# Week-15 (ph-1002).pptx

Student um NTU Faisalabad
28. May 2023
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### Week-15 (ph-1002).pptx

• 1. Week # 15 Electromagnetic induction: Induction experiments, Faraday’s law, Eddy Current & its applications Book: “University Physics” by Young & Freedman 13th edition, page: P-957” Applied Physics (Ph-1003), 4 (3, 3) 1
• 2. Michael Faraday • 1791 – 1867 • Great experimental scientist • Invented electric motor, generator and transformers • Discovered electromagnetic induction • Discovered laws of electrolysis 2
• 3. 3 Faraday’s law of induction states: The induced emf in a closed loop equals the negative of the time rate of change of magnetic flux through the loop. In symbols, Faraday’s law is If we have a coil with N identical turns, and if the flux varies at the same rate through each turn, the total rate of change through all the turns is N times as large as for a single turn. If øBis the flux through each turn, the total emf in a coil with N turns is The negative sign in Faraday’s Law is included to indicate the polarity of the induced emf, which is found by Lenz’ Law
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• 10. 10 Application: Exploring the Brain with Induced emfs Transcranial magnetic stimulation (TMS) is a technique for studying the function of various parts of the brain. A coil held to the subject’s head carries a varying electric current, and so produces a varying magnetic field. This field causes an induced emf, and that triggers electric activity in the region of the brain underneath the coil. By observing how the TMS subject responds (for instance, which muscles move as a result of stimulating a certain part of the brain), a physician can test for various neurological conditions
• 11. Applications of Faraday’s Law – Electric Guitar • A vibrating string induces an emf in a coil • A permanent magnet inside the coil magnetizes a portion of the string nearest the coil • As the string vibrates at some frequency, its magnetized segment produces a changing flux through the pickup coil • The changing flux produces an induced emf that is fed to an amplifier 11
• 12. Applications of Faraday’s Law – Apnea Monitor • The coil of wire attached to the chest carries an alternating current • An induced emf produced by the varying field passes through a pick up coil • When breathing stops, the pattern of induced voltages stabilizes and external monitors sound an alert 12
• 13. 13 Induction stove Figure below, shows two pots of water that were placed on an induction stove at the same time. There are two interesting features in this drawing. First, the stove itself is cool to the touch. Second, the water in the ferromagnetic metal pot is boiling while that in the glass pot is not. How can such a “cool” stove boil water, and why isn’t the water in the glass pot boiling? The water in the ferromagnetic metal pot is boiling. Yet the water in the glass pot is not boiling, and the stove top is cool to the touch. The stove operates in this way by using electromagnetic induction
• 14. 14 Reasoning for Induction stove: The key to this puzzle is related to the fact that one pot is made from a ferromagnetic metal and one from glass. We know that metals are good conductors, while glass is an insulator. Perhaps the stove causes electricity to flow directly in the metal pot. This is exactly what happens. The stove is called an induction stove because it operates by using electromagnetic induction. Just beneath the cooking surface is a metal coil that carries an ac current (frequency about 25 kHz). This current produces an alternating magnetic field that extends outward to the location of the metal pot. As the changing field crosses the pot’s bottom surface, an emf is induced in it. Because the pot is metallic, an induced current is generated by the induced emf. The metal has a finite resistance to the induced current, however, and heats up as energy is dissipated in this resistance. The fact that the metal is ferromagnetic is important. Ferromagnetic materials contain magnetic domains (see Section 21.9), and the boundaries between them move extremely rapidly in response to the external magnetic field, thus enhancing the induction effect. A normal aluminum cooking pot, in contrast, is not ferromagnetic, so this enhancement is absent and such cookware is not used with induction stoves. An emf is also induced in the glass pot and the cooking surface of the stove. However, these materials are insulators, so very little induced current exists within them. Thus, they do not heat up very much and remain cool to the touch.
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• 18. Applications of Induction: Sound Systems, This microphone works by induction; the vibrating membrane induces an emf in the coil 18
• 19. Applications of Induction: Sound Systems, Differently magnetized areas on an audio tape or disk induce signals in the read/write heads. 19
• 20. Applications of Induction: Seismograph A seismograph has a fixed coil and a magnet hung on a spring (or vice versa), and records the current induced when the earth shakes. 20