IB Physics 12

1. IB Physics 12 Mr. Jean February 25th, 2014

2. The plan: • Video clip of the day • Wave Duality

3. Wave Duality: • Small Wave Length: • Behaves like a particle. • Large Wave Length: • Behaves like a wave.

4. Compton scattering: • This was the second big experiment which proved that photons existed. • It also had some interesting results.

5. With collisions we must consider: • Conservation of Energy: • Conservation of momentum:

7. Compton scattering: • Major discovery supporting Quantum Theory

10. Compton Scattering: • Compton scattering depends on how big the wave is as it interacts with the material’s surface in comparison to the Compton wave length.

11. Compton Scattering • http://www.youtube.com/watch?v=fI2C4VlR1OM • http://www.youtube.com/watch?v=fI2C4VlR1OM&list=PLFIIOUbmbUQI6vJD0kvxqcNoRAGXZ-0dx • This is a list of different videos explaining (some with examples) of the Compton scattering model.

12. The Problem: • Electrons moving around a nucleus in roughly circular or elliptical paths would feel a centripetal force and would therefore have centripetal acceleration. • But accelerating charges produce electric magnetic radiation, therefore these electrons should continuously emit energy, slow down, and eventually collapse into the nucleus. • Obviously, this does not happen.

13. Limitation of the nuclear model of the atom: • According to classical physics, an orbiting electron is accelerating, and accelerating bodies radiate energy. This would mean that electrons would radiate energy as they orbit the nucleus. This contradicts observations for two reasons: • 1. Electrons would lose energy and spiral into the nucleus. This would destroy all matter. • 2. Electrons would radiate energy as light in a continuous spectrum of colors. This contradicts experimental observation since the emission spectra of atoms are observed to consist of only well-defined discrete wavelengths.

14. Bohr's Explanation: • Laws of electromagnetism do not apply at atomic level. • Within an atom energy is only absorbed or emitted when electrons change energy levels and this energy is QUANTIZED. • Electrons are either in lowest energy state (ground state) or certain allowed levels (excited state).

15. Conclusion: • Observations of atomic emission and absorption spectra indicate that: • electrons do not radiate energy when in stable orbits. Stable orbits only occur at certain radial distance from the nucleus. Thus, electrons in these orbits have a well-defined discrete amount of energy. • electrons only radiate or absorb energy only when they move (transition) between stable orbits. This energy is quantized and fixed by the energy differences between the allowed orbital levels.

16. Emission Spectrum • 1. Low pressure gas is energized by applying a potential difference across it causing it to heat up. • 2. The hot gas emits light energy only at certain well-defined frequencies, as seen through a diffraction grating (spectroscope) or prism.

18. Absorption Spectra • 1. Light is shone through a cool low pressure gas. • 2. A diffraction grating or prism is used to determine what frequencies pass through the gas and which are absorbed.

21. To move from one level to another, electrons must emit or absorb a photon with a certain amount of energy (E = hf). • This absorbed or emitted photon had an energy value equal to the difference between energy levels.

25. Energy Levels: • Bohr found that for the hydrogen atom, the energy associated with a particular level was given by: • En = Energy from change (J) • n = Quantum level

26. Where n is principle quantum number (energy level) and energy is negative because energy is being added to pull the electron away from the nucleus.

28. Changing Energy Levelsto Electrons: • This is a great website highlighting states of energy in matter. • http://www.kcvs.ca/martin/astro/au/unit2/61/chp6_1.html

29. For the remainder of class: • Work on Quest work