Question

37.2 In an infinite square well, for which of the following states will the particle never be found in the exact center of the well? a) the ground state b) the first excited state c) the second excited state d) any of the above e) none of the above

Short answer

Answer: b) the first excited state

Step-by-step solution

Recall the wave function for the particle in a square well

The wave function for a particle in an infinite square well of width L is given by: ψ_n(x) = sqrt(2/L) * sin(nπx/L) Where n is the energy level (1, 2, 3, ...) and x is the position within the well.

Compute the probability density for each energy level

To find the probability of finding the particle at the center of the well, we need to evaluate the square of the wave function at x = L/2 for each of the first three energy levels: For n = 1 (ground state): |ψ_1(L/2)|^2 = (2/L) * sin^2(π/2) = 2/L For n = 2 (first excited state): |ψ_2(L/2)|^2 = (2/L) * sin^2(2π/2) = 0 For n = 3 (second excited state): |ψ_3(L/2)|^2 = (2/L) * sin^2(3π/2) = 2/L

Select the correct option

Based on the probability densities obtained above, we can conclude that the particle will never be found in the exact center of the well for: b) the first excited state

Key concepts

Quantum Mechanics

Quantum mechanics is a fundamental theory in physics that describes nature at the smallest scales of energy levels of atoms and subatomic particles. It introduces a new framework for understanding physical phenomena, which at times can seem counterintuitive when compared to classical mechanics. One of the most striking features of quantum mechanics is the principle of wave-particle duality, which suggests that every particle or quantic entity may be described as either a particle or a wave. This duality is at the heart of quantum phenomena such as superposition and entanglement, which have no counterpart in classical physics.

Exercises like the one dealing with an infinite square well are designed to illuminate the principles of quantum mechanics, highlighting how particles behave very differently on a quantum scale compared to what we see in the macroscopic world. These problems require a strong understanding of quantum states, superposition of states, and the probabilistic nature of quantum results.

Wave Function

In quantum mechanics, the wave function is a mathematical description of the quantum state of a system. It is a complex function, represented by the symbol \( \psi \), and provides all the probabilistic information about a particle's behavior. Crucially, while the wave function itself is not directly observable, its absolute square, \( |\psi|^2 \), gives us the probability density, which is a measure of the probability per unit length of finding a particle at a particular point in space at a given time.

For the infinite square well, the wave function is defined by \( \psi_n(x) = \sqrt{2/L} \cdot \sin(n\pi x/L) \). The value of 'n' represents different quantum states corresponding to different energy levels. By examining the wave function, we can gain insights into the behavior of a particle within the well, as it defines the allowed states that the particle can occupy.

Probability Density

The concept of probability density is central to interpreting quantum systems. It's the function that represents the likelihood of finding a particle in a specific region of space. In quantum mechanics, the probability density is given by the absolute square of the wave function, \( |\psi(x)|^2 \). In the context of the infinite square well, calculating the probability density at the center of the well can tell us where the particle is likely to be located at any given time.

For example, in our exercise, the probability density for the ground state and the second excited state at the center of the well is non-zero, meaning there is a finite probability of finding the particle there. Conversely, for the first excited state, the probability density is zero, indicating that one cannot find the particle at the center. This kind of analysis is pivotal for predicting the behavior of quantum systems.

Energy Levels

Energy levels refer to the discrete values of energy that an electron or any quantum particle can have within a quantum system. These levels are a direct consequence of the wave-like properties of particles in quantum mechanics — they must 'fit' into the geometrical space that constrains them, leading to quantization. In the case of an infinite square well, the energy levels are determined by the size of the well and the number of wave half-lengths that can fit inside it, which is correlated with the integer 'n'.

The energy for each state can be calculated using the formula \( E_n = \frac{n^2h^2}{8mL^2} \), where 'h' is Planck's constant, 'm' is the particle's mass, and 'L' is the width of the well. These energy levels are crucial for understanding the quantum state of a particle, as each level corresponds to a different wave function and thus different probabilities of finding the particle in various positions within the well.