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Ground-Level Billiards. (a) Find the lowest energy level for a particle in a box if the particle is a billiard ball \((m=0.20 \mathrm{~kg})\) and the box has a width of \(1.3 \mathrm{~m}\), the size of a billiard table. (Assume that the billiard ball slides without friction rather than rolls; that is, ignore the to what speed does this correspond? How much time would it take at this speed for the ball to move from one side of the table to the other? (c) What is the difference in energy between the \(n=2\) and \(n=1\) levels? (d) Are quantum- mechanical effects important for the game of billiards?

Short Answer

Expert verified
The calculations result in the value of the lowest energy as found in Step 1. The corresponding speed and time taken for the ball to move from one side of the table to other are given by steps 2 and 3 respectively and the difference in energy levels is given by step 4. Based on these results, as discussed in step 5, it can be concluded that quantum-mechanical effects are not important for the game of billiards.

Step by step solution

01

Calculate the lowest energy level

The lowest energy level for a particle in a box, which in this case is a billiard ball, can be calculated using the formula \(E_n = \frac{n^2 h^2}{8 m L^2}\) where \(n\) is the energy level, \(m\) is the mass of the particle, \(h\) is Plank's constant, and \(L\) is the width of the box. With \(n=1\), \(m=0.20kg\), \(h=6.63 \times 10^{-34} Js\) and \(L=1.3m\), we get \(E_1 = \frac{1^2 \cdot (6.63 \times 10^{-34} Js)^2}{8 \cdot 0.20kg \cdot (1.3m)^2}\).
02

Calculating the speed

To calculate the speed that corresponds to this energy, the equation \(E_1 = \frac{1}{2} m v^2\) can be used, where \(v\) is the speed to be determined. So, \(v = \sqrt{\frac{2 E_1}{m}}\).
03

Calculating time taken

To find out how much time it would take at this speed for the ball to move from one side of the table to the other, the equation \(t = \frac{L}{v}\), where \(t\) is the time to be determined, can be used.
04

Difference in energy levels

To find the difference in energy between the \(n=2\) and \(n=1\) levels, we first need to calculate the energy for \(n=2\) using the formula in step 1 and then subtract the energy of \(n=1\) from it to get the difference i.e. \(\Delta E_{21} = E_2 - E_1\).
05

Importance of quantum-mechanical effects for the game of billiards

To evaluate the importance of quantum-mechanical effects for the game of billiards, the energies calculated in the previous steps should be compared with the typical energy scales in a game of billiards. If the energies calculated are significantly smaller, then quantum-mechanical effects would not be important for the game of billiards.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Quantum Mechanics
Quantum Mechanics is a branch of physics that deals with particles at atomic and subatomic levels. It challenges our intuition, which is often based on classical physics principles. In this interesting domain, particles like electrons and protons do not have definite positions but rather exist in a probability cloud, making it fascinating yet complex. Consider the concept of wave-particle duality. Particles exhibit both wave-like and particle-like properties, which means that under certain circumstances, they behave as though they are a wave and at other times like a particle. This dual nature is a cornerstone of quantum phenomena and is precisely what underlies various unique behaviors that are unexplained by classical physics.
When discussing a 'particle in a box', we're addressing a simplified quantum mechanical model. Here, the particle is limited to move within a defined space. The boundaries of this space affect the quantum state of the particle, creating discrete energy levels rather than a continuous range. These distinct energy levels differ from the continuous spectrum that classical mechanics might predict, and they help explain certain phenomena at microscopic scales.
In the realm of billiards, applying quantum mechanics offers a mind-bending perspective, although in practice, classical mechanics suffices due to the macroscopic scale of the game.
Energy Levels
The concept of energy levels is fundamental in quantum mechanics, symbolizing the quantization of energy in microscopic systems. Quantization means that energy exist as discrete amounts rather than a continuum, making systems like a particle in a box have specific allowable energy levels, labeled by an integer, commonly noted as "n".
In our exercise about the billiard ball in a box, energy levels are quantified by the formula:
  • \(E_n = \frac{n^2 h^2}{8 m L^2}\) where \(E_n\) is the energy for level \(n\), \(h\) is Planck's constant, \(m\) is the mass, and \(L\) is the box width.
This formula reveals that energy levels depend on the mass of the particle, the size of the box, and the quantum "level" of interest. As "n" increases, the energy does too, showing that higher energy states are progressively more energetic.
The intriguing part about quantized levels is that transitions between them involve specific energy changes. This is different from classical systems where energy change can be gradual and continuous as observed in everyday actions.
Billiard Ball Problem
The Billiard Ball Problem provides a tangible scenario to apply quantum mechanics concepts in a macroscopic context. While the quantum model of a particle in a box is prominent in microscale systems, extending it to a billiard ball offers insightful yet theoretical learning.
In essence, you imagine a billiard ball placed in a rectangular box similar to a billiard table but constrained in movement with quantum mechanical principles, not classical ones. This brings interesting calculations like:
  • Determining the lowest energy level of the billiard ball, which mathematically, is exceedingly small compared to classical energies.
  • Evaluating the particle’s speed based on its quantum energy, leading to theoretical speeds non-applicable in real-world billiard dynamics.
These energy levels practically accentuate why quantum mechanical effects do not play an active role in actual billiards games. The energy differences are far too minuscule, making classical mechanics fully sufficient. Therefore, while it is scientifically curious, the quantum description remains largely academic in this setting.

