Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Electronics Fundamentals 8th edition Floyd/Buchla© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.chapter 7electronics fundamentalscircuits, devices, and applicationsTHOMAS L. FLOYDDAVID M. BUCHLAElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.Magnetic fields are described by drawing flux lines that represent the magnetic field. Where lines are close together, the flux density is higher.Where lines are further apart, the flux density is lower.Magnetic QuantitiesElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.Magnetic fields are composed of invisible lines of force that radiate from the north pole to the south pole of a magnetic material. Field lines can be visualized with the aid of iron filings sprinkled in a magnetic field.The Magnetic FieldElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.The magnetic field lines surrounding a magnet actually radiate in three dimensions. These magnetic field lines are always associated with moving charge.The Magnetic FieldIn permanent magnets, the moving charge is due to the orbital motion of electrons. Ferromagnetic materials have minute magnetic domains in their structure.Electronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.In ferromagnetic materials such as iron, nickel, and cobalt, the magnetic domains are randomly oriented when unmagnetized. When placed in a magnetic field, the domains become aligned, thus they effectively become magnets.Magnetic MaterialsElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.The unit of flux is the weber. The unit of flux density is the weber/square meter, which defines the unit tesla, (T), a very large unit. Flux density is given by the equationwhere B = flux density (T) = flux (Wb)A = area (m2) B=AF l u x l i n e s ( A r e a ( m )2Magnetic QuantitiesElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.To measure magnetic fields, an instrument called a gaussmeter is used. A typical gaussmeter is shown.Magnetic QuantitiesThe gauss, (G) is a unit of flux density and is a much smaller unit than the tesla (104 G = 1 T). Gaussmeters are commonly used for testing motors, classifying magnets, mapping magnetic fields, and quality control by manufacturers of motors, relays, solenoids, and other magnetic devices.Electronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved. What is the flux density in a rectangular core that is 8.0 mm by 5.0 mm if the flux is 20 Wb?( ) ( )623 320 10 Wb0.50 Wb/m =8.0 10 m 5.0 1 0 T0 m.50 B-- -�= =� �Magnetic Quantities B=AExpress this result in gauss.410 G0.5 T T5000 G� �=� �� �Electronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.• Magnetic flux lines surround a current carrying wire. • The field lines are concentric circles.Magnetic Quantities• As in the case of bar magnets, the effects of electrical current can be visualized with iron filings around the wire – the current must be large to see this effect.Current-carrying wireIron filingsElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.• Permeability () defines the ease with which a magnetic field can be established in a given material. It is measured in units of the weber per ampere-turn meter.Magnetic Quantities• Relative Permeability (r) is the ratio of the absolute permeability to the permeability of a vacuum.• The permeability of a vacuum (0) is 4 x 10-7 weber per ampere-turn meter, which is used as a reference.0rElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.• Reluctance (R) is the opposition to the establishment of a magnetic field in a material. R= reluctance in A-t/Wbl = length of the path = permeability (Wb/A-t m).A = area in m2 Magnetic QuantitiesAlRElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.Fm = NI• Recall that magnetic flux lines surround a current-carrying wire. A coil reinforces and intensifies these flux lines. • The cause of magnetic flux is called magnetomotive force (mmf), which is related to the current and number of turns of the coil.Fm = magnetomotive force (A-t)N = number of turns of wire in a coil I = current (A)Magnetic QuantitiesElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.• Ohm’s law for magnetic circuits is Magnetic Quantities• flux () is analogous to current• magnetomotive force (Fm) is analogous to voltage• reluctance (R) is analogous to resistance. What flux is in a core that is wrapped with a 300 turn coil with a current of 100 mA if the reluctance of the core is 1.5 x 107 A-t/Wb?2.0 WbRmFElectronics Fundamentals 8th edition Floyd/BuchlaChapter 7Chapter 7© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.The magnetomotive force (mmf) is not a true force in the physics sense, but can be thought of as a cause of flux in a core or other material.Current in the coil causes flux in the iron core.What is the mmf if a 250 turn coil has 3 A of current? 750 A-tMagnetic QuantitiesIron coreElectronics Fundamentals