Optical disk

Optical disk is a secondary storage unit which can store extremely large amount of data in a limited space. It is a random access system which consists of a rotating disk which is coated with a highly reflective material. For recording and retrieving data, laser beam technology is employed.  For this reason, optical disks are also known as laser disks. Optical disk has one long spiral track which starts at outer edge and spirals inward to the center. This is a main difference between this disk and magnetic disk where several concentric tracks exist. The optical disk track is split into various sectors.  Optical disks are very economical and with very small sizes ranging from 5 inches to 12 inches in diameter. The factors which influence storage capacity of an optical disk are number of sectors and number of bytes per sector. An optical disk has to be mounted on an optical disk drive before it can be used for reading and writing of information. This disk drive has provision to include all the mechanical, electrical and electronic components which facilitates reading and writing tasks. The different categories of optical disks are compact disk read only memory (CD-ROM), compact disk recordable (WORM-CD-R), compact disk rewritable (CD-RW) and digital video disk with read only memory (DVD-ROM). Since there is no mechanical heads in optical disks, damage due to crash is forbidden. The cost per bit of storage is very low and storage life is more than 30 years. Optical disks are ideal for storing and reading musical data. Optical disks are easy to handle, store and is portable. The main limitation of optical disk is its low data access speed. Optical disk drive mechanism is complicated and the option of reusability is forbidden in it.

Soil dynamics and earthquake engineering

The term soil has originated from the latin word solum which has different meanings. Soil dynamics deals with the study of material properties of soil under dynamic stress. Knowledge of classical dynamics of elastic continuum and dynamics of vibrations are required for the effective soil dynamics study. Since soils and rocks are naturally occurring materials, it is always preferred to have test on theses before use. Relying fully on structural handbook is not advisable. Earth quake motion dynamics is usually expressed in terms of acceleration – time trace, velocity-time trace and displacement-time trace. There exist two methods for the evaluation of soils- deterministic and non-deterministic. Since the soil dynamics and earth quake engineering are very much inter related, it lead to the emergence of new branch of engineering- geotechnical earthquake engineering. In majority of the cases, motion of the soil causes earthquakes. The motion of the soil is transmitted to the structure leading to damages. In seismic zone, study of the motion of soil or soil dynamics is very significant and for any earthquake resistant design of structures, seismic data plays a major role.

Different hydro power plants

Run-of-river plants: Here stream should be perennial with minimum flow which is adequate to generate sufficient power. Thus power generation is achieved with the utilization of minimum flow.

Storage plants: Here reservoir water storage is achieved by constructing a dam across a river. Storage is done during high rainfall season.

Cascade plant: Here available water is used more than once in a series of hydel power plants constructed to utilize the available drop in the head.

Pumped storage plant: Here power generation is carried out using the water stored in the upper reservoir during the period of low rainfall.

Tidal power: Water can be stored in a basin during high tide which can be made to fall into sea during low tide through some turbines to produce water.