What sort of materials are ohmic and what sort of materials are non-ohmic? I have tried looking around on the internet and have not found any clear way to differentiate between ohmic and non-ohmic materials based on their properties. Would I be correct in saying that metals are ohmic; where as semiconductors and non-metals are non-ohmic? (assuming that they are at a constant temperature).
[Physics] Ohmic and non-ohmic conductors
electrical-resistancematerial-science
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The I-V characteristics of materials and devices should always be measured at the same thermodynamic conditions, i.e. at the same temperature. Mixing the actual isothermal I-V characteristic with the temperature dependence doesn't lead to any useful data for the purposes of physics (but it is occasionally done in electrical engineering and electronics design for certain parts like NTC heaters and breakers).
A pure semiconductor at a constant temperature would be a pretty good Ohmic conductor, i.e. the current will be proportional to the applied voltage. This is a lot harder to measure properly on semiconductors than on metals, though, because of junctions formed with the metal wires that one has to attach for the measurement.
The conduction characteristics of semiconductor devices with one or multiple different materials forming junctions, on the other hand, is highly non-linear and can be made very complex. These devices will also have a temperature dependence, but it can be tuned very finely with appropriate material combinations and geometries.
Pure metals have typically increasing resistance with increasing temperature, but alloys can be made that have almost constant temperature characteristic (i.e. they are both Ohmic and temperature independent). One can also make metal alloys with negative characteristics, if necessary. Both constant and negative temperature characteristic is of enormous importance for the design of electronics, almost none of which would function properly if we couldn't make these near zero-TC metal alloys for resistors and NTC's for temperature measurement and compensation.
Non-metallic materials with very strong negative temperature characteristics often use percolation phenomena, i.e. on grain boundaries in sintered crystal powders, where conduction can only happen in very few narrow points in the material. As the material expands, these points of contact may get lost and the resistance may increase by many orders of magnitude over the technical temperature range of the material. The physics of these systems is very different from that of metals and semiconductors.
I think it would be better to say that power lines are designed to avoid ohmic heating rather than that they make use of it. I am not sure about the potential advantages of the heating for lines that may otherwise be weighed down and damaged or destroyed by snow and ice in cold climates, though. One would have to look at the design requirements for these power systems to understand if their designers make explicit use of these otherwise unwanted losses.
You are correct that one can trade current for voltage and vice versa by adjusting the resistance in circuits. Much of electronics design is a repeated application of that principle.
As for the question of how to design materials that have nearly temperature independent characteristics, that would require a very deep dive into solid state physics and materials research and I will leave that to someone who actually has the necessary detail knowledge. The guiding principle in many of these practical applications is that one tries to offset a positive gradient of one material with the negative gradient of another or one tries to combine multiple materials in such a way that the physical effects (like the formation of defects in the mixed material) offset bulk effects like the increase in the number of conduction band electrons in either of the constituents.
The short answer is: Because metals are really absorptive (which comes from the fact that the nearly free electrons in the metal follow the oscillations of the radiation thereby depleting its energy), but some only in part of the visible range.
The reflectivity of a material is given by the Fresnel equations in terms of the index of refraction. They describe the angle dependency and further tell you that the higher the difference in the index of refraction the more light will be reflected at the interface.
It is important to understand, that the index of refraction in general can be a complex number. The imaginary part of the index of refraction describes the absorption of the material, while the (well known) real part describes the usual "optical density" causing refraction. So there are two possibilities for a material to reflect strongly: Either because it has a large real part of the index of refraction (like diamond) or because it absorbs light strongly (like metals). The latter effect can also be seen with lines written using a dark overhead transparency marker: they reflect in the colour range that does not pass through.
So, the reflection on the surface of metals is mainly due to the imaginary part of the index of refraction (that is, the absorptivity). For coloured metals like copper or gold the so called "plasma frequency" of the metal above which the metal begins to loose its strong absorptivity is in the visible range (or in the near UV). Therefore such metals only reflect a portion of the spectrum, well you get a tinted reflection.
The other materials (plastic, glass, apples) have one thing in common: they have a relatively low absorptivity (while for metals the wave only enters a few nanometers, the other materials range from transparent to waves entering at least several micrometers; the absorption caused by pigments in the material is typically much weaker than the one in metals). This means that the reflection is caused by the change of the real part of the index of refraction. As most materials are only slightly dispersive in the optical range, this means that all frequencies are reflected more or less equally, therefore the reflection is not tinted.
Best Answer
Ohmic conductors are conductors where the current is proportional to the voltage applied across it with all other physical conditions held constant (Temperature being the main one). This is an accurate definition of Ohm's law. The constant of proportionality in this relationship is the same as the resistance.
Any conductor which follows this rule is ohmic, and any conductor that does not is non-ohmic. Simply stating that ohmic conductors should follow the equation V=IR at a constant temperature does not suffice as this relation will be followed by definition of resistance (R=V/I). The real defining quality of an ohmic conductor is that its resistance does not change at a constant temperature.