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We learned that to be normalizable, a wave function must not itself diverge and must fall to 0 faster than |x|-1/2as x get large. Nevertheless. We find two functions that slightly violate these requirements very useful. Consider the quantum mechanical plane wave Aei(kx-ax)and the weird functionx0(x)pictured which we here call by its proper name. the Dirac delta function.

(a) Which of the two normalizability requirements is violated by the plane wave, and which by Dirac delta function?

(b) Normalization of the plane wave could be accomplished if it were simply truncated, restricted to the region -b<x<+bbeing identically 0 outside. What would then be the relationship between b and A, and what would happen to A as b approaches infinity?

(c) Rather than an infinitely tall and narrow spike like the Dirac delta function. Consider a function that is 0 everywhere except the narrow region-E<x<+ where its value is a constant B. This too could be normalized, What would be the relationship between s and B, and what would happen to B as s approaches 0? (What we get is not exactly the Dirac delta function, but the distinction involves comparing infinities, a dangerous business that we will avoid.)

(d) As we see, the two "exceptional" functions may be viewed as limits of normalizable ones. In those limits, they are also complementary to each other in terms of their position and momentum uncertainties. Without getting into calculations, describe how they are complementary.

Short Answer

Expert verified

(a) The function of plane wave is never fall off but Dirac delta function is diverged.

(b) b Approaches to infinity, a goes to zero.

(c) As the goes to 0 B approaches to infinity.

(d) The nature of the two functions is complementary.

Step by step solution

01

Explanation

It鈥檚 given that a wave function must not be diverge and must not fall to 0 or faster thanx-1/2 .

A plane wave is actually a monochromatic radiation in a field of infinite extent within space travelling in a specified direction. The wave function for the plane wave is always sinusoidal and depends on time.

For the limit of zero width Dirac delta function is used. Thisfunction alwaysdiverges but the wave function for the planewave neverfallsoff.

Thus, the function of plane wave never fall off but Dirac delta function is diverged.

02

To determine

It鈥檚 given that: It鈥檚 given that a wave function must not be diverge and must not fall to 0 or faster than x-1/2.

Formula used:

Write the expression for the wave function of plane wave.

(x)=Aei(kx-蝇迟)鈥︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌.(1)

Here, (x)is the wave function for the plane wave, A is the amplitude of the wave, K is the propagation constant, x is the position of the plane wave, is the angular frequency and t is the time.

Write the expression for the complex conjugate of the wave function (x).

*(x)=Ae-i(kx-蝇迟)鈥︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌︹赌..(2)

Now, multiply equation (1) and (2).

(x)*(x)=(Aei(kx-蝇迟))(Ae-i(kx-蝇迟))

(x)2=A2

Integrate both sides of the above equation.

(x)2dx=A2-bbdx

Substitute 1 for(x)2dx due to normalization and solve the above integration.

1=A2(b-(-b))A2=12bA=12b

Here, b is some position of the plane wave.

Thus as b approaches to infinity, A goes to zero.

03

relationship between s and B.

A function is infinitely tall and has narrow spike like a delta function.

Write the expression for the amplitude of the function.

B=12鈥︹︹︹︹︹︹︹︹︹︹︹︹︹︹.. (3)

Here, B is the amplitude of the wave andis some postion of the wave.

Substitute 0 for in equation (3),

B=

Thus, as the goes to 0 B approaches to infinity.

04

Define the complementary nature of the plane wave function and delta function.

A plane wave is actually a monochromatic radiation in a field of infinite extent within space and travelling in a specified direction. The wave function for the plane wave is always sinusoidal and depends on time.

For the limit of zero width Dirac Delta function is used. This function is always diverges but the wave function for plane wave is never fall off.

The uncertainty of the momentum of plane wave is well defined (螖辫=0)so the uncertainty in position is closed to infinity (螖虫=)according to Heisenberg principle.

The uncertainty of the position of delta function is well defined (螖虫=0), so the uncertainty in momentum is closed to infinity(螖虫=)according to Heisenberg principle.

Thus, the nature of the two function is complementary.

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Most popular questions from this chapter

Sketch(x) . Would you expect this wave function to be the ground state? Why or why not?

Figure 5.15 shows that the allowed wave functions for a finite well whose depth U0was chosen to be62/mL2.

(a) Insert this value in equation (5-23), then using a calculator or computer, solve for the allowed value of kL, of which there are four.

(b) Usingk=2mEfind corresponding values of E. Do they appear to agree with figure 5.15?

(c) Show that the chosenU0implies that 122L2k2.

(d) DefiningLandCto be 1 for convenience, plug your KLand values into the wave function given in exercise 46, then plot the results. ( Note: Your first and third KLvalues should correspond to even function of z, thus using the form withCOSKZ, while the second and forth correspond to odd functions. Do the plots also agree with Figure 5.15?

There are mathematical solutions to the Schr枚dinger equation for the finite well for any energy, and in fact. They can be made smooth everywhere. Guided by A Closer Look: Solving the Finite Well. Show this as follows:

(a) Don't throw out any mathematical solutions. That is in region Il (x<0), assume that (Ce+ax+De-ax), and in region III (x>L), assume that(x)=Fe+ax+Ge-ax. Write the smoothness conditions.

(b) In Section 5.6. the smoothness conditions were combined to eliminate A,Band Gin favor of C. In the remaining equation. Ccanceled. leaving an equation involving only kand , solvable for only certain values of E. Why can't this be done here?

(c) Our solution is smooth. What is still wrong with it physically?

(d) Show that

localid="1660137122940" D=12(B-kA)andF=12e-L[(A-Bk)sin(kL)+(Ak+B)cos(kL)]

and that setting these offending coefficients to 0 reproduces quantization condition (5-22).

A study of classical waves tells us that a standing wave can be expressed as a sum of two travelling waves. Quantum-Mechanical travelling waves, discussed in Chapter 4, is of the form (x,t)=Aei(kx=t). Show that the infinite well鈥檚 standing wave function can be expressed as a sum of two traveling waves.

Show that the uncertainty in the position of a ground state harmonic oscillator is 1/22/mk1/4.

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