MDCAT Physics · Physics · Physics of solids and electronics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A force that stretches a solid produces stress (force per unit area) and strain (extension per unit length). Up to the proportional limit, stress is proportional to strain. Their ratio is Young's modulus, which belongs to the material and not to the size of the wire. Up to the elastic limit the wire returns to its original length. Beyond it the change is permanent (plastic).
Band theory: in a solid the electron energy levels form bands. In conductors the valence and conduction bands overlap. In insulators a wide forbidden gap separates them. In semiconductors the gap is small, about 1 eV, so heating frees more electrons and the resistance falls. Adding a small amount of impurity (doping) gives n-type or p-type material.
Students say Young's modulus changes when the wire is made longer or thicker. The extension changes, but the modulus stays the same for the same material. In the same way, n-type silicon is not negatively charged: it is neutral.
Answer: Extension = 3.0 mm.
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