Why conductors offer resistance?
Answer:
Resistance is the property of a component which restricts the flow of electric current. Energy is used up as the voltage across the component drives the current through it and this energy appears as heat in the component.
In a metal, the atoms are arranged in a crystal-like configuration. The type of metal will determine how the bonds are arranged, and how closely the atoms are grouped. Electrons can inhabit energy levels. Generally, only the "outer" electrons in an atom interact to form the bonds with other atoms. These outer electrons are held to the atom with a relatively small amount of energy. Normally, they inhabit an energy level we call the valence band. This is their "ground" state. The addition of energy can raise these electrons out of the valence band and into the "conduction" band. In the conduction band they are free to move about within the crystal structure. The application of an electric potential will influence them to move in a particular direction.
Now, in a metal, the valence band is relatively close to the conduction band - that is, very little energy is necessary to cause electrons to jump from their valence state into the conduction band. In fact, we think of metals as having a large population of free electrons in the conduction band all the time. So the application of electric potential will cause them to move - a current flow. So, metals generally have a relatively low (though not zero) resistance. In a material such as glass, there is a large energy gap between the valence and conduction band. This means there are very few free electrons available for current flow, and it takes a large input of energy to raise any electrons into the conduction band.
Within a metal conductor, even though there are free electrons, there is still resistance to current flow. This can be described by simple models, but apparently only quantum electron theories accurately deal with the behavior of metals under extreme conditions such as very low temperatures. Replacing the idea of electrons as particles with electrons as waves solves the problems of the simpler models. You can picture these electron waves oscillating through the metal lattice (which can also be pictured as a wave-like structure) - the interference of the lattice structure with the electrons causes resistance. This resistance is caused mainly by two things. One is impurities in the metal, which cause irregularities in the periodicity of the lattice. The other is the disturbance or "vibration" of the lattice caused by heat. Since some heat is always present (except at absolute zero) there is always some resistance from this source which prevents the electrons from sailing through.
Author: Keith Welch, Radialogical Controls Group...
They are CALLED conductors because they offer much less electrical resistance than materials that we don't call conductors. The level of resistance is the most important part of the definition of the word "conductor". Ignoring the extremely narrow circumstances that permit the existence of "superconductors", all substances offer resistance.
If you asked "Why are liquids so runny?", the answer would be that being "runny" is the defining feature of the state of a substance we call "liquid".
Conductors have resistance because nothing ideal exists. A perfect conductor has no resistance but no such thing exists in the real world. The ideal case can often be used as a good approximation in many cases. There is always loss in any system because nothing is perfectly efficient. The resistance of a conductor is not desired which is why copper is chosen since it does have low resistance.
because the conductor isn't pure
Conductors :
All conductors contain movable electric charges which will move when an electric potential difference (measured in volts) is applied across separate points on a wire (etc) made from the material. This flow of charge (measured in amperes) is what is meant by electric current. In most materials, the amount of current is proportional to the voltage (Ohm's Law) provided the temperature remains constant and the material remains in the same shape and state. The ratio between the voltage and the current is called the resistance (measured in ohms) of the object between the points where the voltage was applied. The resistance across a standard mass (and shape) of a material at a given temperature is called the resistivity of the material. The inverse of resistance and resistivity is conductance and conductivity.
Most familiar conductors are metallic. Copper is the most common material for electrical wiring, and gold for high-quality surface-to-surface contacts. However, there are also many non-metallic conductors, including graphite, solutions of salts, and all plasmas. See electrical conduction for more information on the physical mechanism for charge flow in materials.
Non-conducting materials lack mobile charges, and so resist the flow of electric current, generating heat. In fact, all materials offer some resistance and warm up when a current flows. Thus, proper design of an electrical conductor takes into account the temperature that the conductor needs to be able to endure without damage, as well as the quantity of electrical current. The motion of charges also creates an electromagnetic field around the conductor that exerts a mechanical radial squeezing force on the conductor. A conductor of a given material and volume (length x cross-sectional area) has no real limit to the current it can carry without being destroyed as long as the heat generated by the resistive loss is removed and the conductor can withstand the radial forces. This effect is especially critical in printed circuits, where conductors are relatively small and close together, and inside an enclosure: the heat produced, if not properly removed, can cause fusing (melting) of the tracks.
Since all conductors have some resistance, and all insulators will carry some current, there is no theoretical dividing line between conductors and insulators. However, there is a large gap between the conductance of materials that will carry a useful current at working voltages and those that will carry a negligible current for the purpose in hand, so the categories of insulator and conductor do have practical utility
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