like firing a 16 shell at a piece of tissue paper and seeing it bounce back. - E Rutherford

Transcription

1 Black body radiation [const.] λ max = T E radiated surf. area Planck E f hf = T E W KE 4 ROY G BIV Einstein Photo-electric effect- Photons Thompson electron identification B + F v R +q 2 mv F= =qvb=f R F E =qe B Atom structure like firing a 16 shell at a piece of tissue paper and seeing it bounce back. - E Rutherford.

2 6000 K 10-1 [const.] λ max = T 1893 Wiens Law 2000 K T( K) Black Body Radiation Stefan Boltzman Law E radiated surf. area = T K

3 Spectroscopy (emission) prism Black body spectrum hot dense body or diffraction grating emission or bright line spectrum hot (dilute) gas prism 10-2

4 Black Body Radiation & Stars Temperature of star determines light (color) it emits 10-3

5 10-3a

6 Betelgeuse (T~3,200 K) Belt Orion Sword Sirius 10-3b Rigel (T~11,000 K) Orion emission Nebula (red Balmer line)

7 black body radiation spectrum with Microwave background left over from the Big Bang Bell labs 10-3c Holmdel NJ, ~1965 Horn Antenna Penzias & Wilson

8 10-4

9 Photoelectric Effect Einstein (1905) explained/ predicted with photons = light particles (bundles). 1 mv 2 = K 2 f - i metale- Voltage source V + + V - i I=0 for f<f 0 I 1 2. K max = mv 2 3. K m 2 max 1. i i I-light intensity (for f>f 0 ) I K max depends on f (not I) ie measure retarding V needed to stop i K max = ev 0 4. Low I i: high I No I below f 0 f 0 f 0 Another material 10-5

10 10-5a

11 10-5b

12 10-6 hf 0 0

13 hf

14 Quantum theory of photoelectric effect : Einstein 1905 (his Nobel Prize) e - K K= KE of e - after escape E Photon energy in Energy Conservation hf K 0 Q: What is velocity of e - (for Na next page) 1 v 2 m K 2 v 2(K) 2(1.19)(10) m 9.1(10) v 0.511(10) 6 m s e - KE of e - after escape v <<c OK! 0 work function Potential energy e- has to climb to escape Work function Threshold frequency K 0 i 0 I very low hf 0 0 even 1 photon or will give i!! 10-8

15 Photoelectric Effect see next page f 14 1 Example: Na: metal 410nm f Work function=2.28ev e - KE 7.31(10) KE electron =? s E hf [6.63(10) ]7.31(10) [ Js]/ s 10-9 f (10) J E f 4.84(10) J 3.02eV 19 J 1.6(10) ev E W KE f 3.02eV 2.28eV KE KE.74 ev.74ev1.6(10) KE 1.19(10) 19 J 19 J ev

16 c f 8 m 3(10) c f s 9 410(10) m (10) f 7.31(10) 14 1 s 10-9a

17 10-9a

18 Thompson electron identification

19 Thompson crossed E and B experiment gives e/m Milliken Oil Drop experiment gives e charge 10-11

20 Rutherford back scattering target detector source 10-12

21 -J.J. Thomson-cathode rays- particles (e - )much smaller than the atom Thomson Model: Plum Pudding -the plum-pudding model of an atom e - are embedded in the atom like raisins in the pudding Picture of the Atom ~1900 positive charge is equally and uniformly distributed inside the atom 10-13

22 What Rutherford Expected Projectiles (very fast He nuclei called alpha particles) will be slightly deflected by gold atoms Picture of the Atom ~1900 What Rutherford Saw Occasionally (rarely) the projectile scattered at huge angles! 10-14

23 a

24 like firing a 16 shell at a piece of tissue paper and seeing it bounce back. - E Rutherford. Rutherford Atom An atom's mass must be concentrated in a small positively charged nucleus as only a very small number of alpha particles either deflected or rebounded off the foil. Atom:Yankee Stadium :: Nucleus:grain of sand. Most of the atom must be empty space. This space must contain the electrons. The electrons orbit the nucleus like planets around the sun

25 The Atom _ electrons (-) _ orbit around massive nucleus Like solar system on 1A scale. _ + _ Force that binds atom together Electric Force 1/r 2 like gravity but times stronger neutron proton (+) nucleus mass proton p + = mass n mass proton p + ~ 2000 mass electron 10-16

26 Gigantic problem e - in circular orbit e - accelerating should emit EM radiation!!!! e - should lose kinetic energy!!!! e - should spiral into the nucleus!!!! R B + F a v +q Atoms should not exist except briefly There must exist some new physical "stationary state" for e - in its orbit. New physics needed a

27 The Atom + + nucleus by weight + _ + _ + proton 10-17