free or bound inside molecules. Also, intrinsic magnetic moments can be modeled as bound currents because they arise from the quantum spin of charged particles. Materials that act strongly like collections of free currents are known as conductors, as stated previously. Materials that act strongly like collections of bound currents are known as (ferro)magnetic materials. Most magnetic materials are also conductors. Important magnetic materials include iron, steel, and nickel. History and Applications As a broad field of study, electromagnetism has been devel- oped by a large variety of contributors. Ancient Greeks pursued basic studies of electrostatics and magnetostatics using amber and lodestones. In the eighteenth and early nineteenth centu- ries, scientists such as Benjamin Franklin, Charles Augustin de Coulomb, Jean-Baptiste Biot, and Félix Savart determined the nature of electric charge, electric currents, electrostatic fields, + ward ' s science