Eletrodinâmica Griffiths manual de soluções

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  • Contents Preface 2 1 The Wave Function 3 2 Time-Independent Schrdinger Equation 14 3 Formalism 62 4 Quantum Mechanics in Three Dimensions 87 5 Identical Particles 132 6 Time-Independent Perturbation Theory 154 7 The Variational Principle 196 8 The WKB Approximation 219 9 Time-Dependent Perturbation Theory 236 10 The Adiabatic Approximation 254 11 Scattering 268 12 Afterword 282 Appendix Linear Algebra 283 2nd Edition 1st Edition Problem Correlation Grid 299
  • 2 Preface These are my own solutions to the problems in Introduction to Quantum Mechanics, 2nd ed. I have made every eort to insure that they are clear and correct, but errors are bound to occur, and for this I apologize in advance. I would like to thank the many people who pointed out mistakes in the solution manual for the rst edition, and encourage anyone who nds defects in this one to alert me (grith@reed.edu). Ill maintain a list of errata on my web page (http://academic.reed.edu/physics/faculty/griths.html), and incorporate corrections in the manual itself from time to time. I also thank my students at Reed and at Smith for many useful suggestions, and above all Neelaksh Sadhoo, who did most of the typesetting. At the end of the manual there is a grid that correlates the problem numbers in the second edition with those in the rst edition. David Griths c 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
  • CHAPTER 1. THE WAVE FUNCTION 3 Chapter 1 The Wave Function Problem 1.1 (a) j 2 = 212 = 441. j2 = 1 N j2 N(j) = 1 14 (142 ) + (152 ) + 3(162 ) + 2(222 ) + 2(242 ) + 5(252 ) = 1 14 (196 + 225 + 768 + 968 + 1152 + 3125) = 6434 14 = 459.571. (b) j j = j j 14 14 21 = 7 15 15 21 = 6 16 16 21 = 5 22 22 21 = 1 24 24 21 = 3 25 25 21 = 4 2 = 1 N (j)2 N(j) = 1 14 (7)2 + (6)2 + (5)2 3 + (1)2 2 + (3)2 2 + (4)2 5 = 1 14 (49 + 36 + 75 + 2 + 18 + 80) = 260 14 = 18.571. = 18.571 = 4.309. (c) j2 j 2 = 459.571 441 = 18.571. [Agrees with (b).] c 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
  • 4 CHAPTER 1. THE WAVE FUNCTION Problem 1.2 (a) x2 = h 0 x2 1 2 hx dx = 1 2 h 2 5 x5/2 h 0 = h2 5 . 2 = x2 x 2 = h2 5 h 3 2 = 4 45 h2 = 2h 3 5 = 0.2981h. (b) P = 1 x+ x 1 2 hx dx = 1 1 2 h (2 x) x+ x = 1 1 h x+ x . x+ x + = 0.3333h + 0.2981h = 0.6315h; x x = 0.3333h 0.2981h = 0.0352h. P = 1 0.6315 + 0.0352 = 0.393. Problem 1.3 (a) 1 = Ae(xa)2 dx. Let u x a, du = dx, u : . 1 = A eu2 du = A A = . (b) x = A xe(xa)2 dx = A (u + a)eu2 du = A ueu2 du + a eu2 du = A 0 + a = a. x2 = A x2 e(xa)2 dx = A u2 eu2 du + 2a ueu2 du + a2 eu2 du = A 1 2 + 0 + a2 = a2 + 1 2 . 2 = x2 x 2 = a2 + 1 2 a2 = 1 2 ; = 1 2 . c 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
  • CHAPTER 1. THE WAVE FUNCTION 5 (c) A xa (x) Problem 1.4 (a) 1 = |A|2 a2 a 0 x2 dx + |A|2 (b a) 2 b a (b x)2 dx = |A|2 1 a2 x3 3 a 0 + 1 (b a)2 (b x)3 3 b a = |A|2 a 3 + b a 3 = |A|2 b 3 A = 3 b . (b) xa A b (c) At x = a. (d) P = a 0 ||2 dx = |A|2 a2 a 0 x2 dx = |A|2 a 3 = a b . P = 1 if b = a, P = 1/2 if b = 2a. (e) x = x||2 dx = |A|2 1 a2 a 0 x3 dx + 1 (b a)2 b a x(b x)2 dx = 3 b 1 a2 x4 4 a 0 + 1 (b a)2 b2 x2 2 2b x3 3 + x4 4 b a = 3 4b(b a)2 a2 (b a)2 + 2b4 8b4 /3 + b4 2a2 b2 + 8a3 b/3 a4 = 3 4b(b a)2 b4 3 a2 b2 + 2 3 a3 b = 1 4(b a)2 (b3 3a2 b + 2a3 ) = 2a + b 4 . c 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
