RESISTANCE AND CONDUCTANCE

When a circuit with a potential difference at its terminals is closed, an electric current begins to flow. Owing to the electric forces of the field, free electrons move along the conductor, encounter atoms of the conductor material, impart to them some of their kinetic energy and slow down their flow. Then the field speeds them up again and the cycle is repeated. Every conductor always offers some resistance to the flow of free electrons and therefore heats up.

That property of a body by virtue of which it resists the flow of electricity causing a dissipitation of electrical energy as heat is termed resistance and is designated by the letter R.

The device which introduces resistance into a circuit is known as a resistor. A resistor may be variable (it is called rheostat) and is used to vary the current in a circuit. A rheostat is a length of wire wound on an insulating core.

There are some factors which affect the resistance of a conductor: 1. length: 2. cross-sectional area; 3. material; 4. temperature. The property of a conductor is the passage of electric current which is called conductance. A circuit with large conductance has low resistance and vice versa. Therefore, resistance and conductance are reciprocal quantities. Conductance is presented by the letter G or g.

G. GRAMMAR REVISION. (Complex Object, Complex Subject).

Underline Complex object in the following sentences.

1. We know Morse to have been a painter by profession.

2. Scientists expect lasers to solve the problem of controlled thermonuclear reaction.

3. M. Faraday supposed a beam of light to reserve its polarization as it passed through a magnetized crystal.

4. Designers expect dirigibles to be used for exploration of new territories.

5. Japanese designers believe a new ceramic engine to replace the conventional one.

6. Engineers suppose a new ‘night vision’ system to enable drivers to see better after dark.

7. Scientists believe new laser devices to be widely used in medicine.

8. We know the first digital optical disks to have been produced in 1982 as disks for music.

H. Translate the sentences with Complex subject.

1. The phenomenon of superconductivity appears to have been discovered as early as 1911.

2. Before 1911 superconductivity was assumed to be impossible.

3. Recent discoveries in superconductivity made scientists look for the new conducting materials and for practical applications of the phenomenon.

4. The latest achievements in the field of superconductivity are certain to make a revolution in technology and industry.

5. Recommendations from physicists will allow the necessary measures to be taken to protect the air from pollution.

6. Lasers are sure to do some jobs better and more economically than other devices.

7. A Dutch physicist found a superconducting material to return to normal state when

a strong magnetic field was applied.

8. The electric resistivity of a mercury wire was found to disappear when cooled to -269*C.

9. Additional radio transmitters let the pilot make his approach to an airport by watching his flight instruments.

10. There seems to be a lot of alloys and compounds that become superconductors under certain conditions.

I. Read and translate the given text, using the new lexical minimum.

According to the prominent scientist V.L. Ginzburg, the latest world achievements in the field of superconductivity mean a revolution in technology and industry. Superconductivity was discovered by a Dutch physicist K. Onnes in 1911, who was awarded the Nobel Prize in 1913 for his low temperature research. He found the electrical resistivity of a mercury wire to disappear suddenly when cooled below a temperature of 4 Kelvin (-269*C). Absolute zero is known as to be OK. This discovery was a completely unexpected phenomenon. He also discovered that

a superconducting material can be turned to the normal state either by passing a sufficiently large current through it or by applying a sufficiently strong magnetic field to it. But at that time there was no theory to explain this.

In 1950 physicists Landau and Ginzburg made a great contribution to the development of superconductivity theory. They introduced a model which proved to be useful in understanding electromagnetic properties of superconductors. Research in superconductors became especially active since a discovery made in 1986 by IBM scientists in Zurich. They found a metallic ceramic compound to become a superconductor at a temperature well above the previously achieved record of 23 K.

In 1987 American physicist Paul Chu informed about a much more sensational discovery:

he and his colleagues produced superconductivity at an unbelievable before temperature

98 K in a special ceramic material. At once in all leading laboratories throughout the world superconductivity of critical temperature 100 K and higher (that is, above the boiling temperature of liquid nitrogen) were obtained.

J. Answer the questions to the text H.

1. What is this text about?

2. What is the phenomenon of superconductivity?

3. Who was the first to discover the phenomenon?

4. What scientists worked in this field?

5. Is it possible to return superconducting materials to the normal state?

6. How can it be done?

K. Control yourself. Can you translate the following worlds without a vocabulary?

current circuit resistance

flow load affect

charge source speed

sphere external wire

acid internal cable

solid conductor copper

base insulator rheostat

salt resistor device

L. Translate the following words and word combinations from Russian into English.

температурная зависимость, соли, основания, кислоты, медь, свинец, олово, повышение, понижение, сплавы, сопротивление, реостат, показания, электролиты, амперметр, омметр, описывать, проводимость, охлаждать, сложные соединения, заряд, суперпроводник, поток, электрический ток, свойства, металлы, химические элементы, низкий, высокий, жидкий, твердый, легкий, явление, преимущество, недостаток.

М. Translate the text without a vocabulary.

Superconductivity is a state of matter that chemical elements, compounds and alloys are cooled to temperatures near to absolute zero. A superconductor is a solid material that loses all resistance to the flow of electric current when cooled below a characteristic temperature.

This temperature differs for different materials but is within the absolute zero (-273*C).

Superconductors have thermal, electric and magnetic properties that differ from their properties at higher temperatures and from properties of non-superconductive materials.

Now hundreds of materials are known to become superconductors at low temperatures.

About 26 of the chemical elements are superconductors. Among these are known metals such

as aluminium, tin, lead and mercury. Most of the known superconductors are alloys and compounds.

N. Tell your group-mates what this lesson is about, using new words and word

combinations and Grammar units. Be ready to ask and answer the questions.

Date: 2014-12-28; view: 1513