Two questions from the exam

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Maurice Rafe Watkins
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1 Two qestions from the exam 3. When the sn is located near one of the horizons, an observer looking at the sky directly overhead will view partially polarized light. This effect is de to which of the following processes? a) reflection b) refraction c) diffraction d) inteference e) scattering 4. If different filters are sed with an astronomical telescope, which of the following wold give the best resoltion? (Hint: think Rayleigh criterion.) a) red b) green c) ble d) black e) all yield the same resoltion q min = 1.22l/D ble has smallest wavelength of above (know yor spectrm)

2 Another problem 10. In hydrogen, an electron makes a transition from the n = 6 to n = 2 state. What is wavelength of the emitted photon? a) 334 nm b) 372 nm c) 410 nm d) 448 nm e) 486 nm E n = ev/n 2 E 6 = ev/36 = 0.38 ev E 2 = ev/4 = 3.4 ev E 6 -E 2 =Eg=3.02 ev=hc/l l=hc/eg=(6.62x10-34 J.s)(3X10 8 m/s) /[3.02 ev X 1.6 X10-19 J/eV)] =410 nm

Bands and band gaps If we have a single atom, then we ve learned abot the energy levels possible for electrons in the atom If we have many atoms close together (like in a solid), the energy levels

3 Bands and band gaps If we have a single atom, then we ve learned abot the energy levels possible for electrons in the atom If we have many atoms close together (like in a solid), the energy levels smsh ot (technical term) into bands electrons in the solid can have energies within the bands and the energies between the bands are forbidden (band gaps) highest filled band (in grond state) is called valence band next higher empty band is called condction band

This is possible becase the condction band is close in energy to the valence band and

4 Condctors and inslators Sppose I apply a voltage across a condctor. What happens? Some of the electrons can accelerate and gain energy. This is possible becase the condction band is close in energy to the valence band and there are empty energy states to jmp into. This can t happen with inslators where there is too large of an energy gap between.

Semi-condctors In between a condctor and an inslator is a semicondctor

size of the band gap energy E g depends on the temperatre of the

light-emitting diode, electrons jmping from the condction band back to

5 Semi-condctors In between a condctor and an inslator is a semicondctor where there is a small band gap of abot 1 ev silicon gallim arsenide The size of the band gap energy E g depends on the temperatre of the semicondctor the lower the temperatre, the smaller the band gap In a light-emitting diode, electrons jmping from the condction band back to the valence band emit a photon of energy E g E photon =hc/l l=hc/e for E~few ev, l is visible

Back to the ncles In 1919, Rtherford starts to collect the first data indicating that there is another strctre within the ncles- the proton. Two years later, James Chadwick and E.S.

6 Back to the ncles In 1919, Rtherford starts to collect the first data indicating that there is another strctre within the ncles- the proton. Two years later, James Chadwick and E.S. Bieler conclde that some strong force holds the ncles together. Why? If protons in ncles (and not electrons) then what keeps the replsive force from driving the ncles apart? Ncles confined to a very small volme (<10-14 m in diameter), 0.01% of diameter of atom s density is kg/m 3

So here we are in 1930 There were three fndamental particles- electron, proton, and photon. There were three fndamental forces- gravity, electromagnetic, and the strong nclear force.

7 So here we are in 1930 There were three fndamental particles- electron, proton, and photon. There were three fndamental forces- gravity, electromagnetic, and the strong nclear force. Gravity we know is mch weaker than the other forces strong force is abot 100X stronger than EM force s natrally occring elements only have p to 100 protons s strong force between protons satrates bt EM replsive force doesn t s for more than 100 protons replsive force wins There were still some details to work ot bt many felt like Max Born, who said, "Physics as we know it will be over in six months." Who is Max Borns granddaghter?

8 Olivia Newton-John as far as I know, John Travolta is not related to any famos physicist

9 The ncles l The ncles had ceased to be fndamental. It was composed of positively charged protons and netrally charged netrons (not actally discovered ntil 1931 by Chadwick).

