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UT Arlington PHYS 1444 - Lecture Notes

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PHYS 1444 – Section 003 Lecture #18AnnouncementsMagnetic Materials - FerromagnetismB in Magnetic MaterialsSlide 5HysteresisSlide 7Magnetically Soft MaterialInduced EMFElectromagnetic InductionSlide 11Magnetic FluxFaraday’s Law of InductionLenz’s LawInduction of EMFExample 29 – 2Example 29 – 2, cnt’dEMF Induced on a Moving ConductorWednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu1PHYS 1444 – Section 003Lecture #18Wednesday, Nov. 2, 2005Dr. Jaehoon Yu•Magnetic Materials – Ferromagnetism •Magnetic Fields in Magnetic Materials; Hysteresis•Induced EMF•Faraday’s Law of Induction•Lenz’s Law•EMF Induced on a Moving ConductorWednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu2Announcements•The 2nd term exam–Date: Monday, Nov. 7–Time: 1 – 2:20pm–Location: SH 103–Coverage: from CH 26 to CH29 – 3•Your textbooks–UTA bookstore agreed to exchange your books with the ones that has complete chapters•You need to provide a proof of purchase–Receipts, copy of cancelled checks, credit card statement, etc.Wednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu3Magnetic Materials - Ferromagnetism•Iron is a material that can turn into a strong magnet–This kind of material is called ferromagneticferromagnetic material•In microscopic sense, ferromagnetic materials consists of many tiny regions called domainsdomains–Domains are like little magnets usually smaller than 1mm in length or width•What do you think the alignment of domains are like when they are not magnetized?–Randomly arranged•What if they are magnetized?–The domains aligned with the external magnetic field direction grows while domains not aligned reduce–This gives magnetization to the material•How do we demagnetize a bar magnet?–Hit the magnet hard or heat it over the Curie temperatureWednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu4B in Magnetic Materials•What is the magnetic field inside a solenoid?• –Magnetic field in a long solenoid is directly proportional to the current.–This is valid only if air is inside the coil•What do you think will happen to B if we have something other than the air inside the solenoid?–It will be increased dramatically, when the current flows•Especially if a ferromagnetic material such as an iron is put inside, the field could increase by several orders of magnitude•Why?–Since the domains in the iron aligns permanently by the external field.–The resulting magnetic field is the sum of that due to current and due to the iron0B =0nImWednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu5B in Magnetic Materials•It is sometimes convenient to write the total field as the sum of two terms• –B0 is the field due only to the current in the wire, namely the external field•The field that would be present without a ferromagnetic material–BM is the additional field due to the ferromagnetic material itself; often BM>>B0•The total field in this case can be written by replacing 0 with another proportionality constant , the magnetic permeability of the material–  is a property of a magnetic material–  is not a constant but varies with the external fieldB =rB nIm=0B +rMBrWednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu6Iron Core ToroidHysteresis•What is a toroid?–A solenoid bent into a shape•Toroid is used for magnetic field measurement–Why?–Since it does not leak magnetic field outside of itself, it fully contains all the magnetic field created within it.•Consider an un-magnetized iron core toroid, without any current flowing in the wire–What do you think will happen if the current slowly increases?–B0 increases linearly with the current.–And B increases also but follows the curved line shown in the graph–As B0 increases, the domains become more aligned until nearly all are aligned (point b on the graph)•The iron is said to be approaching saturation•Point b is typically at 70% of the maxWednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu7Hysteresis•What do you think will happen to B if the external field B0 is reduced to 0 by decreasing the current in the coil?–Of course it goes to 0!!–Wrong! Wrong! Wrong! They do not go to 0. Why not?–The domains do not completely return to random alignment state•Now if the current direction is reversed, the external magnetic field direction is reversed, causing the total field B pass 0, and the direction reverses to the opposite side–If the current is reversed again, the total field B will increase but never goes through the origin•This kind of curve whose path does not retrace themselves and does not go through the origin is called the HysteresisHysteresis.Wednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu8Magnetically Soft Material•In a hysteresis cycle, much energy is transformed to thermal energy. Why?–Due to the microscopic friction between domains as they change directions to align with the external field•The energy dissipated in the hysteresis cycle is proportional to the area of the hysteresis loop•Ferromagnetic material with large hysteresis area is called magnetically hard while the small ones are called soft–Which ones do you think are preferred in electromagnets or transformers?•Soft. Why?•Since the energy loss is small and much easier to switch off the field •Then how do we demagnetize a ferromagnetic material?–Keep repeating the Hysteresis loop, reducing the range of B0.Wednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu9Induced EMF•It has been discovered by Oersted and company in early 19th century that –Magnetic field can be produced by the electric current–Magnetic field can exert force on electric charge•So if you were scientists at that time, what would you wonder?–Yes, you are absolutely right. You would wonder if magnetic field can create electric current.–An American scientist Joseph Henry and an English scientist Michael Faraday independently found that it was possible•Though, Faraday was given the credit since he published his work before Henry did–He also did a lot of detailed studies on magnetic inductionWednesday, Nov. 2, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu10Electromagnetic Induction•Faraday used an apparatus below to show that magnetic field can induce current•Despite his hope he did not see steady current induced on the other side when the switch


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