ch9 - Electricity And Electronics Fundamentals TECH 175...

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Unformatted text preview: Electricity And Electronics Fundamentals TECH 175 Fall Semester 2007 Chapter 9 Magnetism Technology is every minute! this? Changing What is 1951 First large-scale largeelectronic computer Punched cards Magnetic tapes Internal memory made up from vacuum tubes Very Slow Tremendou s amount of heat! 3 The Nature of a Magnetic Field Magnetism refers to the force that acts between magnets and magnetic material. Flux lines show the direction and intensity of this field at all points. The field is strongest at the poles; the direction is from N to S. 4 Laws of Magnetism Like poles repel each other Unlike poles attract each other 5 6 1 Laws of Magnetism The magnetic force increases as the distance between the magnets decreases Magnetic force, either attractive or repelling, varies inversely with the square of the distance between the poles Magnets 7 8 Magnetism refers to the force that acts between magnets and magnetic materials. materials. Magnets attract pieces of iron, deflect compass needles, attract/repel other magnets! This force acts without the need for direct physical contact! Magnetic field is a force field What happens when two magnets are brought close together. Unlike poles attract & flux lines pass from one magnet to the other "Attraction" Attraction" Like poles repel, and flux lines are pushed back as flattening of the field between magnets. "Repulsion" Repulsion" Magnetic materials attracted by magnets such as iron, nickel, cobalt, & their alloys are called FERROMAGNETIC materials. Attraction Magnetic Flux: Repulsion 9 Flux lines "lines of force" showing force" direction and intensity of the fields at all points take the longer but easier path through soft iron, rather than the shorter path that they normally take. Materials that are attracted by magnets are called ferromagnetic materials. These materials provide an easy path for magnetic flux. Iron, nickel, cobalt, and their alloys are ferromagnetic. Nonmagnetic materials such as plastic, wood, and glass have no effect on the field. 10 Flux and Flux Density Flux, Total number of lines If a magnet is strong, the lines of flux will be more dense Flux and Flux Density Units for magnetic flux are webers (Wb) Area is measured in square meters Units for flux density Wb/m2 or teslas (T) 1 tesla = 10 000 gauss Flux density, B, Number of lines per unit area Divide total flux passing perpendicularly through an area by the area B = /A 11 B may also be measured in gauss We will work only with teslas 12 2 Flux and Flux Density Electromagnetism Many applications of magnetism involve magnetic effects due to electric currents A magnetic field exists around a conductor carrying an electric current Direction of magnetic field may be determined by the Right Hand Rule 13 14 Right-hand rule Place your right hand around the conductor with your thumb pointing in the direction of the current. Your fingers then point in the direction of the field. If you reverse the direction of the current, the direction of the field reverses. 15 Right-hand rule Curl the fingers of your right hand around the coil in the Direction of the current and your thumb will point in the Direction of the field. If the direction of the current is Reversed, the field also reverses 16 Ferromagnetic core flux 1. 2. Learning Check A magnetic field is a ____ field. The density of flux lines represents the ____ of the field and their direction represents the _____ of the field. The direction of a magnetic field is from ___ to ___ outside a magnet. Flux density B is defined as ratio /A, where A is area (parallel, perpendicular) to . 18 3. 4. 17 3 Magnetic Circuits Practical applications Use structures to guide and shape magnetic flux Called magnetic circuits Magnetic Circuits Playback heads on tape recorders VCRs and disk drives pick up the varying magnetic field and convert it to voltage 19 20 Magnetic Circuits Recording System Magnetic Circuits Playback System 21 22 Series Elements and Parallel Elements Magnetic circuits may have sections of different materials Cast iron, sheet steel, and an air gap Series Elements and Parallel Elements A magnetic circuit may have elements in parallel Sum of fluxes entering a junction is equal to the sum leaving For this circuit, flux is the same in all sections Circuit is a series magnetic circuit 23 Similar to series/parallel electric circuits 24 4 Magnetic Circuits with dc Excitation Current through a coil creates magnetic flux Magnetomotive Force (MMF) = NI N is the number of turns of the coil Reluctance Opposition of the circuit Depends on the dimension of the core and its material Reluctance = l/A (At / Wb) Wb) 25 26 Magnetic Circuits with dc Excitation Ohm's Law for magnetic circuits: Ohm' = / Magnetic Field Intensity The magnetic field intensity, H, is intensity, the magnetomotive force (mmf) per unit length. H = /l = NI/l The units are Ampereturns/meter. Ampere NI = Hl 27 28 Magnetic Field Intensity The Relationship between B and H B and H are related by the equation B = H, where is the permeability of the core. Permeability is the measure of how easy it is to establish flux in a material. The larger the value of , the larger flux density for a given H. B = H 29 30 5 If B =1.4 T for sheet steel, what is H? SheetSteel Cast Steel Cast Iron Cast Iron 31 32 Example: (a) If the core shown is cast iron and =0.1mWb, I=? Series Magnetic Circuits One Material NI = 250 At. Determine 33 34 Forces Due to an Electromagnet Electromagnets Relays, doorbells, lifting magnets, etc. Electromagnetic Relay Force computed from flux density, the gap area, and the permeability F = B g Ag 20 2 Bg - flux density in the air-gap (Tesla) Ag - gap area (m2) F - the force (Newton) 35 The force due to the current-carrying coil close the contacts 36 6 Control a Load with a Relay Electromagnetic Relay A relay can be chosen to open or close a circuit Normally closed (NC) Normally open (NO) 37 38 Electromagnetic Relay Time relays Designed to have an adjustable time delay Time-delay on TimeAfter the relay is energized, there is a pause before the contact close Electromagnetic Relay Magnetic reed switch Time-delay off TimeAfter the relay is de-energized, there is a detime delay before the contacts open 39 40 Magnetic reed switch Magnetic Circuit Breaker Magnetic circuit breaker 41 42 7 Magnetic Levitation Transportation Use of magnetism to elevate a train car Propel the car on the rails using magnetism Rails are made up of a long series of solenoids mounted along the track When the solenoids are energized, the train is elevated above the track Computer energizes the solenoids one by one so that the car can gradually increase its speed to over 200 miles per hour 43 Properties of Magnetic Materials Atoms produce small, atomic-level atomicmagnetic fields. For nonmagnetic materials, these fields are randomly arranged. For ferromagnetic materials, the fields do not cancel, but instead form into domains. domains. If the domain fields in a material line up, the material is magnetized. 44 Nonmagnetized Ferromagnetic Material Magnetization Process 45 46 Hysteresis Loop Demagnetization by successfully shrinking the Hysteresis Loop Magnetic Field Measurement 47 48 8 Applications Applications Speakers and microphones 49 50 Applications Magnetic Resonance Imaging (MRI) Magnetic Resonance Imaging (MRI) 51 52 The toroid has a circular cross section and = 628 Wb. If r1 = Wb. 8cm and r2 = 12cm, what is the flux density in Tesla? For the iron core, flux density B2 = 0.6 T. Compute B1 and B3 53 54 9 ...
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