Slides on Problems for classical physics | PHY 102, Assignments of Physics

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Major Concepts in PhysicsLecture 17.^ Prof Simon Catterall^ Office 309 Physics, x 5978^ [email protected]^ http://physics/courses/PHY102.08Spring

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This week … „^ Homework 4 – due in lab this week „^ Homework 5 goes out

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Partial resolution „^ Planck/Einstein say that EM waves can^ sometimes

be thought of as stream of photons each carrying energy

E=hf

„^ h – new fundamental constant – Planck’sconstant h=6.6x

-34^ Js

„^ h parametrizes quantum effects –^ ‰^ h=

Æ^ Newton correct ‰ h large^ Æ^ world would be very different ….

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Line emission demo „^ Each atom/element has its own characteristicpattern of emission lines – unique EMfrequencies emitted after atom is excited

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Absorption spectrum „^ Pass gas from white light source through amaterial – see characteristic line are

removed

or absorbed from spectrum. „ Emission of light linked to motion/behavior ofelectrons in atom – thus see that sometimeselectrons move so as to emit light, sometimeslight is absorbed by electrons resulting inchange in their motion

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Before the structure of the atom was known, an empiricalresult was derived for the wavelengths of the spectral linesof hydrogen in the visible portion of the spectrum (theBalmer series).

⎞ ⎟ ⎠ (^111) ⎛ −= R ⎜^24 n λ⎝

Where R = 1.

7,^ × 10 m -1^ is the Rydberg constant and n

≥3.

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Steps to an understanding of atom „^ Atoms electrically neutral – equal numbers ofelectrons and some heavier positivelycharged particle (proton) „^ Simplest model – Thomson plum pudding „^ Shown to be incorrect – Rutherford scatteringexperiment …

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Rutherford scattering

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Rutherford atom „^ Very dense central nucleus surrounded bycloud of electrons „^ Nucleus maybe 10

-5^ smaller than atom but

contains almost all the mass „ Planetary picture of atom …

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Bohr model of H atom (1913) „^ Assume that electron is (represented by) awave „^ Assume wave must fit around a circular orbitif stable …

2 π^ r=n

λ

n – integer, r radius of circular orbit,

λ

wavelength

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What is

„^ Assume Planck relation:

E=hf=hc/

λ

„^ Assume photons are massless particleswhere E=pc (see later) „^ Implies

λ=h/p for photons „^ Assume

same for electrons ….^ (De Broglie relation)

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Allowed energies for H

E=-(me

2 /4πε

(^2) h)(1/2n 0

2 )

When atom is heated electron can move toa state of higher energy. Subsequently when it decays to groundstate again it emits a photon whoseenergy is the difference in those energylevels

hf=En

-E^0

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• Fig. 27.
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• Fig. 27.