Conductors, insulators, and semiconductors
A current flowing through a wire heats it. This familiar phenomenon occurs in the heating coils of an electric range or in the hot tungsten filament of an electric light bulb. This ohmic heating is the basis for the fuses used to protect electric circuits and prevent fires; if the current exceeds a certain value, a fuse, which is made of an alloy with a low melting point, melts and interrupts the flow of current. The power P dissipated in a resistance R through which current i flows is given by:
where P is in watts (one watt equals one joule per second), i is in amperes, and R is in ohms. According to Ohm’s law, the potential difference V between the two ends of the resistor is given by V = iR, and so the power P can be expressed equivalently as:
In certain materials, however, the power dissipation that manifests itself as heat suddenly disappears if the conductor is cooled to a very low temperature. The disappearance of all resistance is a phenomenon known as superconductivity.
As mentioned earlier, electrons acquire some average drift velocity v under the influence of an electric field in a wire. Normally the electrons, subjected to a force because of an electric field, accelerate and progressively acquire greater speed. Their velocity is, however, limited in a wire because they lose some of their acquired energy to the wire in collisions with other electrons and in collisions with atoms in the wire. The lost energy is either transferred to other electrons, which later radiate, or the wire becomes excited with tiny mechanical vibrations referred to as phonons. Both processes heat the material.
The term phonon emphasizes the relationship of these vibrations to another mechanical vibration—namely, sound. In a superconductor, a complex quantum mechanical effect prevents these small losses of energy to the medium. The effect involves interactions between electrons and also those between electrons and the rest of the material.
It can be visualized by considering the coupling of the electrons in pairs with opposite momenta. The motion of the paired electrons is such that no energy is given up to the medium in inelastic collisions or phonon excitations. One can imagine that an electron about to “collide” with and lose energy to the medium could end up instead colliding with its partner so that they exchange momentum without imparting any to the medium.
5. Проверьте усвоенную информацию ответив на следующие вопросы на английском языке:
1. What are fuses used for?
2. What phenomenon is known as the disappearance of all resistance?
3. What happens when electrons collide with other electrons and atoms under the influence of an electric field in a wire?
4. What must be the temperature of the material in order to exhibit the superconducting property?
5. How was the problem of costly liquefied helium solved?
6. Подставьте соответствующие формы глагола to be в пробелы, где требуется, и переведите предложения:
1. The electrons …… influenced by an external electric field are free to move. 2.Electrophoresis is an interesting application ……. based on the mobility of particles ……. suspended in an electrolytic solution. 3. The difference in speed can …….. utilized to separate the contents of the suspension. 4. According to Ohm’s law, the potential difference V ……. given by V = iR. 5. The electrons ……. subjected to a force. 6. The velocity …… limited. 7. The lost energy …… transferred to other electrons. 8. The wire becomes …….. excited with tiny mechanical vibrations ….. referred to as phonons. 9. It can …….. visualized by considering the coupling of the electrons. 10. This material must ……. cooled. 11. Cooling requires the use of ……. liquefied helium.
7. Раскройте скобки, подставляя правильную форму глагола в предложения:
1. Resistance to the flow of charge ……. to increase with temperature. (to tend). 2. The resistivity of semiconductors such as silicon and germanium …….. rapidly with temperature (decrease). 3. The capability of different materials to conduct electricity……….. (vary). 4. The sodium and chlorine ions in the solution ………. the charge carriers (provide). 5. The large mass of each sodium and chlorine ion ………. (increase). 6. A current ………. through a wire heats it (flow). 7. This phenomenon …….. in the hot tungsten filament of an electric light bulb (occur). 8. If the current exceeds a certain value, a fuse …………. the flow of current. (interrupt). 9. The electrons …………. speed (accelerate).