Magnetism

Most material is unmagnetized, as its domains cancel. N In magnetic substances, an external field aligns domains. Slit Slit Slit Interference Incident wave Diffracted wave Moving a magnet toward (or away from) a simple ferrite core generates the electrical jumps. Toroidal-core current transformer Ferrite magnet An amplifier makes the Barkhausen jumps audible. B ar kh au se n no is e (m V ) Sponsored by from the online learning library series of Sponsored by UpBrushingMagnets and electromagnetics U nderstanding the forces of nature and putting this knowledge to work is the crux of modern engineering. Physics is at the heart of this knowledge base and deserves a review from time to time, especially by those involved in harnessing its power. Because magnetic forces are the basis of many general motion, electromagnetic, and electrical designs today, we review this phenomenon here, in this Magnets and electromagnetics eBook from Motion System Design. Colorfully illustrated chapters include an entertaining review of magnetism and its discovery, the Father of Magnetism William Gilbert, practical applications in specialty magnetic slurries and wire, plus a fascinating tour of visible electromagnetic radiation — light. 2 PHYSICS eBook Sponsored by HAYDON KERK MOTION SOLUTIONS 3HAYDONKERK.COM • PHYSICS eBook Electromagnetics Table of contents A message from our sponsor UpBrushing Moving a magnet toward (or away from) a simple ferrite core generates the electrical jumps. Toroidal-core current transformer Ferrite magnet An amplifier makes the Barkhausen jumps audible. B ar kh au se n no is e (m V ) Slit Slit Slit Interference Incident wave Diffracted wave 4Magnetism basics: Magnetism is a material’s response to moving electrical charge. Ferromagnetism, that relating to material makeup, is the most familiar incarnation; electric current can also cause a magnetic field. 8 William Gilbert: This scientist quite literally wrote the book on magnetism. Prior to his work, magnetism was the stuff of myth and magic. Gilbert changed all that when he showed systematically that magnetism is a form of energy that acts with attractive or repulsive force, depending on the alignment (or polarity) of the objects involved. 10Visible electromagnetic radiation: Light is a free-spirited but manageable form of energy. It is both predictable and repeatable, lending itself to many practical applications. Anyone working in the field of engineering and using optical encoders, beam interrupters, or photoelectric sensors (not to mention payloads such as lasers or plain, hard optics) needs to know something about light’s properties. 16Wiegand wire, and the man who invented it: The Wiegand effect, a phenomenon discovered in the 1970s, is the unusually useful behavior of magnetic fields in specially designed wire that outputs voltage. 22Magnetorheologic (MR) fluid: Jacob Rabinow of the U.S. National Bureau of Standards first developed his slurry in the 1940s and dubbed it magnetorheologic (MR) fluid for the way it changes properties under the influence of an electromagnetic field. 24 Rare-earth magnets: Magnets made of rare-earth metals are particularly powerful alloys with crystalline structures that have high magnetic anisotropy — which means that they readily align in one direction, and resist it in others. Most material is unmagnetized, as its domains cancel. N In magnetic substances, an external field aligns domains. Haydon Kerk Motion Solutions believes in continually nurturing interest among young people in the sciences and engineering. The world is becoming ever more automated, leading to a greater need for scientists, engineers, and technologists of all types. A perfect avenue for students interested in engineering is the FIRST® Robotics Competition. As the world gets more technology based, there is an ever greater need to develop an interest in engineering and science at a young age. Haydon Kerk Motion Solutions supports this effort as a Silver Supplier for the FIRST® Robotics Competition (FRC®) organized and managed by FIRST (For Inspiration and Recognition of Science and Technology), a non-profit organization that inspires young people’s interest and participation in science and technology. “With its generous contribution and ongoing support to the FIRST Robotics Competition at the highest Silver Supplier level, Haydon Kerk Motion Solutions makes it possible for us to turn the FIRST vision of inspiring young people’s interest and participation in science and technology into a reality,” notes FIRST president Jon Dudas. “The participation of innovative companies such as Haydon Kerk Motion Solutions is a major reason why FIRST continues to experience growth.” On January 8, 2011, FIRST teams were shown the playing field and received a kit of parts made up of