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PHYSICS 002C Lecture 21

PHYSICS 002C Lecture 21 - PHYSICS 002C Lecture 21 Serway...

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PHYSICS 002C Lecture 21 May 18, 2009 Serway and Jewett Chapter 28 – Quantum Mechanics Prelude to quantum physics– What is wave? What is particle? A wave is a propagating disturbance of some kind that transmits energy and momentum. Standing waves have only energy. If the wave is in a material medium such as water or air or earth or even fire (a plasma) [the four elements of yore], the parts of the medium move to & fro but have no net movement associated with the wave. A particle is an idealization of a material object such as a marble, a baseball, a planet, star, or galaxy, in which we ignore all parameters except e.g. the mass, charge, velocity, and sometimes angular momentum associated with its spin. Euclid’s notion of a “point” is “that which has no extent”. Oops – correction. The transformation in space-time for a moving frame is a contrary rotation of the x and ct axes. The concepts of mass points and continuous waves do not work perfectly, and are bad descriptions of light and atoms, and especially large objects such as buckyballs and tubs of liquid helium. Little massive objects such as electrons and massless ones such as the particles of light that we call photons both exhibit Point interactions Interference effects So they are both point-like and wave-like. To understand means to be able to predict the course of events from an initial set of circumstances, as in understand the stock market or understand why someone would behave in a certain way. Even so, we keep asking WHY is this the way the world works? WHAT is the underlying machinery of it? What was wrong with the old physics – wasn’t that hard enough? I will ignore that comment and proceed to Chap 28.1 Blackbody radiation – hot things emit heat and light – IR, visible and UV radiation. DEMO A Black Body Q I have made an enclosed box with a total inside surface area A = 0.1 m 2 from Al foil which has an emissivity of =0.9. There is a hole in the side of the box with area a= 2 cm 2 . Assuming light entering the hole can only escape if it bounces off the back wall and happens to pass through the hole after the first bounce, what is the approximate intensity of light emerging from the hole compared to the light intensity reflecting from the outside of the box? (a) 10 -1 ; (b) 10 -3 ; (c) 10 -5 ; (d) 10 -10 ; (e) 10 -30 . 1
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You have learned in Physics 2B that Classical Thermodynamics predicts that in equilibrium every degree of freedom will have a mean energy kT 2 1 for every component of the potential or kinetic energy. How many degrees of freedom (how many different waves) are there for light waves in a cubical box made from mirrors with sides of length a ?
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