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Transcript
NOTES ON LESSON ENGINEERING PHYSICS - II UNIT – I CONDUCTING MATERIALS Materials can be broadly classified in to three types based on conductivity. They are, 1. Conductors (Example: metals), Semi – conductors (Example: germanium, silicon) and Insulators (Example: wood, mica, glass). Conductivity of a Material: Consider a solid material of length and cross sectional area A. The no. of free electrons in 1m3 of the solid is n. Total no. of electrons in the solid N is = Volume no.of electrons. Electron theory of metals. Classical free electron theory. Quantum free electron theory Band theory Drawbacks of Classical Free Electron Theory: Contradiction occurs between classical and Quantum free electron theories on the energy absorption of free electrons. Wiedemann - Franz law (Lorentz Number) The ratio of thermal conductivity to electrical conductivity of a metal is directly proportional to the absolute temperature of the metal . UNIT -II. SEMI CONDUCTING MATERIALS. Introduction The materials whose conductivity and resistivity lies in between conductors and insulators are called semi - conducting materials. They have almost filled valence and empty conduction bands. They have small energy gap. Based on the current conducting property there are two types of semi- conductors. 1) Intrinsic or pure semi- conductor 2) Extrinsic or impure semi – conductor Carrier concentration in an intrinsic semiconductor: Density of charge carries in an intrinsic semi conductor Mobility and conductivity of intrinsic Semi- conductors When and electric field is applied to a Semi- conductor, the charge carries attain drift velocity ). is proportional to the applied field E. conducting materials.Also is different for different Semi - is different for different charge carriers. Extrinsic Semi - Conductor: It is in impure form.It is formed by doping impurities to an intrinsic Semi - Conductor. Depending upon the type of impurity, there are two types of extrinsic Semi - Conductor. N - type extrinsic Semi - Conductors and P - type extrinsic Semi – Conductor Hall effect: When a current carrying conductor ( metal or Semi - conductor ) is placed in a transverse magnetic field, an electric field is produced inside the conductor in a direction normal to both the current and the magnetic field. This phenomen is calle Hall effect and the voltage generated is called hall voltage . UNIT - III MAGNETIC & SUPER CONDUCTING MATERIALS Introduction: Super Conducting Phenomena Super Conductivity was first observed in 1911 by the Dutch Physicist Heike Kamerlingh onnes. In 1908, H.K onnes liquefied Helium gas at 4.2K. Also He studied the properties of Hg. at very low temperatures. He found that at 4.2k, the resistively of Hg suddenly becomes Zero (10-5 Ω cm)i.e., Hg becomes a super conducting material.At very low temperature, the ordinary conducting material has some receptivity.But the super conducting material has zero receptivity as shown in figure. UNIT IV DIELECTRIC MATERIALS Introduction Dielectrics are insulators. They have no free electrons. At abnormal high temperature, they allow a small current. They have high specific resistance ( ). They have negative temperature co- efficient of resistance of resistance ( ). They have large insulation resistance. UNIT – V MODERN ENGINEERING MATERIALS INTRODUCTION In the past few decades, all spheres of science and technology.Consequently, sophisticated areas like aerospace, computers electronics communications require new materials for their technological development.For this, right materials must be selected.And proper application of those materials is essential.