motors, batteries, a control system, a PC, and a mix of other automation components, but no instructions. Working with mentors, students have six weeks to design, build, program, and test their robots to meet that season’s engineering challenge. Haydon Kerk Motion Solutions Commitment to Education Once these young inventors create a robot, their teams participate in competitions that measure the effectiveness of each robot, the power of collaboration and the determination of the students. The FIRST Competition is aptly dubbed a “varsity sport for the mind.” “Haydon Kerk Motion Solutions is proud to be a supporter of the FIRST Robotics Competition. This is a perfect event to develop the science and engineering skills needed for our future technology inventors and leaders,” states Dan Montone, Marketing Director for Haydon Kerk. Haydon Kerk: An “Engineered Motion Solutions” Company Haydon Kerk takes pride in its ability to customize unique solutions to a customer’s motion problem. Haydon Kerk offers a wide range of “base” linear motion assemblies, and then works closely with its customer to customize the system to the needs of the application. There are different degrees of customization. It can be something as simple as a special wire harness or adapter plate or as complex as the creation of a special lead screw nut that encompasses multiple molded features that replaces several components in a customer’s final system. Haydon Kerk’s motion toolbox consists of linear actuators, lead screws, antibacklash lead screw nuts, motorized linear rail systems, and programmable drives. For more information, visit our website at www.HaydonKerk.com. UpBrushing Magnetism is a material’s response to moving electrical charge. Ferromagnetism, that relating to material makeup, is the most familiar incarnation. However, in 1821, Danish scientist Hans Christian Oersted showed that electric current through a wire could also cause a magnetic field. Ampere discovered that magnetism is a force between electric currents: Parallel currents in the same direction attract (for paramagnetism) while opposing flows cause repulsion, also called diamagnetism. Magnetism basics Earth’s magnetic poles The 17th-century scientist William Gilbert was the first to recognize that Earth itself has magnetic poles. In a solenoid, current is applied to a coil to create a magnetic field. This in turn draws a plunger in to close the solenoid. Electromagnetic forces are attractive or repulsive. They affect electrically charged particles through virtual photons. These are not imaginary, but exchanged between charged particles (like electrons) quickly. Macroscopically useful, electromagnetic forces are the basis of all electrical designs today. C ou rt es y M ar ci n B ia łe k 4 PHYSICS eBook Sponsored by HAYDON KERK MOTION SOLUTIONS 5HAYDONKERK.COM • PHYSICS eBook + - Magnetic field Current Then, French physicist and mathematician Andre Marie Ampere (1775-1836) established the relationship between electricity and magnetism. Ampere termed the study of currents electrodynamics; his most important treatise on electricity and magnetism was published in 1826, and this theory became fundamental for 19th century developments in electricity and magnetism. Most material is unmagnetized, as its domains cancel. N In magnetic substances, an external field aligns domains. Fe rro magnetism C u rr en t- in du ced magnetism Th e st ro ng es t m agn et o n Ear th A $2.5 million split magnet system has the potential to revolutionize research across many fields, according to scientists at the National High Magnetic Field Laboratory (Mag Lab) at Florida State University, where the giant magnet recently made its debut. The world-record magnet operates at 25 tesla — equal to 500,000 times the Earth's magnetic field — and surpasses the 17.5 tesla French record set in 1991 for this style of magnet. For many years, scientists have used high magnetic fields to explore the properties of materials under extreme conditions of heat and pressure. The modern Father of Magnetism William Gilbert envisioned the Earth as a giant magnetic globe. To study the planet, he made a sphere from lodestone — his famous terrella or little earth. Read on ... One aspect of magnetism is induction, which occurs when conductors (like the bars in a common ac induction-motor’s rotor) spin through a magnetic field — in this example, that generated by current through the motor’s stator windings. Then an induced electric current flows in the conductors. Flux rotation Rotor rotation + ñ ñ + N S N S The strongest magnet is a $2.5 million unit at the National High Magnetic Field Laboratory at Florida State University. Some motors called pancake motors use Lorentz forces for propulsion: They leverage forces of point charges moving in a wire in a magnetic field from twin flat rotors that flank a stator. Iron has a tendency to magnetize in little steps rather than in a smooth progression. When subjected to an external field, microscopic “neighborhoods” of simi- larly oriented atoms snap into alignment together, and then cause nearby neighborhoods to do the same. Physicist Heinrich Barkhausen discovered this phenomenon in 1919.  