/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Q22P The gaussian wave packet. A free... [FREE SOLUTION] | 91影视

91影视

The gaussian wave packet. A free particle has the initial wave function

Y(x,0)=Ae-ax2

whereAand are constants ( is real and positive).

(a) NormalizeY(x,0)

(b) Find Y(x,t). Hint: Integrals of the form

-+e-(ax2+bx)dx

Can be handled by 鈥渃ompleting the square鈥: Lety=a[x+bl2a], and note that(ax2+bx)=y2-(b2l4a). Answer:

localid="1658297483210" Y(x,t)=(2a)1/4e-ex2l[1+(2ihatlm)]1+(2ihatlm)

(c) Find . Express your answer in terms of the quantity

localid="1658297497509" =a1+(2ihatlm)2

Sketchlocalid="1658124147567" |Y|2(as a function of x) at t=0, and again for some very large t. Qualitatively, what happens to |Y|2, as time goes on?

(d) Find <x>,<p>,<x2>,<p2>,xand P. Partial answer:localid="1658297458579" <p2>=ah2, but it may take some algebra to reduce it to this simple form.

(e) Does the uncertainty principle hold? At what time tdoes the system come

closest to the uncertainty limit?

Short Answer

Expert verified
  • The normalize wave function is A=2aP1/4.
  • The value ofY(x,t) is role="math" localid="1658125710951" 2aP1/4e-ax2l(1+2t/m)1+(2it/m).
  • The value of Y(x,t)2is 2ape-2ax2l(1+2)1+2.
  • The value of xis 0, pis 0, x2is 14w2p2is 2a, xis 12w, and pis a
  • The uncertainty principle holds and it is closest to the uncertainty limit at t=0, and at that time it is right at the uncertainty limit.

Step by step solution

01

Identification for the given data

The given data is listed below as follows,

The initial wave function for a free particle is,Y(x,0)=Ae-2x2

02

Definition of wave function

The characteristics of a particular wave form is given by a function known as wave function. Also, wave equation is satisfied by this wave function. It is denoted by .

03

(a) Normalize the given wave function

Normalize the given wave function in the following way,

1-A2-e-2ax2dx1-A2p2aA-2ap1/4

Thus, the normalize wave function is A-2ap1/4.

04

(b) Determination of the value of Y(x,t)

Perform the integration.

-e-ax2-bx2dx=-e-y2+(b2/4a)1ady=1aeb2/4a-e-y2dy=paeb2/4a

Find the value of (k).

role="math" localid="1658128927318" (k)=12pA-e-ax2e-ikxdx=12p2a1/4pae-k2/4a=1(2pa)1/4e-k2/4a

Determine the value of .

Y(x,t)=12p1(2pa)1/4-e-k2/4aeikx-k2/2mdk=12p12pa1/4p14a+it2me-x2/4(14+it/2m)=2ap1/4e-ax2l1+2it/m1+2it/m

Thus, the value of Yx,tis 2ap1/4e-ax2/1+2i/m1+2it/m.

05

(c) Determination of the value of |Yx,t|2 and plot |Y|2 on the graph

Assume that=2atm

Substitute for 2atmin Y(x)2ap1/4e-ax2/(1+2it/m)1+2it/m.

Yx,t=2ape-ax2/1+ie-ax2/(1-i)1+i(1-i)

Simplify the exponent.

-ax21+i-ax2(1-i)=-ax21-i+1i1-i(1+)=-2ax21+2

Use the above value and find in Y(x,t)2.

Y(x,t)2=2ape-2ax2l(1+2)1+2

It is known that w=a1+2so, Y(x,t)2=2pwe-2w2x2. Now plot the graph for Yx,t2when t=0

It is known that as the value of tincreases, the graph of Y2flattens out and broadens. So, plot the graph for Y(x,t)2when t>0.

Thus, the value of Y(x,t)2is 2ape-2ax2/(1-e2)1+2.

06

(d) Determination of the value of <x>,<p>,<x2>,<p2>,σx  and σp

Determine the value of xin the following way,

x=-xY2dx

It is the odd integrand.

Determine the value of pin the following way,

p=mdxdt=0

Determine the value of x2in the following way,

x2=2pw-x2e-2w2x2dx=2pw14w2p2w2=14w2

Determine the value of p2in the following way,

p2=-2-y=d2ydx2dx

Write the value of y.

y=Be-bx2

Here, B=2ap1/411+iand b=21+i.

Double differentiate y=Be-bx2with respect to x.

d2ydx2=-2bB(1-2bx2)e-bx2

Multiply y*with the above expression.

y*d2ydx2=-2bB2(1-2bx2)e-(b-b*)x2

It is known that b+b*=2w2B2=2pw.

