p2020chap10 - PHYS-2020: General Physics II Course Lecture...

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Unformatted text preview: PHYS-2020: General Physics II Course Lecture Notes Section X Dr. Donald G. Luttermoser East Tennessee State University Edition 3.3 Abstract These class notes are designed for use of the instructor and students of the course PHYS-2020: General Physics II taught by Dr. Donald Luttermoser at East Tennessee State University. These notes make reference to the College Physics, 9th Edition (2012) textbook by Serway and Vuille. X. Interaction of Photons with Matter A. The Classical Point of View. 1. A system is a collection of particles that interact among them- selves via internal forces and that may interact with the world outside via external fields. a) To a classical physicist, a particle is an indivisible mass point possessing a variety of physical properties that can be measured. i) Intrinsic Properties : These don’t depend on the particle’s location, don’t evolve with time, and aren’t influenced by its physical environment ( e.g. , rest mass and charge). ii) Extrinsic Properties : These evolve with time in response to the forces on the particle ( e.g. , posi- tion and momentum). b) These measurable quantities are called observables . c) Listing values of the observables of a particle at any time = ⇒ specify its state . (A trajectory is an equivalent way to specify a particle’s state.) d) The state of the system is just the collection of the states of the particles comprising it. 2. According to classical physics, all properties, intrinsic and ex- trinsic, of a particle could be known to infinite precision = ⇒ for instance, we could measure the precise value of both position and momentum of a particle at the same time. X–1 X–2 PHYS-2020: General Physics II 3. Classical physics predicts the outcome of a measurement by cal- culating the trajectory ( i.e. , the values of its position and mo- mentum for all times after some initial (arbitrary) time t ◦ ) of a particle: { vector r, vector p, t ; t ≥ t ◦ } ≡ trajectory , (X-1) where the linear momentum is, by definition, vectorp ≡ mvectorv , (X-2) with m the mass of the particle. a) Trajectories are state descriptors of Newtonian physics. b) To study the evolution of the state represented by the trajectory in Eq. (X-1), we use Newton’s Second Law: ma =- ΔPE Δ r , (X-3) where PE is the potential energy of the particle. c) To obtain the trajectory for t > t ◦ , one only need to know PE and the initial conditions = ⇒ the values of vector r and vector p at the initial time t ◦ . d) Notice that classical physics tacitly assumes that we can measure the initial conditions without altering the motion of the particle = ⇒ the scheme of classical physics is based on precise specification of the position and momentum of the particle ....
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This note was uploaded on 03/21/2012 for the course PHY 2020 taught by Professor Staff during the Fall '08 term at University of Florida.

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p2020chap10 - PHYS-2020: General Physics II Course Lecture...

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