Text 8. Resistance and resistivity

1. Прочтите и переведите текст.

Vocabulary:

resistance – сопротивление

resistivity – удельное сопротивление

to depend on – зависить от

to melt – плавить

unit – единица измерения

honor – честь

to define – определять

steady – устойчивый, постоянный

mercury – ртуть

column – столбик

cross-section – поперечное сечение

to compose – составлять

inversely – обратно

to remain – оставаться

nichrome- (сплав) нихром

Every material offers some resistance to the flow of an electric current through it. The resistance of a conductor depends on its material. It also depends on its temperature. Materials change the value of resistance with change in their temperature. Different materials also have different melting points. Good conductors, like the metals copper, silver, and aluminum, offer very little resistance, while non-conductors, like glass, wood, and paper, offer a very high resistance. The resistance of nichrome is rather high.

Resistance of conductors and their resistivity have different units. The unit by which resistance is measured is called the ohm, in honor of the German physicist Ohm.

The unit of resistance is the ohm while the unit of resistivity is the ohm·m. The standard international ohm is defined as the resistance offered to a steady electric current by a column of mercury 1 sq. mm in cross-section and 106.3 cm long at a temperature of 0º.

There are several factors that determine the electrical resistance of any wire: a) the material, which it is composed of; b) the size of the wire; c) its temperature.

In more general terms, the resistance of a wire is proportional to its length and inversely proportional to its cross-sectional area (provided the temperature of a conductor remains constant). This is Ohm’s law.

2. Ответьте на вопросы по тексту:

1. What does the resistance of a conductor depend on?

2. Can material change the value of resistance?

3. What materials have low (high) value of resistance?

4. What is the unit of resistivity?

5. What are the factors that determine the electrical resistance of any wire?

6. What is Ohm’s Law?

3. Переведите следующие словосочетания:

Metal conductor, steel conductors, aluminum and copper conductors, high conductivity, low resistivity, rather high temperature, conducting materials, resistant material, melting point, rather low melting point, the same point, different units, independent units.

4. Закончите следующие предложения:

1. The unit of resistance is the Ohm while … .

2. The resistivity of aluminium is rather low while … .

3. The melting point of steel is rather high while … .

4. Copper, silver and aluminium are good conductors while … .

5. Выберите правильный вариант:

1. The unit of resistivity is …

1. the ohm b) the ohm·m

2. The value of resistivity …

1. depends on the material b) does not depend on the material

3. When the temperature changes, the value of resistance …

1. changes b) does not change

4. The resistivity of nichrome is …

1. rather high b) rather low

5. Bronze and copper have …

1. different melting points b) the same melting point