Substitute all the values in above expression.

y*d2ydx2=-2b2pw(1-2bx2)e-2w2x2

Substitute the above value in p2=-2-y*d2ydx2dx.

p2=2b22pw-(1-2bx2)e-2w2x2dx=2b21-b2w2

It is known that 1-b2w2=a2b, so substitute it in above expression.

p2=2a

Determine the value of xin the following way,

x=12w

Determine the value of pin the following way

p=a

Thus, the value of xis 0,p is 0, x2is 14w2p2is 2a, xis ,12w and pis a

07

(e) Identification of the fact that uncertainty principle holds or not and determination of the value of the system come closest to uncertainty limit

Find the condition that uncertainty principle holds or not.

xp=12wa=21+q2=21+2at/m22

Thus, the uncertainty principle holds and it is closest to the uncertainty limit at t=0, and at that time it is right at the uncertainty limit.

Unlock Step-by-Step Solutions & Ace Your Exams!

  • Full Textbook Solutions

    Get detailed explanations and key concepts

  • Unlimited Al creation

    Al flashcards, explanations, exams and more...

  • Ads-free access

    To over 500 millions flashcards

  • Money-back guarantee

    We refund you if you fail your exam.

Over 30 million students worldwide already upgrade their learning with 91影视!

One App. One Place for Learning.

All the tools & learning materials you need for study success - in one app.

Get started for free

Most popular questions from this chapter

Consider the potential V(x)=-h2a2msech2(ax)where a is a positive constant, and "sech" stands for the hyperbolic secant

(a) Graph this potential.

(b) Check that this potential has the ground state

0(x)and find its energy. Normalize and sketch its graph.

(C)Show that the function 2(x)=A(ik-atanhaxik+a)ekx

(Where k=2mEihas usual) solves the Schr枚dinger equation for any (positive) energy E. Sincetanhz-1asas This represents, then, a wave coming in from the left with no accompanying reflected wave (i.e., no term . What is the asymptotic formk(x) of at large positive x? What are R and T, for this potential? Comment: This is a famous example of a reflectionless potential-every incident particle, regardless of its energy, passes right through.

In Problem 2.7 (d), you got the expectation value of the energy by summing the series in Equation 2.39, but 1 warned you (in footnote 15 not to try it the "old fashioned way,"<H>=(x,0)*H(x,0)dx, because the discontinuous first derivative of(x.0)renders the second derivative problematic. Actually, you could have done it using integration by parts, but the Dirac delta function affords a much cleaner way to handle such anomalies.

(a) Calculate the first derivative of (x.0)(in Problem 2.7), and express the answer in terms of the step function, (x-c1/2)defined in Equation (Don't worry about the end points-just the interior region

(b) Exploit the result of Problem 2.24(b) to write the second derivative of (x,0)in terms of the delta function.

(c) Evaluate the integral (x,0)*H(x,0)dxand check that you get the same answer as before.

-consider the 鈥渟tep鈥 potential:

v(x)={0,ifx0,V0,ifx>0,

a.Calculate the reflection coefficient, for the case E < V0, and comment on the answer.

b. Calculate the reflection coefficient, for the case E >V0.

c. For potential such as this, which does not go back to zero to the right of the barrier, the transmission coefficient is not simply F2A2(with A the incident amplitude and F the transmitted amplitude), because the transmitted wave travels at a different speed . Show thatT=E-V0V0F2A2,for E >V0. What is T for E < V0?

d. For E > V0, calculate the transmission coefficient for the step potential, and check that T + R = 1.


This is a strictly qualitative problem-no calculations allowed! Consider the "double square well" potential (Figure 2.21). Suppose the depth V0and the width a are fixed, and large enough so that several bound states occur.

(a) Sketch the ground state wave function 1and the first excited state localid="1658211858701" 2(i) for the case b = 0 (ii) forbaand (iii) for ba

(b) Qualitatively, how do the corresponding energies(E1andE2)and vary, as b goes from 0 to ? Sketch E1(b)and E2(b)on the same graph.

(c) The double well is a very primitive one-dimensional model for the potential experienced by an electron in a diatomic molecule (the two wells represent the attractive force of the nuclei). If the nuclei are free to move, they will adopt the configuration of minimum energy. In view of your conclusions in (b), does the electron tend to draw the nuclei together, or push them apart? (Of course, there is also the internuclear repulsion to consider, but that's a separate problem.)

Evaluate the following integrals:

(a)-3+1(x3-3x2+2x-1)(x+2)dx.

(b).0[cos(3x)+2](x-)dx

(c)_1+1exp(lxl+3)(x-2)dx

See all solutions

Recommended explanations on Physics Textbooks

View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Study anywhere. Anytime. Across all devices.