Copper has a high conductivity.
Conductors, insulatorsand semiconductors Materials are classified as conductorsinsulatorsor semiconductors according to their electric conductivity. The classifications can be understood in atomic terms.
Electrons in an atom can have only certain well-defined energies, and, depending on their energies, the electrons are said to occupy particular energy levels. In a typical atom with many electrons, the lower energy levels are filled, each with the number of electrons allowed by a quantum mechanical rule known as the Pauli exclusion principle.
Depending on the element, the highest energy level to have electrons may or may not be completely full. If two atoms of some element are brought close enough together so that they interact, the two-atom system has two closely spaced levels for each level of the single atom.
If 10 atoms interact, the atom system will have a cluster of 10 levels corresponding to each single level of an individual atom. In a solidthe number of atoms and hence the number of levels is extremely large; most of the higher energy levels overlap in a continuous fashion except Conductor and insulator of electricity certain energies in which there are no levels at all.
Energy regions with levels are called energy bandsand regions that have no levels are referred to as band gaps. The highest energy band occupied by electrons is the valence band. In a conductorthe valence band is partially filled, and since there are numerous empty levels, the electrons are free to move under the influence of an electric field ; thus, in a metal the valence band is also the conduction band.
In an insulatorelectrons completely fill the valence band; and the gap between it and the next band, which is the conduction band, is large. The electrons cannot move under the influence of an electric field unless they are given enough energy to cross the large energy gap to the conduction band.
In a semiconductorthe gap to the conduction band is smaller than in an insulator. At room temperature, the valence band is almost completely filled. A few electrons are missing from the valence band because they have acquired enough thermal energy to cross the band gap to the conduction band; as a result, they can move under the influence of an external electric field.
For many materials, including metals, resistance to the flow of charge tends to increase with temperature.
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. The values of electric resistivities listed in Table 2 show 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 in Figure 12, for example, has many extremely mobile carriers; each copper atom has approximately one free electronwhich is highly mobile because of its small mass. An electrolytesuch as a saltwater solutionis 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 oxygennitrogenand 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 Earth as well as from extraterrestrial cosmic rays i. Electrophoresis is an interesting application based on the mobility of particles suspended in an electrolytic solution.
Different particles proteinsfor example move in the same electric field at different speeds; the difference in speed can be used to separate the contents of the suspension. 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 pointmelts and interrupts the flow of current.
|Copper has a high conductivity.|
|Does Water Really Conduct Electricity?|
|Conductors and Insulators||Electronic band theory a branch of physics says that a charge flows if states are available into which electrons can be excited. This allows electrons to gain energy and thereby move through a conductor such as a metal.|
|Electronic band theory a branch of physics says that a charge flows if states are available into which electrons can be excited.|
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 secondi is in amperes, and R is in ohms.
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.Sep 09, · A conductor easily transfers energy in the form of electricity and/or heat, while an insulator does not easily transfer these types of energy.
Conductors are the substances which allow the electricity to pass through . Dynamic electricity, or electric current, is the uniform motion of electrons through a conductor.
Static electricity is unmoving (if on an insulator), accumulated charge formed by either an excess or deficiency of electrons in an object. Some common insulator materials are glass, plastic, rubber, air, and wood. Insulators are used to protect us from the dangerous effects of electricity flowing through conductors.
Sometimes the voltage in an electrical circuit can be quite high and dangerous.
Dynamic electricity, or electric current, is the uniform motion of electrons through a conductor. Static electricity is unmoving (if on an insulator), accumulated charge formed by either an excess or deficiency of electrons in an object.
Conductors and Insulators. In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of charge through them.
Conductors and Insulators. In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of .