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Unformatted text preview: Electromagnetic Fields Homework (Chapter 7) Name: Deng xiaolong (a ) ID Number: 200982009 7-5 Find the energy of the charge distribution of Exercise 5-17 by using (7-8). Answer: as we know (7-8) is 1 ( ) ( ) 2 e S U r r da = v v and the potential of Exercise 5-17 is 2 2 1/2 ( ) [( ) ] 2 z z a z = +- We just need calculate the energy produced in the circular disc. So we need only consider the potential at z = , i.e., (0) 2 a = We also have this expression: da rdrd = , where0 ' 2 and0 ' r a S Therefore: 2 1 1 ( ) ( ) 2 2 2 a e S a U r r da rdrd = = v v Finally, we can get the result: 2 2 2 3 1 2 2 2 2 4 e a a a U = = That means the energy of the charge distribution of Exercise 5-17 is: 2 3 4 e a U = . 7-11 The coaxial cylindrical conductors of Figure 6-12 are used as a capacitor with charges per unit length l q and l q- . Find the energy of a length L of this system by using (7-28) and thus show that the capacitance is (6-45).that the capacitance is (6-45)....
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