Read about one practical application on Page 16. In 1892, Hendrik Lorentz authored the modern and correct formula describing electromagnetic force in terms of force from both electric and magnetic fields: F = q [E+(v × B)] where F = Force, N E = Electric field, V/m B = Magnetic field, T q = Electric particle charge, C v = Instantaneous particle velocity, m/sec Lorentz The Lorentz force law is closely related to Faraday’s law of induction: An induced electromotive force is the time rate of change of the magnetic flux through the circuit. 6 PHYSICS eBook Sponsored by HAYDON KERK MOTION SOLUTIONS 7HAYDONKERK.COM • PHYSICS eBook Cu rre nt Mo vem ent Flux Flux Torque Flux UpBrushing William Gilbert Gilbert's scientific insights also peeled back the mystery of the Earth's magnetic field. By likening the Earth to a large magnet, he was able to explain why suspended lengths of magnetized iron automatically aligned in a north-south direction. Until then, people attributed the phenomenon to one of the stars in the Big Dipper or an undiscovered iron-capped mountain range somewhere in the far North. William Gilbert, quite literally, wrote the book on magnetism. Prior to his work, which culminated in his epic treatise, De Magnete, the effects of magnetism were the stuff of myth and magic. Gilbert changed all that when he showed systematically that magnetism is a form of energy that acts with attractive or repulsive force, depending on the alignment (or polarity) of the objects involved. Gilbert tested many substances, classifying them as “electrics” or “non- electrics” depending on whether or not they built up charge when rubbed. He explained this tribo- electrification as the removal of a fluid, or “humour,” which then left an “effluvium,” or atmosphere, surrounding the treated substance. Magnetism was just one of Gilbert's interests, however. He also investigated the amber effect. As far back as the ancient Greeks, people knew that certain materials, such as amber, developed attractive or repulsive forces when rubbed with a piece of fur. Gilbert called this peculiar property “electricity,” and experimentally proved it was of a different nature than magnetism. Centuries before its discovery, Gilbert sensed the existence of positive and negative charge and its associated electric field. 8 PHYSICS eBook Sponsored by HAYDON KERK MOTION SOLUTIONS 9HAYDONKERK.COM • PHYSICS eBook Diffuse reflection Specular reflection Law of reflection According to the law of reflection, if you shine a light on a mirror, the angle of the reflected ray will equal that of the incident ray. The law also holds for curved reflecting surfaces. Diffuse reflection Diffuse reflection is typical of surfaces that are uneven or covered with particulate matter such as sand, flour, or cleanser. Due to the random nature of the reflected light, such surfaces appear uniformly bright from any direction, though the reflections are not particularly intense in any given direction. Specular reflection Visible electromagnetic radiation — light — is a free-spirited but manageable form of energy. It is both predictable and repeatable, lending itself to many practical applications. Anyone working in the field of engineering and using optical encoders, beam interrupters, or photoelectric sensors (not to mention payloads such as lasers or plain, hard optics) needs to know something about light’s properties. Normal Law of reflection Angle of incidence i Angle of reflection r i = r Con- cave mirror Reflected light rays converge to a focal point in front of a concave mirror, creating a real UpBrushing Visible electromagnetic radiation The path of a light ray through a raindrop is no simple matter. First the light refracts, then it strikes the back of the drop. If the angle is greater than the critical angle (48° for a water-air interface) some of the optical energy will be reflected back out through the front of the raindrop separated according to its wavelengths. Red light, for example, emerges at an angle of 42° ... violet light, at 40° ... resulting in a primary rainbow, red on the outer arc, violet along the inner arc. In rare instances, some light (based on its incident angle) will make two internal reflections. Though less intense, this light can form a second, larger rainbow with the color bands reversed. Rainbow 428 518 Rainbow Red Orange Yellow Green Blue Violet Dispersion Sun ligh t Sunlight (containing all wavelengths of light) splits into its fundamental colors in passing through a prism. Although the prism is made from a homogenous material, the indices of refraction vary for different wavelengths of light. Violet light is refracted over the largest angle; red light, the smallest. Smooth polished surfaces produce specular reflections characterized by uniform alignment. Such unidirectional reflections are of maximum intensity, wasting little (if any) energy off-axis. Dispersion Light Magne t ic field Ele ctr ic field Quick Fact: Light (and all electromagnetic radiation) has electric and magnetic fields that oscillate perpendicular to each other and the main direction of propagation. 