2. Decide whether the following statements are true (t) or false (f) in relation to the information in the text. If you feel a statement is false, change it to make it true.

• Viruses cannot be spread through a computer network, only via floppies transported from computer to computer.

• The virus will spread as soon as you put the infected floppy in your PC.

• The infector works by interfering in some way with the normal use of your computer.

• The detonator in Lehigh works by altering what you see on your screen.

• Most viruses spread through pirated games.

• You should run an antivirus program every time you use your computer.

• There are not very many viruses in circulation.

• Virus shields are more effective than virus scanners.

Text 3 Read the text, find passive constructions, translate.

Database management systems

Databases are used within a medical context for many purposes. For example, they are used to hold patient details so they can be accessed from anywhere within a hospital or network of hospitals. With the recent improvements in image compression techniques, X-rays and scan output can also be held in databases and accessed in the same way.

These multi-user databases are managed by a piece of software called a database management system (DBMS). It is this which differentiates a database from an ordinary computer file. Between the physical database itself (i.e. the data as actually stored) and the users of the system is the DBMS. All requests for access to data from users - whether people at terminals or other programs running in batch - are handled by the DBMS.

One general function of the DBMS is the shielding of database users from machine code (in much the same way that COBOL shields programmers from machine code). In other words, the DBMS provides a view of the data that is elevated above the hardware level, and supports user-requests such as 'Get the PATIENT record for patient Smith', written in a higher-level language.

The DBMS also determines the amount and type of information that each user can access from a database. For example, a surgeon and a hospital administrator will require different views of a database.

When a user wishes to access a database, he makes an access request using a particular data-manipulation language understood by the DBMS. The DBMS receives the request, and checks it for syntax errors. The DBMS then inspects, in turn, the external schema, the conceptual schema, and the mapping between the conceptual schema and the internal schema. It then performs the necessary operations on the stored data.

In general, fields may be required from several logical tables of data held in the database. Each logical record occurrence may, in turn, require data from more than one physical record held in the actual database. The DBMS must retrieve each of the required physical records and construct the logical view of the data requested by the user. In this way, users are protected from having to know anything about the physical layout of the database, which may be altered, say, for performance reasons, without the users having their logical view of the data structures altered.

Ask as many questions to the text as you can and answer them.

Text 4 Read and translate the text.

ELECTRON EMISSION

1. The electron tube depends for its action on a stream of electrons that act as current carriers. To produce this stream of electrons a special metal electrode (cathode) is present in every tube. But at ordinary room temperatures the free elec­trons in the cathode cannot leave its surface because of cer­tain restraining forces that act as a barrier. These attractive surface forces tend to keep the electrons within the cathode substance, except for a small portion that happens to have sufficient kinetic energy (energy of motion) to break through the barrier. The majority of electrons move too slowly for this to happen.

2. To escape from the surface of the material the electrons must perform a certain amount of work to overcome the restraining surface forces. To do this work the electrons must have sufficient energy imparted to them from some external source of energy, since their own kinetic energy is inadequate. There are four principal methods of obtaining electron emission from the surface of the material: thermionic emission, photoelectric emission, field emission and secondary emission.

3. Thermionic emission. It is the most important and one most commonly used in electron tubes. In this method the metal is heated, resulting in increased thermal or kinetic energy of the unbound electrons. Thus, a greater number of electrons will attain sufficient speed and energy to escape from the surface of the emitter. The number of electrons re­leased per unit area of an emitting surface is related to the absolute temperature of the cathode and a quantity of the work an electron must perform when escaping from the emitting surface.

4. The thermionic emission is obtained by heating the cathode electrically. This may be produced in two ways: 1) by using the electrons emitted from the heating spiral for the conduction of current (direct heating) or 2) by arranging the heating spiral in a nickel cylinder coated with barium oxide which emits the electrons (indirect heating). Normally, the method of indirect heating is used.

5. Photoelectric emission. In this process the energy of the light radiation falling upon the metal surface is transferred to the free electrons within the metal and speeds them up sufficiently to enable them to leave the surface.

6. Field or cold-cathode emission. The application of a strong electric field (i.e. a high positive voltage outside the cathode surface) will literally pull the electrons out of the material surface, because of the attraction of the positive field. The stronger the field, the greater the field emission from the cold emitter surface.

7. Secondary emission. When high-speed electrons sud­denly strike a metallic surface they give up their kinetic energy to the electrons and atoms which they strike. Some of the bombarding electrons collide directly with free elec­trons on the metal surface and may knock them out from the surface. The electrons freed in this way are known as second­ary emission electrons, since the primary electrons from some other source must be available to bombard the second­ary electron-emitting surface.

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