{ "cells": [ { "cell_type": "markdown", "id": "04e8c89b", "metadata": { "slideshow": { "slide_type": "slide" } }, "source": [ "# M1.D2: Introduction to Quantum Mechanics. Particle in a box.\n", "\n", "\n", "## Learning Objectives\n", "\n", "* Use the expression of the energy of a particle to investigate how the energy levels change by changing a) the mass of the particle, b) the length of the box, c) both the mass and the length. \n", "* Quantify the zero-point energy of the system and identify what variables affect its value.\n", "* Use magnitudes such as energy gaps and zero-point energy to quantify and compare the quantum vs classical behavior of two systems.\n", "* Plot the wavefunctions of a particle in a one-dimensional box for different particle masses and different box lengths. Plot the square of the wavefunctions and interpret it as density of probability to find the particle.\n", "\n", "\n", "**Context and goals**\n", "\n", "We saw that Bohr's hydrogen atom gives us the energy levels that can reproduce the emission spectra of hydrogen. However, this is not a predictible model as it is not giving us the mechanics (how the system moves) of the atom. In order to understand how the atomic system moves we need to introduce quantum mechanics.\n", "\n", "Quantum mechanics is so different from the physics that you probably know, with so many new concepts, that we will have to start slowly with the simplest of the quantum systems: The particle in a box, it's not an atom, but it will help us understand some concepts when we tackle the quantum atom.\n", "\n", "We want to know the equations of motion of the atom of Hydrogen (one proton and one electron) which we know must be studied with Quantum Mechanics. Before we solve that problem we will tackle the simplest of the problems in quantum mechanics, a free particle that does not interact with anything else but it is constained inside a box.\n", "\n", "By solving this simple system we will come up with some terms that are useful to keep in mind, such as.\n", "* Hamiltonian\n", "* Schrodinger equation\n", "* Eigenfunctions and wavefunctions\n", "* Eigenvalues and energy values\n", "* Quantum numbers\n", "* Probability and density\n", "* The quantum/classical limit\n", "* Degeneracy\n", "\n" ] }, { "cell_type": "markdown", "id": "b5079d41", "metadata": { "slideshow": { "slide_type": "slide" } }, "source": [ "# What are the equations to solve?\n", "\n", "The **equations of motion** are the equations that allow us to predict the motion of a system as it evolves over time.\n", "\n", "In Classical mechanics (the non-quantum one) there are three main formulations of such equations of motion: the newtonian, the lagrangian, and the hamiltonian. \n", "\n", "The one that you already know is the newtonian.\n", "\n", "$$ F = \\frac{dp}{dt} = m\\frac{dv}{dt} = m\\frac{d^{2}x}{dt^{2}} $$ \n", "and being force the derivative of the energy over position $$ F=-\\frac{dE(x)}{dr}$$ \n", "we have\n", "$$ -\\frac{dE(x)}{dx} = m\\frac{d^{2}x}{dt^{2}}$$\n", "\n", "Which is a partial diferential equation which solutions would be x(t), that is a function that tell us the position (x) of particles that make up the system at any given time. In the case of a falling particle under constant gravity force the solution is a function x(t) that tell us where the particle is at any given time.\n", "\n", "$$\n", "x = x_0 + v_0 t + \\frac{a}{2}t^2\n", "$$" ] }, { "cell_type": "markdown", "id": "cdd8b07d", "metadata": { "slideshow": { "slide_type": "slide" } }, "source": [ "# The Schrodinger equation for a particle in a box (1-dimension)\n", "\n", "
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