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For many materials, including metals, resistance to the flow of charge tends to increase with temperature. For example, an increase of 5° C (9° F) increases the resistivity of copper by 2 percent. In contrast, the resistivity of insulators and especially of semiconductors such as silicon and germanium decreases rapidly with temperature; the increased thermal energy causes some of the electrons to populate levels in the conduction band where, influenced by an external electric field, they are free to move.
The energy difference between the valence levels and the conduction band has a strong influence on the conductivity of these materials, with a smaller gap resulting in higher conduction at lower temperatures.
There is an extremely large variation in the capability of different materials to conduct electricity. The principal reason for the large variation is the wide range in the availability and mobility of charge carriers within the materials. The copper wire, for example, has many extremely mobile carriers; each copper atom has approximately one free electron, which is highly mobile because of its small mass.
An electrolyte, such as a saltwater solution, is not as good a conductor as copper. The sodium and chlorine ions in the solution provide the charge carriers. The large mass of each sodium and chlorine ion increases as other attracted ions cluster around them. As a result, the sodium and chlorine ions are far more difficult to move than the free electrons in copper.
Pure water also is a conductor, although it is a poor one because only a very small fraction of the water molecules are dissociated into ions. The oxygen, nitrogen, and argon gases that make up the atmosphere are somewhat conductive because a few charge carriers form when the gases are ionized by radiation from radioactive elements on the Earth as well as from extraterrestrial cosmic rays (i.e. high-speed atomic nuclei and electrons).
Electrophoresis is an interesting application based on the mobility of particles suspended in an electrolytic solution. Different particles (proteins, for example) move in the same electric field at different speeds; the difference in speed can be utilized to separate the contents of the suspension.
6. Проверьте усвоенную информацию, ответив на следующие вопросы на английском языке:
1. How does temperature influence the flow of charge?
2. Would materials with smaller band gaps conduct electricity at lower temperatures?
3. Why is the copper wire a good conductor?
4. What materials beside metals conduct electricity?
7. Соотнесите термины и определения:
8. Приведите как можно больше производных слов при помощи суффиксов для следующих глаголов:
resist, conduct, populate, differ, vary, solve, attract, apply, separate