Magnetic Disks

Magnetic Disks

(Hard Disk)

The topic of magnetic disks is one that involves many physics related phenomenon. The intricate structure and design of “Magnetic Disks” (or hard disks) in computers include the principles of Fluid Flow, Rotational Motion, Electromagnetism, and more. This paper will focus mainly on the previously listed physics occurrences, and the design that goes into engineering the magnetic disk to include them. These physics principles are utilized in such a way that makes the hard disk a very common and useful tool, in this day and age. To most people, the magnetic disk is the most important, yet most mysterious, part of a computer system. A hard disk is a seal unit that holds computer data in the form of magnetic patterns.

Before understanding the physics principles, one must understand the physical design that induces them. A magnetic disk is a flat, circular, rigid sheet of aluminum coated with a layer of magnetic material (can be double sided). The material usually is a form of iron oxide with various other elements added. The disk rotates upon a central axis and a movable read/write head writes information along concentric tracks (circular paths traced out by motion of the disk) on it. Multiple disks can be stacked to store more information. Typically (1985) 11 disks with 22 surfaces, of which 20 are used (minus top/bottom), are manipulated to read/write data.

The “head”, or device used to transmit data onto the magnetic disks, is an important part of the hard disk and composes most of the physics happenings. Current is passed through the head or in the physic’s case, the conductor, to produce a magnetic field around the conductor. This magnetic field then can influence the disk’s magnetic material. The head is driven by an electric motor, using electromagnetism, to exert pushing and pulling forces on magnets to the rotating shaft. In some cases the head moves to a required area on the disk, and the motion of the magnetized surface induces tiny voltage. This voltage is concentrated in the coil of the read head, and can be interpreted as the data stored on the magnetic disk. When the direction of the flow of electric current is reversed, the magnetic field’s polarity is reversed.

The head is mounted in a “slipper” (or holder) positioned above the disk at 0.5-2.5 microns from the surface. When the disk is revolving around its axis, an air current creates a velocity gradient with the surface and air. This creates enough lift to oppose the spring pressing the head towards the disk. If there were contact with the disk, it would wear out quickly considering the disk rotates at 100km/hr. This is enough space for the magnetic field to affect the disk and read/write data. The fluid flow of the air forms a pattern typical of laminar flow, in which adjacent layers of fluid slide smoothly past each other and the flow is steady. The air under the head (shaped like an aerofoil) increases in pressure in this region. An upward force is created on the underside that is greater than the downward force on the top of the head. There is a net upward force, or lift. The greatest contribution to this lift is the reduced pressure on the upper head surface.

In order to create the appropriate voltage from the magnetized surface, we must achieve a certain rotational kinetic energy (K). By taking the calculated moment of inertia (I), of I = ЅMR2 (M = mass in kg, R = radius (minus area covered by axis) in meters) and applying it to the equation, “K = ЅWI2” (W = angular speed in rad/sec) we can determine its kinetic energy. Vise versa, we can use the desired kinetic energy to ascertain how fast we need the angular speed to be. A typical disk rotates at 2400 rpm, or using the conversion as follows: w = 2400 rpm = ( 2400 rev/min )( 2р rad / 1 rev )( 1 min / 60 s ), or at 251 rad/s.

Nearly all hard disks in personal computer systems operate on magnetic principles. Purely optical drives often are used as a secondary form of storage, but the computer to which they are connected is likely to use a magnetic storage medium for primary disk storage. Due to the high performance and density capabilities of magnetic storage, optical hard disks and media probably never will totally replace magnetic storage in PC systems.

When a magnetic field is generated in the head, the field jumps the gaps of the read/write head. The field bends it as the path of least resistance to the other side of the gap, because magnetic