36 Magnetism and Magnetic Fields 36.1 Magnetism Static Magnetic Fields. Time-Dependent Electric and Magnetic Fields. Magnetic Flux Density. Relative Permeabilities. Forces on a Moving Charge. Time-Varying Magnetic Fields. Maxwell ' Equations. Dia- and Paramagnetism.Ferromagnetism and Geoffrey Bate Ferrimagnetism. Intrinsic Magnetic Properties. Extrinsic Magnetic Properties. Amorphous Magnetic Materials 6.2 Magnetic Recording Fundamentals of Magnetic Recording. The Recording Mark H. Kryder Process. The Readback Process. Magnetic Recording Carnegie Mellon University Meda· Magnetic Recording heads· Conclusions 36.1 Magnetism Geoffrey bate Static Magnetic Fields To understand the phenomenon of magnetism we must also consider electricity and vice versa. A stationary electric charge produces, at a point a fixed distance from the charge, a static(i.e, time-invariant)electric field A moving electric charge, i.e., a current, produces at the same point a time-dependent electric field and a magnetic field, dH, whose magnitude is constant if the electric current, L, represented by the moving electric charge, is constant Fields from Constant Currents Figure 36. 1 shows that the direction of the magnetic field is perpendicular both to the current I and to the line, R, from the element dL of the current to a point, P where the magnetic field, dH, is being calculated or measured. dh =I dL x r/4Tr3 A/m when I is in amps and dL and r are in meters If the thumb of the right hand points in the direction of the current, then the fingers of the hand curl in the direction of the magnetic field. Thus, the stream lines of H, i.e. the lines representing at any point the direction of the H field, will be an infinite set of circles having the current as center. The magnitude of the field Ho /2R A/m. The line integral of H about any closed path around the current is H dL= I. This relationship lown as Ampere's circuital law) allows one to find formulas for the magnetic field strength for a variety of symmetrical coil geometries, e.g c 2000 by CRC Press LLC© 2000 by CRC Press LLC 36 Magnetism and Magnetic Fields 36.1 Magnetism Static Magnetic Fields • Time-Dependent Electric and Magnetic Fields • Magnetic Flux Density • Relative Permeabilities • Forces on a Moving Charge • Time-Varying Magnetic Fields • Maxwell’s Equations • Dia- and Paramagnetism • Ferromagnetism and Ferrimagnetism • Intrinsic Magnetic Properties • Extrinsic Magnetic Properties • Amorphous Magnetic Materials 36.2 Magnetic Recording Fundamentals of Magnetic Recording • The Recording Process • The Readback Process • Magnetic Recording Media • Magnetic Recording Heads • Conclusions 36.1 Magnetism Geoffrey Bate Static Magnetic Fields To understand the phenomenon of magnetism we must also consider electricity and vice versa. A stationary electric charge produces, at a point a fixed distance from the charge, a static (i.e., time-invariant) electric field. A moving electric charge, i.e., a current, produces at the same point a time-dependent electric field and a magnetic field, dH, whose magnitude is constant if the electric current, I, represented by the moving electric charge, is constant. Fields from Constant Currents Figure 36.1 shows that the direction of the magnetic field is perpendicular both to the current I and to the line, R, from the element dL of the current to a point, P, where the magnetic field, dH, is being calculated or measured. dH = I dL 2 R/4pR3 A/m when I is in amps and dL and R are in meters If the thumb of the right hand points in the direction of the current, then the fingers of the hand curl in the direction of the magnetic field. Thus, the stream lines of H, i.e., the lines representing at any point the direction of the H field, will be an infinite set of circles having the current as center. The magnitude of the field Hf = I/2pR A/m. The line integral of H about any closed path around the current is rH · dL = I. This relationship (known as Ampère’s circuital law) allows one to find formulas for the magnetic field strength for a variety of symmetrical coil geometries, e.g., Geoffrey Bate Consultant in Information Storage Technology Mark H. Kryder Carnegie Mellon University