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Prof. David R. Jackson ECE Dept.

ECE 2317 Applied Electricity and Magnetism. Spring 2014. Prof. David R. Jackson ECE Dept. Notes 16. Sharp Point Property. Q [C]. b. a. q a. q b. A sharp point produces a strong electric field. The system is charged with Q [ C ].

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Prof. David R. Jackson ECE Dept.

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  1. ECE 2317 Applied Electricity and Magnetism Spring 2014 Prof. David R. Jackson ECE Dept. Notes 16

  2. Sharp Point Property Q [C] b a qa qb A sharp point produces a strong electric field The system is charged with Q [C]. Determine how much charge goes to each sphere, and the electric field at the surface of each sphere.

  3. Sharp Point Property (cont.) Q [C] b a qa qb We ignore the effects of the wire on the charge distribution, and we ignore the interaction between the two spheres. Also, Solution:

  4. Sharp Point Property (cont.) Era Erb b so a Also (A stronger electric field exists on the smaller sphere.) Hence,

  5. Sharp Point Property (cont.) The field is strong as we approach a sharp bend or point on a practical conductor http://www.feynmanlectures.info/docroot/II_06.html

  6. Sharp Point Property (cont.) The electric field near a sharp metal point may be much higher than the surrounding field. sharp point + V -

  7. Lightning Rod - - - - - - - - - - - - - - Lightning is attracted to the rod more than the rest of the building. Important to have it well grounded !

  8. Lightning Rod (cont.) A lighting rod in action

  9. Lightning Rod (cont.) For larger structures, it is good to have more than one rod, or make them as high as possible.

  10. Lightning Rod (cont.) Examples of lightning rods on homes and barns

  11. Lightning Rod (cont.) A lightning rod on top of the “purple parking garage” at IAH airport.

  12. Lightning Rod (cont.) A close-up of the grounding system

  13. Lightning Rod (cont.) George Bush Intercontinental Airport (IAH) Purple Parking Garage Lighting rods are also on top of the light posts.

  14. Lightning Rod (cont.) A320 Aircraft “Static discharge wicks” are used on aircraft to help bleed off any charge buildup due to friction with the atmosphere. Static charge buildup on antennas (and subsequent corona) can cause interference with radio communications.

  15. Sharp Edge on Wedge The field approaches infinity as we approach the tip of a conducting wedge. y x On wedge surface:

  16. Knife Edge The field approaches infinity as we approach the tip of a flat metal plate.

  17. Knife Edge (cont.) Field near the edge of a parallel-plate capacitor Very strong field Nearly uniform field Weak field http://www.feynmanlectures.info/docroot/II_06.html

  18. Knife Edge (cont.) Simulation: color scale shows field strength Actual physical flux plot

  19. Microstrip Line The field and current approach infinity as we approach the edges of the line. y w h x This causes increased conductor loss, compared with transmissions lines such as coaxial cables. It also lowers the power handling capability due to dielectric breakdown. y or x w

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