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Magnetism Intro to magnetism and Electromagnetism. “the basic laws governing magnetism and how they relate to electricity” Objectives: At the completion of this unit you will be able to: • Understand the properties of permanent magnets • Utilize terms used to describe magnetism and magnetic quantities • Understand the operation of electromagnets • Determine the polarity of an electromagnet Introduction to Magnetism • One of the most important phenomena in the study of electricity • The force of magnetism has been known for over 2000 years (Greeks) • Stones with magnetic qualities (magnetite) were first found in Magnesia in Asia Minor • In the Dark Ages magnetism was thought to be caused by evil spirits The Earth is a Magnet • Geographic poles vs. magnetic poles • lodestones • Angle of declination THE FIRST COMPASS Permanent Magnets: Magnets that do not require any power or force to maintain their field One of the basic laws of magnetism states: “Energy is required to create a magnetic field, but no energy is required to maintain a magnetic field” The Electron Theory of Magnetism What makes materials such as iron, nickel, or cobalt, a natural magnetic material • Electron spin theory • Electrons become tiny permanent magnets • Electrons in most materials spin in opposite directions • Once “paired” they tend to cancel out any magnetic properties The Electron Theory of Magnetism What makes materials like iron different than most materials: • An Atom of iron contains 26 electrons • 22 of the electrons are “paired” and cancel each other out • The remaining 4 electrons located in the next to the outermost shell do not become “paired” and spin in the same direction • These 4 electrons account for the magnetic properties of iron The Electron Theory of Magnetism What makes materials like iron different than most materials: • As atoms combine to form molecules • They arrange themselves to form a total of 8 valence electrons • In most materials the electrons cancel each other out • In materials such as iron, the magnetic fields “add” rather than cancel • This “additive” effect forms regions in the molecular structure of the metal called: Magnetic Domains or Magnetic Molecules Magnetic Materials The three basic classifications: • Ferromagnetic - metals that are easily magnetized; iron, nickel, cobalt, manganese • Paramagnetic - metals that can be magnetized but not as easily as ferromagnetic; platinum,titanium,and chromium • Diamagnetic - metal or non-metallic materials that cannot be magnetized; copper,brass, and antimony Note: Alloys often make the best permanent magnet materials. Combinations of such metals as aluminum, nickel, cobalt, copper, and iron (Alnico 5) are commonly used in the production of permanent magnets Magnetic Lines of Force • Magnetic lines of force are called “flux” • Magnetic “lines of flux” repel each other & never cross • Lines of flux do not “flow” but it is assumed that they run North to South One of the basic laws of magnetism: Unlike poles attract - Like poles repel Electromagnetics A basic law of physics: Whenever an electric current flows through a conductor, a magnetic field is formed around the conductor • Electromagnets depend on electrical current flow • They do not retain their magnetic field once current flow stops Electromagnetics Terms to remember: • Permeability - a measure of a material’s willingness to become magnetized • Reluctance - a material’s resistance to magnetism • Saturation - the point when an increase in current fails to significantly increase the magnetic field • Residual Magnetism - the amount of magnetism left once current flow has stopped Electromagnetics Factors that effect electromagnetic fields: • Winding the conductor into a coil increases the strength of the magnetic field • A coil with 10 turns of wire will increase the magnetic field around the conductor by 10 • The strength of the magnetic field is proportional to the amount of current flowing through the conductor “Ampere-Turns” is determined by multiplying the number of turns of wire by the current flow Electromagnetics Core Materials: magnetic/non-magnetic materials around which the conductor is coiled Two Types: • “Air core” electromagnets >Core is made of Non-magnetic material • “Iron core” electromagnets >Made of magnetic materials >Greatly increases strength of the magnetic field by increasing the number of flux lines Magnetic Polarity Determining the Polarity of an Electromagnet • The “Right-Hand Rule” Magnetic Devices • • • • • • Electromagnets Measuring instruments Inductors Transformers Motors Generators;Alternators A Speaker uses both a permanent magnet and an electromagnet Summary • Energy is required to create a magnetic field, but no energy is required to maintain a magnetic field • Unlike poles attract, like poles repel • Whenever current flows through a conductor a magnetic field is formed around the conductor • The direction of current flow through a conductor determines the polarity of the magnetic field