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Ch 40: Quantum Mechanics I: Wave Functions
Young & Freedman Calc - University Physics 15th Edition
Young & Freedman Calc15th EditionUniversity PhysicsISBN: 9780135159552Not the one you use?Change textbook
Chapter 39, Problem 30

An electron with initial kinetic energy 5.05.0 eV encoun­ters a barrier with height U0U_0 and width 0.600.60 nm. What is the transmission coefficient if (a) U0=7.0U_0 = 7.0 eV; (b) U0=9.0U_0 = 9.0 eV; (c) U0=13.0U_0 = 13.0 eV?

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Step 1: Understand the problem. The transmission coefficient describes the probability of an electron tunneling through a potential barrier. This is a quantum mechanical phenomenon governed by the Schrödinger equation. The key parameters are the electron's initial kinetic energy (5.0 eV), the barrier height (U₀), and the barrier width (0.60 nm). The transmission coefficient depends on these values and can be calculated using the formula for quantum tunneling.
Step 2: Recall the formula for the transmission coefficient in the case of a rectangular potential barrier. For an electron with energy E encountering a barrier of height U₀ and width L, the transmission coefficient T is approximately given by: T = e - k L where k is the decay constant inside the barrier, defined as: k = 2 m ħ ² ( U - E ) where m is the mass of the electron, ħ is the reduced Planck constant, U₀ is the barrier height, and E is the electron's energy.
Step 3: Substitute the given values into the formula for k. The mass of the electron (m) is approximately 9.11 × 10⁻³¹ kg, and the reduced Planck constant (ħ) is approximately 1.05 × 10⁻³⁴ J·s. Convert the energies (E and U₀) from eV to joules using the conversion factor 1 eV = 1.6 × 10⁻¹⁹ J. For each case (a, b, c), calculate k using the formula: k = 2 m ħ ² ( U - E )
Step 4: Calculate the transmission coefficient T for each case (a, b, c) using the formula: T = e - k L where L = 0.60 nm = 0.60 × 10⁻⁹ m. Substitute the values of k calculated in Step 3 into this formula for each case.
Step 5: Interpret the results. The transmission coefficient T will be a number between 0 and 1, where values closer to 1 indicate higher probabilities of tunneling. Compare the results for cases (a), (b), and (c) to observe how the barrier height U₀ affects the tunneling probability. Higher barrier heights generally result in lower transmission coefficients.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Quantum Tunneling

Quantum tunneling is a phenomenon in quantum mechanics where a particle can pass through a potential energy barrier, even if its energy is less than the height of the barrier. This occurs due to the wave-like nature of particles, allowing for a probability of finding the particle on the other side of the barrier. The likelihood of tunneling is quantified by the transmission coefficient.

Transmission Coefficient

The transmission coefficient (T) is a measure of the probability that a particle will successfully tunnel through a barrier. It is calculated using the properties of the barrier and the particle's energy. A higher transmission coefficient indicates a greater likelihood of tunneling, while a lower coefficient suggests that tunneling is less probable.
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Potential Energy Barrier

A potential energy barrier is a region in space where the potential energy is higher than the energy of a particle, effectively impeding its motion. In quantum mechanics, even if a particle's energy is lower than the barrier height, there is still a non-zero probability of tunneling through it. The height and width of the barrier significantly influence the transmission coefficient.
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Related Practice
Textbook Question

(a) An electron with initial kinetic energy 3232 eV encoun­ters a square barrier with height 4141 eV and width 0.250.25 nm. What is the probability that the electron will tunnel through the barrier?

(b) A proton with the same kinetic energy encounters the same barrier. What is the probability that the proton will tunnel through the barrier?

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Textbook Question

An electron with initial kinetic energy 6.06.0 eV encounters a barrier with height 11.011.0 eV. What is the probability of tunneling if the width of the barrier is (a) 0.800.80 nm and (b) 0.40 0.40 nm?

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Textbook Question

An electron is in a box of width 3.0*10^-10 m. What are the de Broglie wavelength and the magnitude of the momentum of the electron if it is in (a) the n = 1 level; (b) the n = 2 level; (c) the n = 3 level? In each case how does the wavelength compare to the width of the box?

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Textbook Question

(a) Find the excitation energy from the ground level to the third excited level for an electron confined to a box of width 0.3600.360 nm.

(b) The electron makes a transition from the n=1n = 1 to n=4n = 4 level by absorbing a photon. Calculate the wave­length of this photon.

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Textbook Question

An electron is in a box of width 3.0×10103.0\(\times\)10^{-10} m. What are the de Broglie wavelength and the magnitude of the momentum of the electron if it is in (a) the n=1n = 1 level; (b) the n=2n = 2 level; (c) the n=3n = 3 level? In each case how does the wavelength compare to the width of the box?

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Textbook Question

While undergoing a transition from the n=1n = 1 to the n=2n = 2 energy level, a harmonic oscillator absorbs a photon of wavelength 6.506.50 μμm. What is the wavelength of the absorbed photon when this oscillator undergoes a transition (a) from the n=2n = 2 to the n=3n = 3 energy level and (b) from the n=1n = 1 to the n=3n =3 energy level?

(c) What is the value of (k/m)\(\sqrt{(k^{\prime}\)/m)}, the angular oscillation frequency of the corresponding Newtonian oscillator?

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