Phy107Lect22_6per

# Phy107Lect22_6per - HW8 Chap 13 Concept 4 8 12 14(Due 11/3...

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Fri. Oct 29 Phy107 Lecture 22 Quantum mechanics • At very small sizes the world is VERY different! – Energy not continuous, but can take on only particular discrete values. – Light has particle-like properties, so that photons can bounce off objects just like balls. – Particles also have wave-like properties, so that two particles can interfere just like light does. – Physics is not deterministic, but events occur with a probability determined by quantum mechanics. HW8: Chap. 13 Concept 4, 8, 12, 14 (Due 11/3) Problems 2, 8 Fri. Oct 29 Phy107 Lecture 22 The origin of quantum mechanics The photoelectric effect The Blackbody radiation spectrum • Both of these phenomena were impossible to explain with classical physics • Only possible to understand them by postulating that energy is quantized in discrete units. Fri. Oct 29 Phy107 Lecture 22 Energy quantization Example of swinging pendulum. Larger amplitude, larger energy Small energy Large energy Quantum mechanics: Not every swing amplitude is possible energy cannot change by arbitrarily small steps Fri. Oct 29 Phy107 Lecture 22 Energy quantization • Energy can have only certain discrete values Energy states are separated by Δ E = hf. f = frequency h = Planck’s constant= 6.626 x 10 -34 J-s Δ E = hf =3.3x10 -34 J for pendulum = spacing between energy levels d E = mgd =(1 kg)(9.8 m/s 2 )(0.2 m) ~ 2 Joules Δ E min =hf =3.3x10 -34 J << 2 J Quantization not noticeable Period = 2 sec Freq = 0.5 cycles/sec Fri. Oct 29 Phy107 Lecture 22 Light quantization • Quantization also applies to other physical systems – e.g. light – Classically, to change the energy, we change amplitude • Energy of light can change only in steps of hf . f is the frequency of the light Fri. Oct 29 Phy107 Lecture 22 Or could say… • Light comes in particles called photons. • Energy of one photon is E=hf f = frequency of light • Photon is a particle, but moves at speed of light! – This is possible because it has zero mass.

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Phy107Lect22_6per - HW8 Chap 13 Concept 4 8 12 14(Due 11/3...

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