Engineering Physics PDF

1. Conducting Materials Conducto1·s - Classical free electron theory of metals - Electrical 11nd thermal conductivity - Wiedemann - Franz Jaw - Lorentz number - Drawbacks of classical theory - Quantum theory - Fermi distribution funrtion - Effect of temperature on Fermi function - Density of e...

1. Conducting Materials Conducto1·s - Classical free electron theory of metals - Electrical 11nd thermal conductivity - Wiedemann - Franz Jaw - Lorentz number - Drawbacks of classical theory - Quantum theory - Fermi distribution funrtion - Effect of temperature on Fermi function - Density of energy states - Carrier concentration in metals. Introdu ction Conduct ing materials are generally metals and their alloys. In the study of solid state materials, the metals occupy a special position because of their variety of striking properties such as high electrical and thermal conductivities. The conducti ng materials play an importa nt role in the field of engineering and technology. While selecting the conducting material for a particular application, we must know electrical and thermal properties. They arc mainly used for conducting electricity and heat. The characteristic property of such material s is high electrical and thermal conductivities due to the presence of free electrons. 0 CONDUCTORS The experimental measure ments have shown that the metals and their alloys exhibit large electrical conductivity in 8 - 1 - t d t the order of 10 Q m. Hence, they arc known as con uc ors. The conductors or conducting material s have high thermal and electrical conductivities. The low resistiv e materia ls arc also general ly called as conduc ting materia ls.  ClalllflcaUon ol OolMluet I ,naterlale Thr rmd11th~. mllh'Tillll' ""' r.l11MiftNf into th,._.~ r,af n~ L- _... on ronducth-it.y. Th('y lHlf't"ll Arr. 8 ~ follow~ ' (il 1A'l"O n-si!Jtivity m11t~ri11IJ (ii) IA> · ff'.. 'lfivity matBials and (iii) High n-sistivity materials.. ·... · mtStiVlty. ,··,.. · ~ ·,. ~~"': ,~s:.--~-:.:·-r.:; ;""':·,:;,.,:: :f - [ow resistivity ;:~ materialst· ' ' ' ,;1;~ ,~a1s1::·~· (i) Zero resistivity materials The superconducting materials like alloys of alu... Illlnium zinc gallium niobium, etc., are a special class of In.ate ' ' ' rials. These materials conduct electricity almost with zero resistan ce below transition temperature. Thus, they are called as zero resistivity materials. These materials are used for energy saving m power systems, super conducting magnets, memory storage devices. I (ii) Low resistivity materials rI'he metals like silver, aluminium and alloys have very high electrical conductivity. These materials are called as low resistivity materials. They are used as conductors, electrical contact, etc., in electrical devices and electrical power transmission and distribution, winding wires in motors and transformers.  M (IW BIiia rnutir,U, fflalmalt The materials like tun~ten, pl11t.inum, nichrome etc., h... high resistivity and low trm}M'rRturc co-efficient of retnltanee. These materials Arc cnllrd as hi/(la rr1i1tivit:, mnterinlt. Such mrtnls nnd their alloys arc used in the manufacturing of resistors, hcnt.ing clcmrnts, rcsistnncc thermometers etc. The conducting property of n solid docs not depend on the tot.al number of electrons nvnilnblc because only the valence electrons of the atoms take part in conduction. When these Yalcnce electrons detach from the orbit, they arc called as free clectron.s or cond11ction electrons. In a metal, the electrical conductivity is proportional to the number of free electrons available. Hence, the electronic structure of a metal determines its electrical conductivity. Basic definition It is necessary to know the basic relations m electrical conductivity to understand its derivation. Ohm's Law \Vhen an electrical current flows through a conductor, then the voltage drop across the conductor is given by the ohm's law V... (1 ) V = IR (or) l = - R where I is the current in ampere, R is the resistance to the current flow in ohm and V is the voltage drop across the conductor in volt.  ''..... It ct eindudot la I IIOUNbf..., and p1operty depen nt r11ctor of the matertal tlflf. Tl r-c I A... (2) p- Proport.ionn]ity... constant known as 1·l's1sbv1ty (o11111 m).. 11.. p ;;: ~ I- Length of the conductor (m) I Arca of cross section (m 2) (J - Electrical conductivity (ohm - Im - I) 1 (or) (mho m- ) (or) siemen m- 1 (Sm- J) Current Density (J) It is defined as the current per unit area of cross section of a current carrying conductor. If I is the current and A is the area of cross-section, then current density is given by I J = ~ I Its unit is Am- 2 Electrical :Field (E) The electrical field E in a conductor of uniform cross section is defined as the potential drop (voltage) V per unit length. 1 I E = ~I (4) lt s unit is Vm-  11111 C I &lllf tit fte I IUt fl tl1daltal al&III 4'J It tt... acn 1111ft _... (A) ti I c1 aa111 ,. appltNI fllN'tliNll ftflltl (R)...tennfll.. ti ~ l'Olldu