Lecture Slides on Quantum Physics | PHYS 1030, Study notes of Physics

Download Lecture Slides on Quantum Physics | PHYS 1030 and more Study notes Physics in PDF only on Docsity! Quantum physics Comes from idea that physical quantities are discontinuous or quantized. Examples: charge comes in quantities of 1.6x10-19 C All electric charges are some integer multiples of 1.6x10-19 C Let’s call 1.6x10-19C = e Allowed charges are: 1.6x10-19C, 3.2x10-19C, 4.8x10-19C… or 1e, 2e, 3e, 4e, … Cannot have: 1.5e or 2.1x10-19C More examples of quantization • Light occurs is packets called photons. If you dimmed a light bulb enough you could release light, 1 photon at a time. Photons will be further discussed later. • Electrons that orbit the nucleus have discrete (quantized) energies. For ‘big’ objects, anything of noticeable size, the wavelength of the object is amazingly small. The object needs to be very small for its wavelength to be detectable. Quantum physics is more useful at the microscopic level. Quantum mechanics accurately describes the behavior of particles such as electrons and atoms. For larger objects, such as people, the quantum mechanics breaks down to classical mechanics (The rules of physics that we’re use to using.) Turns out that energy in an electric-magnetic field does not come in a continuous values. It is quantized. EM fields that oscillate produce light waves. The energy in these waves is quantized. The amount of energy in the field can only have certain values. These energy values are = 0, hf, 2hf, 3hf, 4hf, … h = Planck’s constant = 6.6x10-34 J*s f is the frequency Remember that v=f or f = c/ Thus the values of energy can be written as: 0, h c/ , 2hc/ , 3hc/ , 4hc/ , … Photons are the quanta (carriers of the quantized energy). The photon will have an amount of energy of Planck’s constant times the frequency of the photon. • E = h f • Different colors have different wavelengths. • Wavelength is related to frequency by f = c/ or =c/f • So different colors have photons of different energies. Photons only exist at the instant of impact between light and an object. When a photon hits a screen and causes it to light up, the entire EM field loses an amount of energy equal to the energy of the photon. This is how light interacts with matter. Light, a wave, sometimes interacts with matter like a particle. Energy level diagram Shows the quantized energy levels that the electrons are allowed to exist in. This will be determined by the type of atom. The energy of the electrons is quantized. For an electron to change energies, it must jump from one state to another. Emission When an electron falls from a higher level to a lower level, a photon is emitted. The photon will have energy equal to the difference in the quantum jump. The atom gives off light. The change in the energy levels is equal to the energy of the photon is E = hf (note: quite often we use a new energy unit called the electron-volt, eV. It’s more convenient than the Joule because the energies dealt with are small) Absorption Atoms can absorb light (photons). If a photon hits an atom and it’s energy matches the change in energy between electron energy levels, the photon is captured by the atom. To “make room” for the energy, and electron is bumped up to a higher level. This process is the opposite of emission. Spectrum spectrum – set of frequencies that are emitted/absorbed White light has a continuous spectrum. White light is made up of all the colors. line spectrum – produced when only precise separated frequencies are emitted. Two types: Emission spectrum – shows the wavelengths that are emitted in form of lines on the spectrum Absorption spectrum – shows the wavelengths of the photons that are absorbed as gaps in a continuous spectrum. http://www.colorado.edu/physics/2000/quantumzone/in dex.html If we make an energy level diagram, we can see the corresponding sprectrum: http://phys.educ.ksu.edu/vqm/free/h2spec.ht ml Uncertainty Principle When doing quantum physics, we deal with probabilities. Examples: what is the probability that an electron has so much velocity. The probability that the electron is a certain distance from the nucleus. Potential energy curve. • Classical particles can’t go to regions where the potential energy is more than the total energy. • Quantum mechanics says otherwise. That there is a probability that the particle can be in a restricted region. • When a particle passes through the classically forbidden domain, it is said to have tunneled through a barrier. Example: An electron, or other tiny particle, is allowed to ‘tunnel’ through a barrier via quantum mechanics. There is a probability the particle can pass through a wall. People are made up of the same tiny particles. However, the probability that a person can pass through a wall, practically nonexistent. Here we can see that quantum mechanics shows that there is a probability that a particle can be where classical physics says it’s forbidden. Setting up a barrier we can see quantum tunneling. go to tab 1 – 1.5 The graph represents the probability that the particle is at that location. http://www.quantum-physics.polytechnique.fr/ The height of the graph at each position is related to the probability that the particle is located at that position.