1 / 37

CONDUCTORS

CONDUCTORS. M V V K Srinivas Prasad K L University. Electrical Conduction in Metals. Ohm’s Law At constant temperature the current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor. Ohm’s Law: Macroscopic form. Resistance.

brooke
Télécharger la présentation

CONDUCTORS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CONDUCTORS M V V K Srinivas Prasad K L University

  2. Electrical Conduction in Metals • Ohm’s Law • At constant temperature the current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor. • Ohm’s Law: Macroscopic form M V V K Srinivas Prasad, K L University

  3. Resistance • The opposing force offered by the material to the flow of current. • Depends on • Nature of the material (ρ). • Temperature. • Geometry/ dimensions (length L, area of cross section A) R = r (L/A) M V V K Srinivas Prasad, K L University

  4. surface area of current flow current flow path length Resistivity • It is a material property. • It defines how difficult is it for current to flow. • Geometry independent. • Temperature dependent. M V V K Srinivas Prasad, K L University

  5. Examples of Resistivity (ρ) • Ag (Silver): 1.59×10-8 Ω·m • Cu (Copper): 1.68×10-8 Ω·m • Graphite (C): (3 to 60)×10-5 Ω·m • Diamond (C): ~1014 Ω·m • Glass: ~1010 - 1014 Ω·m • Pure Germanium: ~ 0.5Ω·m • Pure Silicon: ~ 2300Ω·m M V V K Srinivas Prasad, K L University

  6. Current Density (J) • It is the current flowing through unit area of cross section. M V V K Srinivas Prasad, K L University

  7. Ohm's Law -- Microscopic Form M V V K Srinivas Prasad, K L University

  8. Experimental verification of ohm’s law M V V K Srinivas Prasad, K L University

  9. Electrical conductivity varies between different materials by over 27 orders of magnitude, the greatest variation of any physical property  (.cm)-1 Metals:  > 107 (.m)-1 Semiconductors: 10-6 <  < 105 (.m)-1 Insulators:  < 10-6 (.m)-1 M V V K Srinivas Prasad, K L University

  10. Energy Band Structures in Solids M V V K Srinivas Prasad, K L University

  11. In most of solids conduction is by electrons. • σ depend on no. of electrons available for conduction. • The no. of electrons available for conduction depends on • Arrangement of electrons states or levels with respect to energy. • The manner in which these states are occupied by electrons. M V V K Srinivas Prasad, K L University

  12. Isolated Atom M V V K Srinivas Prasad, K L University

  13. f02_02_pg18 M V V K Srinivas Prasad, K L University

  14. M V V K Srinivas Prasad, K L University

  15. WHY ENERGY BANDS ARE FORMED? M V V K Srinivas Prasad, K L University

  16. Electrons of one atom are perturbed by the electrons and nuclei of the adjacent atoms. • Results in splitting of atomic states into a series of closely spaced electron states to from what are called ELECTRON ENERGY BAND. • Extent of splitting depends on interatomic separation. M V V K Srinivas Prasad, K L University

  17. Electronic Band Structures From Fig. 17.2 Callister’s Materials Science and Engineering, Adapted Version. M V V K Srinivas Prasad, K L University

  18. Band Structure • Valence band – filled – highest occupied energy levels • Conduction band – empty – lowest unoccupied energy levels Conduction band valence band from Fig. 17.3 Callister’s Materials Science and Engineering, Adapted Version. M V V K Srinivas Prasad, K L University

  19. With in each band the energy states are discrete. • No. of states with in each band will equal the total of all states contributed by the N atoms. • s band consists of N states • p band consists of 3N states • Electrical properties of a solid depends on its electron band structure. M V V K Srinivas Prasad, K L University

  20. M V V K Srinivas Prasad, K L University

  21. Energy Band Structure M V V K Srinivas Prasad, K L University

  22. Energy Band Structure M V V K Srinivas Prasad, K L University

  23. Conductors M V V K Srinivas Prasad, K L University

  24. Conductors M V V K Srinivas Prasad, K L University

  25. M V V K Srinivas Prasad, K L University

  26. Partially filled band Overlapping bands Energy Energy empty band empty GAP band partly filled filled band band filled states filled states filled filled band band Conduction & Electron Transport • Metals (Conductors): -- for metals, empty energy states are adjacent to filled states. • thermal energy excites electrons into empty higher energy states. • two types of band structures for metals - partially filled band - empty band that overlaps filled band M V V K Srinivas Prasad, K L University

  27. Energy Band Structures Metals Semiconductors and Insulators M V V K Srinivas Prasad, K L University

  28. Electron movement An electron moves about randomly in a metal being frequently and randomly scattered by thermal vibrations of the atoms. In the absence of an applied field there is no net drift in any direction. M V V K Srinivas Prasad, K L University

  29. Applied Field – net drift • The electrons scatter by collisions with atoms and vacancies that lose the KE and drastically change their direction of motion. • Electronsmove randomly but with a net drift in the direction opposite to the electric field. • In the presence of an applied field, there is a net drift along the x-direction. • After many scattering events the electron has been displaced by a net distance, Δx, from its initial position toward the positive terminal. M V V K Srinivas Prasad, K L University

  30. Scattering of electrons is because of • Imperfections • Impurity atoms • Vacancies • Interstitial atoms • Dislocations • Thermal vibrations M V V K Srinivas Prasad, K L University

  31. Electron Mobility • Force on electron is -eE, e = charge • No obstacles  electron speeds up in an electric field. Vacuum (TV tube) or perfect crystal • Real solid: electrons scattered by collisions with imperfections and thermal vibrations  friction  resistance  net drift velocity of electrons vd = eE e– electron mobility [m2/V-s]. 1 / Friction Transfers part of energy supplied by electric field into lattice asheat. M V V K Srinivas Prasad, K L University

  32. Electron Mobility • Electrical conductivity proportional to number of free electrons per unit volume, Ne, and electron mobility, e  = Nee e Nmetal >> Nsemi metal < semi metal >> semi M V V K Srinivas Prasad, K L University

  33. Electrical resistivity of metals M V V K Srinivas Prasad, K L University

  34. Total resistivity tot (Matthiessen rule) total = thermal+impurity+deformation Increases with T, with deformation, and with alloying. M V V K Srinivas Prasad, K L University

  35. M V V K Srinivas Prasad, K L University

  36. M V V K Srinivas Prasad, K L University

  37. M V V K Srinivas Prasad, K L University

More Related