  • 6 CHAPTER 1. THE WAVE FUNCTION Problem 1.5 (a) 1 = ||2 dx = 2|A|2 0 e2x dx = 2|A|2 e2x 2 0 = |A|2 ; A = . (b) x = x||2 dx = |A|2 xe2|x| dx = 0. [Odd integrand.] x2 = 2|A|2 0 x2 e2x dx = 2 2 (2)3 = 1 22 . (c) 2 = x2 x 2 = 1 22 ; = 1 2 . |()|2 = |A|2 e2 = e2/ 2 = e 2 = 0.2431. ||2 + x .24 Probability outside: 2 ||2 dx = 2|A|2 e2x dx = 2 e2x 2 = e2 = e 2 = 0.2431. Problem 1.6 For integration by parts, the dierentiation has to be with respect to the integration variable in this case the dierentiation is with respect to t, but the integration variable is x. Its true that t (x||2 ) = x t ||2 + x t ||2 = x t ||2 , but this does not allow us to perform the integration: b a x t ||2 dx = b a t (x||2 )dx = (x||2 ) b a . c 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
  • CHAPTER 1. THE WAVE FUNCTION 7 Problem 1.7 From Eq. 1.33, d p dt = i t x dx. But, noting that 2 xt = 2 tx and using Eqs. 1.23-1.24: t x = t x + x t = i 2m 2 x2 + i V x + x i 2m 2 x2 i V = i 2m 3 x3 2 x2 x + i V x x (V ) The rst term integrates to zero, using integration by parts twice, and the second term can be simplied to V x V x V x = ||2 V x . So d p dt = i i ||2 V x dx = V x . QED Problem 1.8 Suppose satises the Schrodinger equation without V0: i t = 2 2m 2 x2 + V . We want to nd the solution 0 with V0: i 0 t = 2 2m 2 0 x2 + (V + V0)0. Claim: 0 = eiV0t/ . Proof: i 0 t = i t eiV0t/ + i iV0 eiV0t/ = 2 2m 2 x2 + V eiV0t/ + V0eiV0t/ = 2 2m 2 0 x2 + (V + V0)0. QED This has no eect on the expectation value of a dynamical variable, since the extra phase factor, being inde- pendent of x, cancels out in Eq. 1.36. Problem 1.9 (a) 1 = 2|A|2 0 e2amx2 / dx = 2|A|2 1 2 (2am/ ) = |A|2 2am ; A = 2am 1/4 . (b) t = ia; x = 2amx ; 2 x2 = 2am + x x = 2am 1 2amx2 . Plug these into the Schrodinger equation, i t = 2 2m 2 x2 + V : V = i (ia) + 2 2m 2am 1 2amx2 = a a 1 2amx2 = 2a2 mx2 , so V (x) = 2ma2 x2 . c 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
  • 8 CHAPTER 1. THE WAVE FUNCTION (c) x = x||2 dx = 0. [Odd integrand.] x2 = 2|A|2 0 x2 e2amx2 / dx = 2|A|2 1 22(2am/ ) 2am = 4am . p = m d x dt = 0. p2 = i x 2 dx = 2 2 x2 dx = 2 2am 1 2amx2 dx = 2am ||2 dx 2am x2 ||2 dx = 2am 1 2am x2 = 2am 1 2am 4am = 2am 1 2 = am . (d) 2 x = x2 x 2 = 4am = x = 4am ; 2 p = p2 p 2 = am = p = am . xp = 4am am = 2 . This is (just barely) consistent with the uncertainty principle. Problem 1.10 From Math Tables: = 3.141592653589793238462643 (a) P(0) = 0 P(1) = 2/25 P(2) = 3/25 P(3) = 5/25 P(4) = 3/25 P(5) = 3/25 P(6) = 3/25 P(7) = 1/25 P(8) = 2/25 P(9) = 3/25 In general, P(j) = N(j) N . (b) Most probable: 3. Median: 13 are 4, 12 are 5, so median is 4. Average: j = 1 25 [0 0 + 1 2 + 2 3 + 3 5 + 4 3 + 5 3 + 6 3 + 7 1 + 8 2 + 9 3] = 1 25 [0 + 2 + 6 + 15 + 12 + 15 + 18 + 7 + 16 + 27] = 118 25 = 4.72. (c) j2 = 1 25 [0 + 12 2 + 22 3 + 32 5 + 42 3 + 52 3 + 62 3 + 72 1 + 82 2 + 92 3] = 1 25 [0 + 2 + 12 + 45 + 48 + 75 + 108 + 49 + 128 + 243] = 710 25 = 28.4. 2 = j2 j 2 = 28.4 4.722 = 28.4 22.2784 = 6.1216; = 6.1216 = 2.474. c 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
  • CHAPTER 1. THE WAVE FUNCTION 9 Problem 1.11 (a) Constant for 0 , otherwise zero. In view of Eq. 1.16, the constant is 1/. () = 1/, if 0 , 0, otherwise. 1/ /2 0 3/2 () (b) = () d = 1 0 d = 1 2 2 0 = 2 [of course]. 2 = 1 0 2 d = 1 3 3 0 = 2 3 . 2 = 2 2 = 2 3 2 4 = 2 12 ; = 2 3 . (c) sin = 1 0 sin d = 1 ( cos )| 0 = 1 (1 (1)) = 2 . cos = 1 0 cos d = 1 (sin )| 0 = 0. cos2 = 1 0 cos2 d = 1 0 (1/2)d = 1 2 . [Because sin2 + cos2 = 1, and the integrals of sin2 and cos2 are equal (over suitable intervals), one can replace them by 1/2 in such cases.] Problem 1.12 (a) x = r cos dx = r sin d. The probability that the needle lies in range d is ()d = 1 d, so the probability that its in the range dx is (x)dx = 1 dx r sin = 1 dx r 1 (x/r)2 = dx r2 x2 . c 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
  • 10 CHAPTER 1. THE WAVE FUNCTION (x) xr 2r-r-2r (x) = 1 r2x2 , if r < x < r, 0, otherwise. [Note: We want the magnit