10 PHYSICS eBook Sponsored by HAYDON KERK MOTION SOLUTIONS 11HAYDONKERK.COM • PHYSICS eBook Source Total internal reflection Critical angle e2 Refraction Low-to-high density High-to-low density N or m al N or m al e e Total internal reflection When light traveling in optical media reaches an outer surface, the angle of approach determines what happens. Based on the properties of the material, there is a certain angle below which all or part of the light escapes, and above which none of the light escapes, but instead reflects back into the material. At precisely the “critical angle,” some of the light bounces back into the material, while the refracted component (although it doesn’t escape) travels parallel to the surface. Refraction Refraction is what makes a pencil seem to bend when it’s placed in a glass of water. The effect is due to the difference in the indices of refraction in air and water, and the fact that the light is originating in the less optically dense medium (air). Light moving from a low density medium to a high density (air to water, for example) bends toward the normal to the surface, while in the other direction, light bends away from the normal. Interference Slit Slit Slit Interference Incident wave Diffracted wave Spherical aberration An image will appear blurry if parallel rays converge to different points along the optical axis. The finite size of the lens as well as geometric imperfections are the cause of this type of aberration. Interference experiments demonstrate the wavelike nature of light. The most familiar test employs a double- slit aperture, producing an interference pattern of alternating dark and light areas. o Concave mirror o'f f Convex mirror o o'f Convex mirror Light rays bounce off a convex mirror in such a way that they appear to originate from a point behind the mirror. Concave mirror Reflected light rays converge to a focal point in front of a concave mirror, creating a real image. The image may be employed in any number of ways, or simply for viewing as in the case of a video projector or screen. 12 PHYSICS eBook Sponsored by HAYDON KERK MOTION SOLUTIONS 13HAYDONKERK.COM • PHYSICS eBook Warm air Warm air Mirage Looming Cool air Cool air Atmospheric refraction Mirages are common when driving on hot days because the light reflected from distant objects must pass through what amounts to two optical media; warm air and cooler air. Differences in the refractive indices make it look like the objects are inverted. The familiar water mirage is actually an inverted image of the sky (or air space) just above ground level, which is exactly what you would see from a reflective watery surface such as a shimmering pool. If, on the other hand, there’s an air temperature inversion, with cooler air near the ground, you may be able to see objects below the horizon. The conditions at sunrise and sunset often produce this “looming” effect; it also occurs on snow-covered plains and cold lakes. When people use the phrase “looming on the horizon,” they are unknowingly referring to this optical phenomenon. Virtual image What’s often called a “reflection” is more precisely termed a “virtual image.” The image appears to be as far behind the reflective surface as the actual object is in front. The illusion of depth is caused by the divergent nature of the reflected light rays. Our visual system is accustomed to processing light rays traveling in straight lines, and is thus tricked when light rays fan out as they do from a reflective surface. Image Virtual image Mirror Object 14 PHYSICS eBook Sponsored by HAYDON KERK MOTION SOLUTIONS 15HAYDONKERK.COM • PHYSICS eBook Why the sudden change? Wiegand wire’s sudden polarity flip is due in part to iron’s tendency to magnetize in little steps rather than in a smooth progres- sion. When subjected to an external field, microscopic “neighborhoods” of similarly oriented atoms snap into alignment to- gether, and then cause nearby neighbor- hoods to do the same. Physicist Heinrich Barkhausen discovered this phenomenon in 1919.  His ladies sung to him Before he had an oscilloscope to see pulses, it was Wiegand’s perfec