Electrical conductivity | physics | dayline.info
Electrical conductivity: high-pressure phenomena: Effects on electric and Demystified · Quizzes · Galleries · Lists · On This Day · Biographies · Newsletters Moreover, the equations developed to express the relationships between the various be expected with the flow of heat to the surface accompanied by gravitational. The electrical conductivity and density of Ag are 7 X 7. (Ωm) /Vs. b) Assuming the Wiedemann Franz law is obeyed, what is the thermal conductivity of Silver In the case of solids, this relationship breaks down, however its general form. Heat Transfer Questions and Answers – Thermal Conductivity of The average thermal conductivities of water and air conform to the ratio.
This means heat transfer always occurs from a body at a higher temperature to a body at a lower temperature, and will continue until thermal equilibrium is reached. A transfer of thermal energy occurs only through 3 modes: Each mode has a different mechanism and rate of heat transfer, and thus, in any particular situation, the rate of heat transfer depends on how much a certain mode is prevalent.
Conduction involves the transfer of thermal energy by a combination of diffusion of electrons and phonon vibrations — applicable to solids.
Radiation involves the transfer of thermal energy by electromagnetic radiation. The sun is a good example of energy transfer through a near vacuum. This TLP focuses on conduction in crystalline solids. The best metallic thermal conductors are pure copper and silver. At room temperature, commercially pure copper typically has a conductivity of about Wm-1K-1 although the thermal conductivity of a single crystal of copper was measured at 12, Wm-1K-1 at a temperature of In metals, the movement of electrons dominates the conduction of heat.
The bulk material with the highest thermal conductivity aside from the superfluid helium II is, perhaps surprisingly, a non-metal: The high conductivity is even used to test the authenticity of a diamond. Strong covalent bonds within the molecule are responsible for the high conductivity even though there are no free electrons, heat is conducted by phonons. Most natural diamonds also contain boron atoms that replace carbon atoms in the crystal matrix, which also have high thermal conductance.
Even with advanced models, this rapidly becomes far too complicated to model adequately for a material of macroscopic scale. Additionally, the electrons move in straight lines, do not interact with each other, and are scattered randomly by nuclei.
Rather than model the whole lattice, two statistically derived numbers are used: The Drude model can be visualised using the following simulation. With no applied field, it can be seen that the electrons move around randomly.
Use the slider to apply a field, to see its effect on the movement of the electrons. This animation requires Adobe Flash Player 8 and later, which can be downloaded here. However, it is important to note that for non-metals, multivalent metals, and semiconductors, the Drude model fails miserably.
To be able to predict the conductivity of these materials more accurately, quantum mechanical models such as the Nearly Free Electron Model are required.
These are beyond the scope of this TLP Superconductors are also not explained by such simple models, though more information can be found at the Superconductivity TLP. Factors affecting electrical conduction Electrical conduction in most metallic conductors not semiconductors!
There are three important cases: Pure and nearly pure metals For pure metals at around room temperature, the resistivity depends linearly on temperature. Consequently, it is lower in annealed, large crystal metal samples, and higher in alloys and work hardened metals. You might think that at higher temperatures the electrons would have more energy to be able to move through the material, so perhaps it is rather surprising that resistivity increases and conductivity therefore decreases as temperature increases.
The reason for this is that as temperature increases, the electrons are scattered more frequently by lattice vibrations, or phonons, which causes the resistivity to increase. The temperature dependence of the conductivity of pure metals is illustrated schematically in the following simulation.
Use the slider to vary the temperature, to see how the movement of the electrons through the lattice is affected. You can also introduce interstitial atoms by clicking within the lattice. This animation requires Adobe Flash Player 10 and later, which can be downloaded here. Alloys - Solid solution As before, adding an impurity in this case another element decreases the conductivity. Thus, solute atoms with a higher or lower charge than the lattice will have a greater effect on the resistivity.
Thermal conduction metals Metals typically have a relatively high concentration of free conduction electrons, and these can transfer heat as they move through the lattice. Phonon-based conduction also occurs, but the effect is swamped by that of electronic conduction. The following simulation shows how electrons can conduct heat by colliding with the nuclei and transferring thermal energy.
Wiedemann-Franz law Since the dominant method of conduction is the same in metals for thermal and electrical conduction i. The Wiedemann-Franz law states that the ratio of thermal conductivity to the electrical conductivity of a metal is proportional to its temperature.
The thermal conductivity increases with the average electron velocity since this increases the forward transport of energy.
However, the electrical conductivity decreases with an increase in particle velocity because the collisions divert the electrons from forward transport of charge. Ionic conduction For certain materials, there is no net movement of electrons, yet they still conduct electricity. The mechanism is that of ionic conduction, whereby some charged ions can move through the bulk lattice by the usual diffusion mechanisms, except with an electric field driving force. Such ionic conductors are used in solid oxide fuel cells — though for the example of yttria stabilised zirconia YZToperational temperatures are between and degrees C.
Because they conduct by a diffusion like mechanism, higher temperatures lead to higher conductivity, the reverse of what the simple Drude model would predict. Breakdown voltage There is an important, and potentially lethal mechanism by which an insulator can become conductive. In air, it may be commonly recognised as lightning. Gases are commonly ionised in domestic lighting devices. The most common are fluorescent tubes and neon lights.
To initially excite the mercury vapour in a fluorescent tube type light, a voltage spike exceeding the breakdown voltage is needed. This can be noticed when switching such a light on as a sudden ignition, with an associated radio interference spike.
Physics for Kids: Electrical Conductors and Insulators
A faulty tube may not fully ionise, leading to only a small glow at the ends. Under high voltages, even plexiglass may conduct. The temporarily ionised path is opaque on cooling, giving a Lichtenberg figure in this case. For non metals, there are relatively few free electrons, so the phonon method dominates. Metals are good electrical conductors because there are lots of free charges in them. The free charges are usually negative electrons, but in some metals, e. When a voltage difference exists between two points in a metal, it creates an electric field which causes the electrons to move, i.
Of course, the electrons bump into some of the stationary atoms actually, 'ion cores' of the metal and this frictional 'resistance' tends to slow them down.
The resistance depends on the specific type of metal we're dealing with.
The greater the distance an electron can travel without bumping into an ion core, the smaller is the resistance, i. The average distance an electron can travel without colliding is called the 'mean free path. The electrons which are free to respond to the electric field have a thermal speed a sizable percentage of the speed of light, but since they travel randomly with this high speed, they go nowhere on average, i.
Conductance and Resistance Another way to think of conductance is as the opposite of resistance. The resistance of a material is a measurement of how well a material opposes the flow of electric current.
Sometimes conductance is represented by the letter "G" where G is the inverse of resistance, R. It has an electrical resistance of zero. All of the superconductors that have been discovered by scientists to date require a very cold temperature on the order of minus degrees C in order to become superconductors. Electrical Insulators The opposite of a conductor is an insulator. An insulator opposes the flow of electricity. Insulators are important to keep us safe from electricity.
The wire that carries electricity to your computer or television is covered with a rubber-like insulator that protects you from getting electrocuted. Good insulators include glass, the air, and paper. Semiconductors Some materials behave in between a conductor and an insulator.