Text of Chapter 12 Magnetism and Magnetic Circuits. 12.1The nature of a magnetic field [page 461] Magnetism...
Chapter 12 Magnetism and Magnetic Circuits
12.1The nature of a magnetic field [page 461] Magnetism refers to the force that acts betwewen magnets and magnetic materials, e.g. iron. Magnetic field is a force field. Magnetic flux [Fig. 12-1, page 462] the field is strongest at the poles of the magnet. Its directionis from north (N) to south (S) external to the magnet the flux lines never cross Field patterns due to attraction or repulsion [Fig. 12-2, page 462] unlike poles attract, like poles repel
Ferromagnetic materials (p. 462) magnetic materials e.gon, nickel, cobalt. Provide an easy path for magnetic flux [Fig. 12-3, page 463] nonmagetic materials (glass, wood, plastic) have no effect on the field In Figure 12-3,magnetic field follows the path through the path through the iron. It is not affected by the plastic. Application: loudspeaker [Fig. 12-4, page 463] field is created by permanent magnet iron circuit guide the flux to air gap to provide intense field required by the voice coil
12.2Electromagnetism (p. 464) most applications of magnetism involve magnetic effects due to electric currents. Field about a current-carrying conductor [Fig. 12-5, page 464] right-hand rule Field produced by a coil [Fig. 12-6, page 464] if the current direction is reversed, the field direction will reverse for ferromagnetic materials, almost all the flux is confined to the core [Fig. 12-7, page 465]
12.3Flux and Flux Density (p. 465) Symbol unit Flux Weber, Wb Flux density B Teslas, T (or Wb/m 2 ) B = / A the greater the flux density, the stronger the field [Fig. 12-8, page 465] Example 12-1 For the magnetic core of Figure 12-9 [page 465], the flux density at cross section 1 is B 1 = 0.4 T. Determine B 2. Solution = B 1 x A 1 = (0.4 T) (2 x 10 -2 m 2 ) = 0.8 x 12 -2 Wb. Since all flux is confined to the core, the flux at cross section 2 is the same as at cross section 1. Therefore, B 2 = /A 2 = (0.8 x 10 -2 Wb) / (1 x 10 -2 m 2 ) = 0.8 T Figure 12-8Concept of flux density. 1 T = 1 Wb/m 2.
12.4Magnetic Circuits (p. 467) use magentic structures to guide and shape magnetic field found in computer disk drive, tape recorder, motor, generator Recorder head [Fig. 12-10, page 467] current from amplifier pass throgh its coil, creatign a magnetic field that magnetizes the moving tape. Playback system [Fig. 12-11, page 468] voltages induced in the playback coil are amplified and applied to a speaker current from amplifier passes through flexible coil, creating a varying field that interacts with the fixed field of the speaker magnet, causing cone to vibrate
12.5Air Gaps, Fringing, and Laminated Cores (p. 468) fringing occurs at gaps causign a decrease in flux density in gap [Fig. 12-12a, page 468] laminations [Fog. 12-12b, page 468]: effective cross-sectional area