Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Published by Pearson
ISBN 10: 0321740904
ISBN 13: 978-0-32174-090-8

Chapter 39 - Wave Functions and Uncertainty - Exercises and Problems - Page 1178: 50

Answer

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Work Step by Step

$$\color{blue}{\bf [a]}$$ The value of the wave function must be the same at $x = 0$ because the wave function must be a continuous function. Noting that the point $x = 0$ mm is included in both the left interval $-1\; \rm{ mm} \leq x \leq 0\; \rm{ mm}$ and the right interval $0 \;\rm{ mm} \leq x \leq 1 \;\rm{ mm}$), So, at $x=0$, $$ \frac{b}{1 +x^2} = c(1 +x)^2 $$ $$ \frac{b}{1 +0^2} = c(1 +0)^2 $$ Thus, $$ \boxed{b = c }$$ $$\color{blue}{\bf [b]}$$ Since $c=b$, the wave function is given by $$\psi(x) = \begin{cases} \dfrac{b}{(1 + x^2)} & -1 \, \text{mm} \leq x < 0 \, \text{mm} \\\\ b(1 + x)^2 & 0 \, \text{mm} \leq x \leq 1 \, \text{mm}\\\\ 0&\rm {elsewhere} \end{cases} $$ And Since $c=b$, the probability density function is given by $$|\psi(x)|^2 = \begin{cases} \dfrac{b^2}{(1 + x^2)^2} & -1 \, \text{mm} \leq x < 0 \, \text{mm} \\\\ b^2(1 + x)^4 & 0 \, \text{mm} \leq x \leq 1 \, \text{mm}\\\\ 0&\rm {elsewhere} \end{cases} $$ See the two graphs below. $$\color{blue}{\bf [c]}$$ To the right from the origin means from 0 to 1 mm since at $x\gt 1$, $|\psi(x)|^2=0$. So to find this probability, we need first to find $c$. We know that $$\text{Prob}(0\leq x\leq 1)= \int_{-\infty}^{\infty} |\psi(x)|^2 dx =1\tag 1$$ So, $$\int_{-1}^{0} \frac{c^2}{(1 + x^2)^2} dx + \int_{0}^{1} c^2 (1 + x)^4 dx = 1 \tag 2$$ Solving the first one: $$ c^2 \int_{-1}^{0} \frac{dx}{(1 + x^2)^2} = c^2 \left[ \frac{x}{2(1 + x^2)} + \frac{\tan^{-1} x}{2} \right]_{-1}^{0} $$ $$c^2 \int_{-1}^{0} \frac{dx}{(1 + x^2)^2} = \left( \frac{1}{4} + \frac{\pi}{8} \right) c^2 = 0.643c^2\tag 3 $$ Solving the second one: $$ \int_{0}^{1} c^2 (1 + x)^4 dx = c^2 \int_{0}^{1} (x^4 + 4x^3 + 6x^2 + 4x + 1) dx$$ $$ \int_{0}^{1} c^2 (1 + x)^4 dx = c^2 \left[ \frac{x^5}{5} + \frac{4x^4}{4} + \frac{6x^3}{3} + \frac{4x^2}{2} + x \right]_{0}^{1} $$ $$ \int_{0}^{1} c^2 (1 + x)^4 dx = \frac{31}{5} c^2 = 6.20c^2 \tag 4$$ Plug (3) and (4) into (2); $$ \left( \frac{1}{4} + \frac{\pi}{8} \right) c^2 + \frac{31}{5} c^2 = 1 $$ Thus, $$c=b \approx \pm \bf 0.382 $$ Hence, from (1) $$\text{Prob}(0\leq x\leq 1)= \int_{-\infty}^{\infty} |\psi(x)|^2 dx = \int_{0}^{1 } c^2 (1 + x)^4 dx $$ $$\text{Prob}(0\leq x\leq 1)= \int_{0}^{1 } c^2 (1 + x)^4 dx $$ $$\text{Prob}(0\leq x\leq 1) = c^2 \int_{0}^{1 \; } (x^4 + 4x^3 + 6x^2 + 4x + 1) dx $$ $$ \text{Prob}(0\leq x\leq 1) = (0.382)^2 \left[ \frac{x^5}{5} + \frac{4x^4}{4} + \frac{6x^3}{3} + \frac{4x^2}{2} + x \right]_{0}^{1 } $$ $$ \text{Prob}(0\leq x\leq 1) = (0.382)^2 \left[ \frac{1}{5} + 1 + 2 + 2 + 1 \right] $$ $$ \text{Prob}(0\leq x\leq 1) = 0.905 =\color{red}{\bf 90.5\%}$$
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