Transcript Slide 1
The world of the atom In the 1920s and 1930s physicists discovered that the world of the atom was very different than our common sense world. To understand how things work at very tiny scales, like the for the electron and proton, Quantum Physics was created. Quantum Physics describes the characteristics of particles. Using it scientists were able to understand how matter works. All the solid state technology we have today was made possible because of Quantum Physics. This includes, computers, flat screen TVs, ipods, iphones, digital cameras. Particle-Wave Duality, the big surprise Matter is composed of protons, neutrons and electrons. But what are they? Einstein showed that mass and energy are really the same thing. E = mc2 Mass can be changed into energy and energy can be changed into mass. Because of this, matter can be thought of as bound up energy. So what is an electron. Is it a particle that is solid, kind of like a tiny BB? Or is it bound up energy? Which is it? It’s both. Experiments on the nature of the electron showed that some times the electron acts like a particle (localized). Sometimes it acts like a wave (de-localized). Experiments that were conducted to test if the electron was a particle, showed that it was a particle. But test conducted to see if the electron was a wave, showed that it was a wave The two are very different. A particle is compact and solid. It can be hit and will respond by moving around at whatever velocity is imparted to it. A wave is a spread out disturbance which has very specific characteristics. In a nutshell. (Remember this) When an electron is confined to a very small volume of space it has wave characteristics. When an electron is free to move around it has particle characteristics. So far we have only talked about free electrons. Free electrons can have any velocity and move along any path. They are like balls, bullets or BBs. Electrons that are bound to an atom act like waves. In particular, standing waves. Harmonics of a standing wave 2-D standing wave 2-D standing waves in water 3-D standing wave Hydrogen Orbitals Electron bound to the Atom The electron bound to the atom is NOT free to orbit however it wants. It is a standing 3D wave and can only have certain energies. This means that when outside energy is available to the BOUND electron, it can only absorb the energy if that energy exactly matches what is needed to make the electron oscillate as a standing wave. This is not true for a FREE electron. A FREE electron can absorb any energy and responds by gaining that amount of kinetic energy. One last thing… The electron can absorb energy in two ways. It can collide with another charged particle and gain energy. It can absorb EM radiation and gain energy. What happens in this case is that the electric field in the radiation pushes on the electron, which has its own electric field. This makes the electron move. Quiz At the center of the Sun nuclear fusion is producing a lot of energy. This energy moves out into the gas that makes up the rest of the Sun and causes the particles to move around very fast. These same particles interact causing them to give up their kinetic energy to radiant energy. This radiant energy finally escapes the Sun, out into outer space. What would happen to the Sun if more energy was being produced at the center than was escaping the Sun as radiation? Explain. Hydrogen Emission When the electron is in an excited state it can give up that energy in the form of radiant energy. BUT to go from an excited state to a less excited state it can only give up the difference in energy between the two states. This is a very specific amount of energy. Since we know that the energy in an EM wave is inversely proportional to the wavelength, the light that is emitted also has a very specific wavelength. E = hc/λ Three types of spectra Continuous Spectra. This produced by free electrons. The free electrons can have any motion. As a result they emit light at all wavelengths. (called free-free absorption) Emission Spectra. A hot gas in which the electrons are bound can only emit light with very specific energies, which correspond to a change in energy levels. Absorption Spectra. A cool gas in which the electrons are bound can absorb the same specific energies when a continuous light source passes through the gas. What element is in the star? 30 1. 30 2. 3. H (Hydrogen) Hg (Mercury) Ne (Neon) 33% 1 2 3 4 5 6 7 8 9 10 21 22 23 24 25 26 27 28 29 30 11 12 13 33% 14 15 16 33% 17 18 19 0 1 2 3 20 Sun's Spectrum Nuclear Fusion via the proton-proton chain Some things to consider Energy is released when hydrogen is converted into helium. Where does the energy come from? All elements, other than hydrogen have multiple protons locked together in the nucleus. But protons have like charges and repel each other fiercely. How can they remain so close together? Some things to consider Energy is released when hydrogen is converted into helium. Where does the energy come from? The p-p chain reaction shows that 4H He + 2ν + energy But if you add up the mass of 4 H it is greater than the mass of He. The missing mass is converted into energy by E = mc2 Some things to consider All elements, other than hydrogen have multiple protons locked together in the nucleus. But protons have like charges and repel each other fiercely. How can they remain so close together? There is another, stronger force, holding them together. This is the strong nuclear force. It very strong on nuclear scales but only acts over distances about the size of a Uranium nucleus. Then it becomes very weak. To make the protons “stick” together they have to get extremely close to each other. How fast reactions can occur depends on two things. How fast the particles are moving. In other words, it depends on temperature. The collisions have to be head-on. If not the two particles will simple shoot off in other directions when they get close. SO, the more dense the number of particles, the more often reactions can occur. The Sun is neither shrinking nor expanding. How does the amount of energy being produced by nuclear fusion relate to the amount of energy leaving the Sun through radiation? 80 30 1. 2. 3. There is more being produced by fusion than is leaving the sun through radiation There is less being produced by fusion than is leaving the Sun by radiation. The amount of energy produced by fusion is equal to the amount leaving the Sun as radiation. 1 2 3 4 5 6 7 8 9 10 21 22 23 24 25 26 27 28 29 30 0 33% 11 12 13 1 33% 14 15 16 2 33% 17 18 19 3 20 Crucial concept. Since the two are equal, if we know how much energy is leaving the Sun every second, then we know how much energy is being produced each second in the center of the Sun through nuclear fusion. We need to determine the Sun’s luminosity.