10 Characterizing atoms Nclei of atoms consist of protons and netrons Srronding the ncles are the electrons Characterize nclei by following characteristics Atomic nmber Z: # of protons in ncles (also eqal to # electrons for normal atom) Netron nmber N: # of netrons in ncles Mass nmber A: # of ncleons (protons + netrons) in ncles A ZX : X is symbol for element, A is atomic mass and Z is atomic nmber Isotopes of an element have the same atomic nmber bt different atomic weight same nmber of protons and electrons s same chemical properties different nmber of netrons s different nclear properties

11 Masses E R = mc 2 note the factor of 2000 difference nified mass nit: 12C has a mass of 12 am 1 = X kg

12 Radis of ncles Rtherford fond an expression for how close an alpha particle can come to a ncles For a head-on collision all of kinetic energy is changed to potential energy 1 2 (2e)(Ze) 2 mv = k d d is the distance of closest approach

13 Solve for d 4kZe 2 d = mv 2 or a distance of abot 3.2 X m for alpha particles on a gold ncles From these reslts, Rtherford conclded that the positive charge in a ncles is concentrated in a sphere whose radis is no greater than ~10-14 m or abot 10 fermis (1 fermi = 1 X m) Nclear radii go as: r=r o A 1/3 A is the # of ncleons and r o =1.2X10-15 m all nclei have roghly the same density

14 The ncles can exist only becase the strong nclear force between ncleons in a ncles is stronger than the electrostatic replsive force between the protons Light elements have roghly the same nmber of protons and netrons Heavier elements have more netrons than protons (netrons contribte to the strong binding force inside the ncles bt not to the replsive EM force) Isotopes within the shaded region are stable Stable nclei

15 The heavier elements in the niverse were all made in spernova explosions Unstable isotopes are prodced (too many netrons) which then decay to the stable isotopes One of areas of concentration of NSCL (and of RIA) Unstable nclei

16 Binding Energy The total mass of a ncles is less than the sm of the masses of the protons and netrons that comprise it This difference is called the binding energy of the ncles and can be thoght of as the energy that mst be added to a ncles to break it apart iron has the highest binding energy

In 1896, Henri Becqerel, while investigating floresence in ranim salts, accidentally discovered radioactivity Work by Cries and others showed that radioactivity was the reslt of the decay or

17 In 1896, Henri Becqerel, while investigating floresence in ranim salts, accidentally discovered radioactivity Work by Cries and others showed that radioactivity was the reslt of the decay or disintegration of nstable nclei Up til that point, atoms were believed to be indestrctible and forever Cleared p a major qestion as to why the interior of the Earth was still molten Shared the 1903 Nobel prize with the Cries Work by Cries and others showed that radioactivity was the reslt of the decay or disintegration of nstable nclei Radioactivity

18 Alpha particles are helim nclei (2 p, 2 n): Beta particles are speedy electrons: Gamma radiation is a stream of photons: Radiation these are spposed to be x s

19 Half-life If a radioactive sample contains N radioactive nclei at some instant, the nmber of nclei that decay in a time Dt is proportional to N DN/Dt a N DN = -lndt where l is a decay constant R = DN/Dt = ln rate of which atoms decay N=N o e -lt T 1/2 (half-life) is time it takes for half of sample to decay Decay constants vary greatly for different radioactive decays and ths so do halflives

What is fundamental? wwhat is it made of? whow is it all held together? The TWO most fundamental questions about the universe are: Fig. 15.1, p.

What is fundamental? wwhat is it made of? whow is it all held together? The TWO most fundamental questions about the universe are: Fig. 15.1, p. What is fndamental? The TWO most fndamental qestions abot the niverse are: wwhat is it made of? whow is it all held together? Fig. 15.1, p. 467 For Fndamental Elements l The Hnt for the answers to those