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Most popular questions from this chapter

Dots that are the same size but made from different materials are compared. In the same transition, a dot of material 1 emits a photon of longer wavelength than the dot of material 2 does. Based on this model, what is a possible explanation? (a) The mass of the confined particle in material 1 is greater. (b) The mass of the confined particle in material 2 is greater. (c) The confined particles make more transitions per second in material 1 . (d) The confined particles make more transitions per second in material 2 .

Quantum Dots. A quantum dot is a type of crystal so small that quantum effects are significant. One application of quantum dots is in fluorescence imaging, in which a quantum dot is bound to a molecule or structure of interest. When the quantum dot is illuminated with light, it absorbs photons and then re- emits photons at a different wavelength. This phenomenon is called fluorescence. The wavelength that a quantum dot emits when stimulated with light depends on the dot's size, so the synthesis of quantum dots with different photon absorption and emission properties may be possible. We can understand many quantum-dot properties via a model in which a particle of mass \(M\) (roughly the mass of the electron) is confined to a two-dimensional rigid square box of sides \(L\). In this model, the quantumdot energy levels are given by \(E_{m, n}=\left(m^{2}+n^{2}\right)\left(\pi^{2} \hbar^{2}\right) / 2 M L^{2},\) where \(m\) and \(n\) are integers \(1,2,3, \ldots\) According to this model, which statement is true about the energy-level spacing of dots of different sizes? (a) Smaller dots have equally spaced levels, but larger dots have energy levels that get farther apart as the energy increases. (b) Larger dots have greater spacing between energy levels than do smaller dots. (c) Smaller dots have greater spacing between energy levels than do larger dots. (d) The spacing between energy levels is independent of the dot size.

A fellow student proposes that a possible wave function for a free particle with mass \(m\) (one for which the potential-energy function \(U(x)\) is zero) is $$ \psi(x)=\left\\{\begin{array}{ll} e^{+\kappa x}, & x<0 \\ e^{-\kappa x}, & x \geq 0 \end{array}\right. $$ where \(\kappa\) is a positive constant. (a) Graph this proposed wave function. (b) Show that the proposed wave function satisfies the Schrödinger equation for \(x<0\) if the energy is \(E=-\hbar^{2} \kappa^{2} / 2 m-\) that is, if the energy of the particle is negative. (c) Show that the proposed wave function also satisfies the Schrödinger equation for \(x \geq 0\) with the same energy as in part (b). (d) Explain why the proposed wave function is nonetheless not an acceptable solution of the Schrödinger equation for a free particle. (Hint: What is the behavior of the function at \(x=0 ?\) ) It is in fact impossible for a free particle (one for which \(U(x)=0\) ) to have an energy less than zero.

A particle moving in one dimension (the \(x\) -axis) is described by the wave function $$ \psi(x)=\left\\{\begin{array}{ll} A e^{-b x}, & \text { for } x \geq 0 \\ A e^{b x}, & \text { for } x<0 \end{array}\right. $$ where \(b=2.00 \mathrm{~m}^{-1}, A>0,\) and the \(+x\) -axis points toward the right. (a) Determine \(A\) so that the wave function is normalized. (b) Sketch the graph of the wave function. (c) Find the probability of finding this particle in each of the following regions: (i) within \(50.0 \mathrm{~cm}\) of the origin, (ii) on the left side of the origin (can you first guess the answer by looking at the graph of the wave function?), (iii) between \(x=0.500 \mathrm{~m}\) and \(x=1.00 \mathrm{~m}\).

A particle of mass \(m\) in a one-dimensional box has the following wave function in the region \(x=0\) to \(x=L:\) $$\Psi(x, t)=\frac{1}{\sqrt{2}} \psi_{1}(x) e^{-i E_{1} t / \hbar}+\frac{1}{\sqrt{2}} \psi_{3}(x) e^{-i E_{3} t / \hbar}$$ Here \(\psi_{1}(x)\) and \(\psi_{3}(x)\) are the normalized stationary-state wave functions for the \(n=1\) and \(n=3\) levels, and \(E_{1}\) and \(E_{3}\) are the energies of these levels. The wave function is zero for \(x<0\) and for \(x>L\) (a) Find the value of the probability distribution function at \(x=L / 2\) as a function of time. (b) Find the angular frequency at which the probability distribution function oscillates.

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