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Particles of energy Eare incident from the left, where U(x)=0, and at the origin encounter an abrupt drop in potential energy, whose depth is -3E.

  1. Classically, what would the particles do, and what would happen to their kinetic energy?
  2. Apply quantum mechanics, assuming an incident wave of the forminc=eikx, where the normalization constant has been given a simple value of 1, determine completely the wave function everywhere, including numeric values for multiplicative constants.
  3. What is the probability that incident particles will be reflected?

Short Answer

Expert verified
  1. Classical particles would continue to the right, their kinetic energy abruptly increasing from E to 4E.
  2. Therefore, the wave constants arerefl=-13e-ikx and x>0=23eik'x.
  3. The required value of the probability is 19.

Step by step solution

01

Concept used

Energy contained by the object by the virtue of its motion is called kinetic energy.

Potential energyis the energy contained by an object by the virtue of its position.

02

(a) Effect on classical particles and their Kinetic energy

Classical particles would continue to the right, their kinetic energy abruptly increasing from E to 4E E--3E.

03

(b) Determining wave function

Consider the function as:

inc=eikx

To the left of the drop,x<0solve as:

=inc+refl=eikx+Be-ikx

To the right of the drop, (x>0):=Beik'x

Here,

k'=2mE--3Eh=22mEh

Here,data-custom-editor="chemistry" must be continuous at data-custom-editor="chemistry" x=0.

eo+Beo=Ceo1+B=C

Here,ddx must be continuous atx=0.

ikeo-ikBeo=-伪颁别ok1-B=k'C

From the first and second conditions solve as:

k1-B=k'1+BB=k-k'k+k'B=2mEh-2mE--3Eh2mEh+2mE--3EhB=-13

So, data-custom-editor="chemistry" C=23.

Hence the required wave functions are given by,

refl=-13e-ikxx>0=23eik,x

04

(c) Probability that incident particles will be reflected

The required probability can be calculated by

B*BA*A=19

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Most popular questions from this chapter

Fusion in the Sun: Without tunnelling. our Sun would fail us. The source of its energy is nuclear fusion. and a crucial step is the fusion of a light-hydrogen nucleus, which is just a proton, and a heavy-hydrogen nucleus. which is of the same charge but twice the mass. When these nuclei get close enough. their short-range attraction via the strong force overcomes their Coulomb repulsion. This allows them to stick together, resulting in a reduced total mass/internal energy and a consequent release of kinetic energy. However, the Sun's temperature is simply too low to ensure that nuclei move fast enough to overcome their repulsion.

a) By equating the average thermal kinetic energy that the nuclei would have when distant,32KBT. and the Coulomb potential energy they would have when 2fm apart, roughly the separation at which they stick, show that a temperature of about 1019K would be needed.

b) The Sun's core is only about 10k. If nuclei can鈥檛 make it "over the top." they must tunnel. Consider the following model, illustrated in the figure: One nucleus is fixed at the origin, while the other approaches from far away with energyE. As rdecreases, the Coulomb potential energy increases, until the separation ris roughly the nuclear radius rnuc. Whereupon the potential energy is Umaxand then quickly drops down into a very deep "hole" as the strong-force attraction takes over. Given then EUmax, the point b, where tunnelling must begin. will be very large compared with rnuc, so we approximate the barrier's width Las simply b. Its height, U0, we approximate by the Coulomb potential evaluated at b2. Finally. for the energy Ewhich fixes b, let us use 432KBT. which is a reasonable limit, given the natural range of speeds in a thermodynamic system.Combining these approximations, show that the exponential factor in the wide-barrier tunnelling probability is

exp[-e24蟺蔚0h4m3kBT]

c)Using the proton mass for , evaluate this factor for a temperature of107K. Then evaluate it at3000K. about that of an incandescent filament or hot flame. and rather high by Earth standards. Discuss the consequences.

A method for finding tunneling probability for a barrier that is "wide" but whose height varies in an arbitrary way is the so-called WKB approximation.

T=exp[2122m(U(x)E)dx]

Here U(x) is the height of the arbitrary potential energy barrier.Whicha particle first penetrates at x=0 and finally exits at x=L. Although not entirely rigorous, show that this can be obtained by treating the barrier as a series of rectangular slices, each of width dx (though each is still a "wide" barrier), and by assuming that the probability of tunneling through the total is the product of the probabilities for each slice.

What fraction of a beam of 50eVelectrons would get through a 200V1nm wide electrostatic barrier?

Show that if you attempt to detect a particle while tunneling, your experiment must render its kinetic energy so uncertain that it might well be "over the top."

A beam of particles of energy E incident upon a potential step ofU0=(5/4)E is described by wave function:inc(x)=eikx

  1. Determine the reflected wave and wave inside the step by enforcing the required continuity conditions to obtain their (possibly complex) amplitudes.
  2. Verify the explicit calculation the ratio of reflected probability density to the incident probability density is 1.
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