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23 - 726 Chapter 15 Magnetic Circuits and Transformers 8...

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Unformatted text preview: 726 Chapter 15 Magnetic Circuits and Transformers 8. The B—H relationship for iron takes the form of a hysteresis loop, which displays saturation in the neighborhood of l to 2 T. The area of the loop rep- resents energy converted to heat per cycle. Eddy currents are another cause of core loss. Energy can be stored in magnetic fields. In a magnetic circuit consisting of an iron core with an air gap, most of the energy is stored in the gap. Problems Section 15.]: Magnetic Fields P15.1. What is the fundamental cause of magnetic fields? P15.2. State the right-hand rule as it applies to a. a current-carrying conductor; b. a current-carry— ing coil. P15.3. State Ampére’s law, including the reference directions for the currents. P15.4. State Faraday’s law of magnetic induction and Lenz‘s law. P155. A long copper pipe carries a dc current. Is there a magnetic field inside the pipe due to the current? Outside? Justify your answers. *P15.6. The magnetic field of the earth is approxi- mately 3 x10‘5 T. At what distance from a long straight wire carrying a steady current of 10 A is the field equal to 10 percent of the earth‘s field? *PISJ. A bar magnet is inserted into a single-turn coil as illustrated in Figure P15]. Is the voltage var, positive or negative as the bar approaches the coil? P153. An irregular loop of wire carries an electrical current as illustrated in Figure P158. Is there a net force on the loop due to the magnetic fields created? Justify your answer. (Hint: Consider Newton‘s third law of motion.) * Denotes that answers can he found on the OrCAD CD and on the website ww.prenha1'l .com/hamb'l ey 9. In an ideal transformer. the voltage across each coil is proportional to its number of turns. the net mmf is zero, and the power efficiency is 100 percent. 10. Equivalent circuits for real transformers are shown in Figures 15.28 and 15.29 on pages 721 and 722, respectively. 11. Efficiency and regulation are important aspects of transformer operation. Figure P153 \’ Figure P153 P153. Use the right-hand rule to find the directi _' of the magnetic flux for each coil shown in Ft .! ure P159. Mark the N and S ends of each co“ Do the coils attract or repel one another? 10 AT [Elma (a) (b Figure P153 *P15.10. A 0.5—m length of wire carries a 10-A curre'l' perpendicular to a magnetic field. Dete ' ' ...
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