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Consider the wave function
(x,t)=Ae-|x|e-it

whereA, , and are positive real constants. (We鈥檒l see in Chapter for what potential (V) this wave function satisfies the Schr枚dingerequation.)

(a) Normalize .

(b) Determine the expectation values ofx and x2.

(c) Find the standard deviation of . Sketch the graph of2 , as a function ofx, and mark the points (x+)and (x-), to illustrate the sense in which蟽 represents the 鈥渟pread鈥 inx. What is the probability that the particle would be found outside this range?

Short Answer

Expert verified
  1. The value of ise-xeit
  2. The value ofx=0 andx2=122
  3. The standard deviation is =12. The sketch is shown in the figure, andthe probability isP=0.243 .

Step by step solution

01

The given information

Given wave function is,(x,t)=Ae-|x|e-it

Where the real constants are A, and .

02

The normalization of wave equation 

a)

The condition for normalization of wave equation is:

-+x*dx=1

The value of *(x) is (x)=Ae-|x|e-iex ,So

*(x)=Ae-||eiet

The normalization condition can be written:

|A|2-+e-2|x|dx=1

For being even function, the equation can be written as

2A20+e-2xdx=12|A|2-2e-2(*)-e-2(0)=1|A|2-(0-1)=1A=

So, the normalization of the wave equation is e-xeit.

03

The values of x and x2

(b)

Theaverage value of x is x=-+*xdx.

As=* . So,

x=-+x|y|2dx=|A|2-+xe-2|x|dx

Being xe-2|x|an odd function,the value of x=0.

So, the value of x2 is as follows:

x2=-+*x2dx=|A|2-+x2e-2xdx=20+x2e-2xdx=20+x3-1e-2xdx

According to the gamma function, the value will be 0xn-1e-axdx=(n)an.

So,

x2=2(3)(2)3=22!(2)3x2=122

The expectation value of x is 0 and x2=122.

04

The value of standard deviation and probability

(c)

The standard deviation is given by,

2=x2-x2

After substitution the value of x2 and x is:

2=122-0=122=1(2)

The expression |()|2 have to be calculated to make the graph ||2and to mark the points. So,

()2=|A|2e-2=e-212=e-2=e-1.414=e1.414=0.2431

According to the above value,the graph is shown below,

To determine the likelihood that the particle will be discovered outside of this range,

P=--||2dx+a+||2dx=2||2dx=2|A|20e-2xdx=2e-2x-20

Further solving above equation as,

P=-e-2()-e-2a=-1-e-212=-0-e-2=-(-0.2431)=0.2431

So,the value of P is 0.2431.

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Most popular questions from this chapter

Suppose you wanted to describe an unstable particle, that spontaneously disintegrates with a lifetime in that case the total probability of finding the particle somewhere should not be constant but should decrease at an exponential rate:

p(t)=-[x,t2]dx=e-tt

A crude way of achieving this result is as follows. in equation 1.24 we tightly assumed that is real. That is certainly responsible, but it leads to the conservation of probability enshrined in equation 1.27. What if we assign to in imaginary part

V=V0=i

Where is the true potential energy and is a positive real constant?

  1. Show that now we get

dpdt=2hp.

Solve for and find the lifetime of the particle in terms of

In general, quantum mechanics is relevant when the de Broglie wavelength of the principle in question(h/p) is greater than the characteristic Size of the system (d). in thermal equilibrium at (kelvin) TemperatureT the average kinetic energy of a particle is

p22m=32kBT

(Where kBis Boltzmann's constant), so the typical de Broglie wavelength is

=h3mkBT.

The purpose of this problem is to anticipate which systems will have to be treated quantum mechanically, and which can safely be described classically.

(a) Solids. The lattice spacing in a typical solid is aroundd=0.3nm . Find the temperature below which the free 18electrons in a solid are quantum mechanical. Below what temperature are the nuclei in a solid quantum mechanical? (Use sodium as a typical case.) Moral: The free electrons in a solid are always quantum mechanical; the nuclei are almost never quantum mechanical. The same goes for liquids (for which the interatonic spacing is roughly the same), with the exception of helium below4K .

(b) Gases. For what temperatures are the atoms in an ideal gas at pressure quantum mechanical? Hint: Use the ideal gas law (PV=NkBT)to deduce the interatomic spacing.

(a) Find the standard deviation of the distribution in Example 1.1.

(b) What is the probability that a photograph, selected at random, would show a distance x more than one standard deviation away from the average?

A needle of lengthlis dropped at random onto a sheet of paper ruled with parallel lines a distancelapart. What is the probability that the needle will cross a line?

At time t = 0 a particle is represented by the wave function

(x,0)={A(x,0),0xa,A(bx)/(ba),axb,0,otherwise,where A, a, and b are (positive) constants.

(a) Normalize (that is, find A, in terms of a and b).

(b) Sketch (x,0), as a function of x.

(c) Where is the particle most likely to be found, at t = 0?.

(d) What is the probability of finding the particle to the left of a? Check your result in the limiting cases b = a and b= 2a.

(e) What is the expectation value of x?

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